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Syntheses of chlorin, benzoporphyrin and bacteriochlorin derivatives Yon-Hin, Paul 1989

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SYNTHESES OF CHLORIN, BENZOPORPHYRDV AND BACTERIOCHLORIN DERIVATIVES BY PAULYON-HIN B. Sc., McMaster University, 1983 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF CHEMISTRY We accept this as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1989 ® Paul Yon-Hin, 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ii Dr. D. Dolphin ABSTRACT The syntheses of chlorin, benzoporphyrin, and bacteriochlorin derivatives are presented in this thesis. The key step in each synthesis involved a Diels-Alder reaction of a vinylporphyrin with an appropriate dienophile. The vinylporphyrins 81,101 and 106 were prepared in high yield using a variation of Johnson's regioselective synthesis employing dipyrromethenes 93,94,104, and 144 as crucial building blocks. As the first objective, the Diels-Alder reactions of 81 with 1,2-disubstituted vinyl sulfones were investigated in order to provide a route to chlorin derivatives which could act as intermediates in a proposed synthetic pathway for a model compound of factor 1. The regio-and stereoselectivity of the cycloadditions were examined and the appropriate regioisomers were considered for the continuation of the proposed synthetic plan. The second objective of the work was to provide a general strategy directed towards the synthesis of benzoporphyrin derivatives via a common intermediate, namely the P-unsubstituted-P'-vinylporphyrin 101. Evidence is presented that suggests an isomerization of the initial cycloadduct to another porphyrin en route to the benzoporphyrin 171. The final objective of the work was to synthesize stable bacteriochlorin derivatives to be used as photosensitizers in photodynamic therapy. The key intermediate, an A,C-divinylporphyrin 106, was synthesized via two routes, and its chemistry with olefinic and acetylenic dienophiles was studied. The resulting bis-adducts (e.g. 179 and 183) were isolated in moderate yields and were found to be stable compounds absorbing light in the 730-800 nm region. iv T A B L E OF CONTENTS ABSTRACT ii T A B L E OF CONTENTS iv LIST OF TABLES vii LIST OF FIGURES viii LIST OF SCHEMES xi LIST OF ABBREVIATIONS xiv ACKNOWLEDGEMENTS xvi CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW 1 1.1 Overview 2 1.2 Structural Features 7 1.3 Nomenclature 8 1.4 Electronic Spectra and Light Absorption Properties 11 1.5 Synthetic Aspects of Porphyrins 15 1.5.1 Porphyrins from Monopyrroles 16 1.5.2 Porphyrins from Dipyrromethanes 16 1.5.3 Porphyrins from Dipyrromethenes 18 1.5.4 Porphyrins from Dipyrroketones 19 1.5.5 Porphyrins from Open-chain Tetrapyrroles 20 1.6 Synthetic Aspects of Reduced Porphyrins 22 1.6.1 Reduced Porphyrins from the Reduction of Porphyrins 23 1.6.2 Reduced Porphyrins from the Oxidation of Porphyrins 25 1.6.3 Reduced Porphyrins from the Contraction of the Porphyrin 7t-System via Inter- or Intramolecular Additions 29 1.6.4 Reduced Porphyrins via the Stepwise Approach v 33 CHAPTER 2 RESULTS AND DISCUSSION 41 2.1 Synthetic Challenge and Objective 42 2.2 Synthetic Plan 46 2.3 Monopyrrolic Starting Materials 56 2.4 Synthesis of Pyrrole Precursors of Dipyrrolic Intermediates 59 2.5 Synthesis of Dipyrromethanes 71 2.6 Synthesis of Dipyrromethenes 72 2.6.1 Synthesis of 5-Methyl-5'-unsubstituted dipyrromethenes 72 2.6.2 Synthesis of 5-Bromo-5'-bromomethyldipyrromethenes 75 2.7 Synthesis of Vinylporphyrins.... 78 2.7.1 Synthesis of the p^Methyl-P'-monovinylporphyrin 78 2.7.2 Synthesis of the P-Unsubstituted-P'-monovinylporphyrin 86 2.7.3 Synthesis of the A,C-Divinylporphyrin 89 2.8 Synthesis of Chlorin Derivatives 94 2.8.1 Synthesis of P-Phenylsulfonylacrylate Ester 95 2.8.2 Synthesis of P-Phenylsulfonylacrylonitrile 95 2.8.3 Diels-Alder Reaction of (E)-pVPhenylsulfonylacrylate with Monovmylporphyrin 81 97 2.8.4 Diels-Alder Reaction of (E)-pVPhenylsulfonylacrylonitrile with Monovmylporphyrin 81 109 2.8.5 Diels-Alder Reaction of (Z)-|i-Phenylsulfonylacrylonitrile with Monovmylporphyrin 81 115 2.8.6 Further Chemistry on Chlorin Derivatives 123 2.9 Synthesis of Bervzoporphyrin Derivatives 132 vi 2.10 Synthesis of Bacteriochlorin Derivatives 141 2.10.1 Diels-Alder Reactions of the A.C-olvinylporphyrin 146 2.10.2 Absorption and Fluorescence Data 163 2.10.3 Cytotoxicity of the Synthetic Bacteriochlorin Derivatives 165 2.10.4 The Bacteriochlorin Derivatives as Photosensitizers for PDT 166 2.11 Summary 166 CHAPTER 3 EXPERIMENTAL 168 3.1 General Methods 169 3.2 Nomenclature Used for the Synthesized Compounds 171 3.3 Synthesis of Dienophiles 175 3.4 Synthesis of Monopyrroles 177 3.5 Synthesis of Dipyrromethanes and Dipyrromethenes 207 3.6 Synthesis of a,c-Biladienes 215 3.7 Synthesis of Porphyrins 221 3.8 Synthesis of Chlorin Derivatives 244 3.9 Synthesis of Benzoporphyrin Derivatives 254 3.10 Synthesis of Bacteriochlorin Derivatives 261 REFERENCES ,. 267 vii LIST OF TABLES Table 1.1: Trivial Names of Common Free Base Porphyrins 9 Table 2.1: Chemical Shifts and Coupling Constants for (E> and (Z)-pVPhenylsulfonylacrylonitrile 97 Table 2.2: Results of Difference n.O.e. Experiments on DAEST1 101 Table 2.3: Results of Difference n.O.e. Experiments on DAEST2 107 Table 2.4: Results of Difference n.0.e. Experiments on 160 112 Table 2.5: Results of Difference n.0.e. Experiments on 161 118 Table 2.6: Results of Difference n.0.e. Experiments on 163 126 Table 2.7: Absorption Data of the Synthetic Bacteriochlorin Derivatives 163 Table 2.8: Fluorescence Data of the Synthetic Bacteriochlorin Derivatives 165 LIST OF FIGURES Figure 1.1: Tetrapyrrolic Macrocycle Skeletons 2 Figure 1.2: Uroporphyrinogen JJJ, Biosynthetic Precursor to Tetrapyrroles 3 Figure 1.3: Examples of Recently Isolated Chlorin-type Macrocycles 4 Figure 1.4: Porphyrin Analogs 6 Figure 1.5: The Electron De localization Pathways in Some of the Macrocycles 7 Figure 1.6: Fischer Numbering Scheme 8 Figure 1.7: rLTPAC-IUB Numbering System for Tetrapyrroles 10 Figure 1.8: The Four Classes of Absorption Spectra for Porphyrins 12 Figure 1.9: Typical Visible Spectrum of Metalated Porphyrins 14 Figure 1.10: Visible Spectra of Non-Metalated and Metalated Chlorins and Bacteriochlorins 15 Figure 1.11: Illustration of the Dissection of an Isobacteriochlorin into Two Halves —33 Figure 2.1: Monopyrrolic Starting Precursors and Intermediates 56 Figure 2.2: Crucial Pyrroles Required in Dipyrrolic Synthesis 60 Figure 2.3: The !H-NMR Spectrum of Monovdnylporphyrin 81 85 Figure 2.4: The Electronic Spectrum of Monovinylporphyrin 101 88 Figure 2.5: The Electronic Spectrum of A,C-Divinylporphyrin 106 93 Figure 2.6: Diels-Alder Reaction of 81 with (E)-pVPhenylsulfonylacrylate 99 Figure 2.7: The 1H-NMR Spectrum of Cycloadduct DAEST1 100 Figure 2.8: Information on DAEST1 from Difference n.O.e. Experiments 102 Figure 2.9: The Attached Proton Test Spectrum of 159-D 104 Figure 2.10: The 1H-NMR Spectrum of Cycloadduct DAEST2 106 Figure 2.11: Information on DAEST2 from Difference n.O.e. Experiments 108 Figure 2.12: Diels-Alder Reaction of 81 with (E)-p^Phenylsulfonylacrylonitrile 110 Figure 2.13: The 1H-NMR Spectrum of Cycloadduct 160 I l l ix Figure 2.14: Information on 160 from Difference n.O.e. Experiments 112 Figure 2.15: The Attached Proton Test Spectrum of 160-C 114 Figure 2.16: Diels-Alder Reaction of 81 with (Z)-pVPhenylsulfdnylacrylonitrile 116 Figure 2.17: The 1H-NMR Spectrum of Cycloadduct 161 117 Figure 2.18: The Attached Proton Test Spectrum of 161-D 119 Figure 2.19: Information on 161-D from Difference n.O.e. Experiments 120 Figure 2.20: Transition States in Cycloadditions with (E)- and (Z)-p-Phenylsulfonylacrylonitrile 122 ' Figure 2.21: The Electronic Spectum of 160-C and 163 124 Figure 2.22: The 1H-NMR Spectrum of Hydrogenated Cycloadduct 163 125 Figure 2.23: Information from Difference n.O.e. Experiments 127 Figure 2.24: The 1H-NMR Spectrum of the Desulfonated Adduct 168 130 Figure 2.25: The Electronic Spectrum of the Benzoporphyrin 171 134 Figure 2.26: The 1H-NMR Spectrum of the Benzoporphyrin 171 135 Figure 2.27: The 1H-NMR Spectrum of the Compound 174 138 Figure 2.28: Synthetic Porphyrins Proposed as Potential Photosensitizers 142 Figure 2.29: "Chlorins" Proposed as Potential Photosensitizers 143 Figure 2.30: Synthetic Porphycene as a Photosensitizer 144 Figure 2.31: Phthalocyanines and Naphthalocyanines as Photosensitizers 145 Figure 2.32: Endo-Transition State in the Cycloaddition with N-Phenylmaleimide.... 148 Figure 2.33: The Eletronic Spectrum of the Bis-adduct 179 149 Figure 2.34: The 1H-NMR Spectrum of the Bis-adduct 179 151 Figure 2.35: The Difference n.O.e. Spectra on Bis-adduct 179 152 Figure 2.36: Mode of Approach of a Second Molecule of the Dienophile 154 Figure 2.37: The 1H-NMR Spectrum of Bis-adduct 181 156 Figure 2.38: Relevant 1H N M R Chemical Shifts for Ring A of 182 158 Figure 2.39: The Electronic Spectrum of Bis-adduct 183 159 X Figure 2.40: Comparison of the Relevant iH-NMR Data for Ring A between 182 and 183 160 Figure 2.41: Spacial Representation of Rearranged Bis-adduct 183 in Ring A 161 Figure 2.42: The Difference n.O.e. Experiments on 183 162 Figure 2.43: The Fluorescence Emission Spectrum of Bis-adduct 164 xi LIST OF SCHEMES Scheme 1: Synthesis of Porphyrins via MacDonald's Route 17 Scheme 2: Synthesis of Porphyrins via Dipyrromethenes 18 Scheme 3: Synthesis of Porphyrins from Dipyrroketones 19 Scheme 4: Synthesis of Porphyrins from l,19-Dimethyl-a,c-Biladienes 20 Scheme 5: Synthesis of Porphyrins via a Tripyrrene Intermediate 21 Scheme 6: Johnson's Porphyrin Synthesis via a,c-Biladienes 22 Scheme 7: Raney Nickel Reduction of TPP 23 Scheme 8: Photoreduction of Symmetrically Substituted MetaUoporphyrins 24 Scheme 9: Site-Specific Photoreduction of Unsymmetrically Substituted MetaLlbporphyrins 24 Scheme 10: Photoreduction of Metallochlorins 25 Scheme 11: The Pinacol Rearrangement in the H2Q2 Oxidation 26 Scheme 12: Hydrogen Peroxide Oxidation of OEP 27 Scheme 13: Synthesis of a Model Compound for Heme di 28 Scheme 14: Osmium Tetroxide Oxidation of Porphyrins 29 Scheme 15: Reaction of Pp LX DME with Singlet O2 30 Scheme 16: Reaction of Pp DC DME with Dimethyl Acetylenedicarboxylate 31 Scheme 17: Reaction of Pp II DME with p-Nitrosobenzene 31 Scheme 18: The Intramolecular Cyclization Step in Woodward's Chlorin e$ Synthesis 32 Scheme 19: Formation of a Chlorin via an Intramolecular Addition 32 Scheme 20: The First Stepwise Synthesis of an Isobacteriochlorin 34 Scheme 21: Synthesis of C-Methylated Chlorin via an Irriino-Ether Intermediate 35 Scheme 22: Synthesis of C-Methylated Chlorin via l-Bromo-19-Methyl Intermediate 36 xii Scheme 23: Synthesis of a C-Methylated Chlorin via a Sulfur Contraction Step 37 Scheme 24: The Photochemical Cyclization of a Seco-system 38 Scheme 25: Synthesis of (±) Bonellin Dimethyl Ester 39 Scheme 26: Building Blocks for Sirohydrochlorin Octamethyl Ester 40 Scheme 27: Clezy's First Route to a Benzoporphyrin 43 Scheme 28: Baker's Hypothesis on Benzoporphyrin Formation 44 Scheme 29: Strategy for a Factor 1 Model 47 Scheme 30: Proposed Synthetic Approach to 79 from Monovinylporphyrin 81 48 Scheme 31: Retrosynthetic Analysis of Monovinylporphyrin 81 50 Scheme 32: Retrosynthetic Analysis of Dipyrromethane 91 51 Scheme 33: Retrosynthetic Analysis of the Dipyrromethenes 51 Scheme 34: Retrosynthetic Analysis of Benzoporphyrin 100 53 Scheme 35: Retrosynthetic Analysis of the Bacteriochlorin (Fischer's Approach) 54 Scheme 36: Synthesis of Pyrrole 99 57 Scheme 37: Synthesis of Pyrrole 111 58 Scheme 38: Electrophilic Substitution on the Pyrrole Ring 59 Scheme 39: Transformation of Pyrrole 109 61 Scheme 40: Transformation of Pyrrole 112 64 Scheme 41: Transformation of Pyrrole 109 66 Scheme 42: Transformation of Pyrrole 111 67 Scheme 43: Transformation of Pyrrole 110 68 Scheme 44: p-Modifications of Pyrrole 111 70 Scheme 45: Synthesis of Dipyrromethane 91 71 Scheme 46: Synthesis of Dipyrromethenes 73 Scheme 47: Possible Mechanism of the Acid-catalysed Condensation in the Syntheses of Dipyrromethenes 74 Scheme 48: Synthesis of Ester Porphyrin 88 from Dipyrromethane 91 79 Scheme 49: Synthesis of Ester Porphyrin 88 via Dipyrromethenes 81 Scheme 50: Transformation of Ester Porphyrin to Vinylporphyrin 83 Scheme 51: Synthetic Route to the pVUnsubstirated-p'-Monovmylporphyrin 101 87 Scheme 52: Synthesis of A,C-Divinylporphyrin 106 via Fischer's Approach 90 Scheme 53: Synthesis of A,C-Divinylporphyrin 106 via Johnson's method 92 Scheme 54: Synthesis of (E)-pVPhenylsuIfonylacrylate Ester 95 Scheme 55: Synthesis of (E)-and (Z)-(P)-Phenylsulfonylacrylonitrile 96 Scheme 56: An Oxidative Desulfonation Reaction 128 Scheme 57: Attempted Oxidative Desulfonation on 159-C and 163 128 Scheme 58: Desulfonation of 163 with DBU 129 Scheme 59: Attempted Epoxidation on Adduct 168 131 Scheme 60: Diels-Alder Reaction of 101 with Esters of Acetylenedicarboxylate 133 Scheme 61: Diels-Alder Reaction of 101 with Di-tert-butyl Acetylenedicarboxylate. 137 Scheme 62: The Possible Pathways for the Formation of the Benzoporphyrin 139 Scheme 63: Diels-Alder Reaction of 106 with N-Phenylmaleimide 147 Scheme 64: Diels-Alder Reaction of 106 with (E)-P-Phenylsulfonylacrylonitrile.... 155 Scheme 65: Diels-Alder Reaction of 106 with Diethyl Acetylenedicarboxylate 157 xiv LIST OF ABBREVIATIONS Ac acetyl Ala o^ arrnnolevulinic acid APT attached proton test br broad Bu* tert-butyl d doublet (d) decomposes DBU diazabicyclo[5.4.0]undec-5-ene ddd doublet of doublets of doublets DDQ 2,3-dichloro-5,6-dicyano-l,4-benzoquinone DMF N,N-dimemylforrnamide DMSO dimethylsulfoxide DNOE difference nuclear Overhauser effect Et ethyl ether diethyl ether HpD hematoporphyrin derivatives HMPA hexamethylphosphoramide Hz hertz IR infrared J coupling constant LRMS low resolution mass spectroscopy m multiplet m- meta MCPBA m-chloroperbenzoic acid Me methyl M.P. melting point NMR nuclear magnetic resonance n.O.e nuclear Overhauser effect OEP octaethylporphyrin P- para PBG porphobilinogen PDT photodynamic therapy Ph phenyl Pp protoporphyrin q quartet t triplet TPA trifluoroacetic acid THF tea^ydrofuran tic thin layer chromatography TMS trimethylsilyl tosylate p-toluenesulfonate UV ultraviolet UV-vis ultraviolet and visible ACKNOWLEDGEMENTS xvi I would like to thank sincerely, my supervisor, Dr. David Dolphin for introducing me to the field of porphyrin chemistry, and for his guidance and support throughout this work. I am also indebted to Dr. Tilak Wijesekera for his invaluable suggestions during the course of the synthetic work and for his assistance with the preparation of this thesis. In addition, I would like to acknowledge Dr. Walter Dandliker and Dr. Julia Levy for providing me with the studies on the bacteriochlorin derivatives. Thanks as well to Drs. William Cullen, Tom Money and Gordon Bates, members of my advisory committee, for suggestions made during the course of the work. Special thanks to Mr. Luc Maurice for proofreading, and to Ms. Joette Heuft for proofreading and typing parts of this thesis, and to Mr. Jack Chow for digitizing the electronic spectra. I would also bike to thank past and present members of my group, and NMR, Mass Spectrometry, and Microanalysis staff members. Finally, I would like to acknowledge the Killam trustee for awarding me a pre-doctoral scholarship for two consecutive years. This thesis is dedicated to my parents. 1 C h a p t e r 1 Introduction and Literature Review 2 1.1 Overview Porphyrins, chlorins, bacteriochlorins, isobacteriochlorins and corrins (Fig. 1.1) form the basic macrocyclic skeletons of important natural pigments of life. Each of these compounds consists of four pyrrole-type rings linked by rnethine bridges. Observed structural similarities of the macrocycles and the presence of common peripheral substituents led to the suggestion that these compounds are derived from a single biosynthetic precursor which is now known to be Uroporphyrinogen UJ (Fig. 1.2).1 Uroporphyrinogen LTJ is formed from the condensation of four pyrroles, porphobilinogen (PBG) which in turn is derived from two molecules of ^aminolevulinic acid (ALA). Porphyrin Chlorin Corrin Bacteriochlorin Isobacteriochlorin Figure 1.1: Tetrapyrrolic Macrocycle Skeletons t*1™* d i Siroheme Heme Figure 1.2: yroporphyrinogen UJ, Biosynthetic Precursor to Tetrapyrroles 4 Given their frequency of occurence in nature, it is not surprising that the polypyrrolic pigments should play important roles in diverse biological processes.2 Figure 1.2 shows that the polypyrrolic pigments contain metal ions which are crucial to their functioning. An example is heme, the iron (II) complex of protoporphyrin LX, a porphyrin which is the prosthetic group of hemoglobins, myoglobins, cytochromes, catalases and peroxidases. These heme proteins are involved in the transportation (hemoglobins) and storage (myoglobins) of oxygen, in the electron-transfer process (cytochromes in the respiratory chain), and in oxygenation reactions (catalases and peroxidases).3 Another pigment is chlorophyll a, a magnesium complex of a dihydroporphyrin (a chlorin) which assists plants in harvesting, and subsequently, transforming solar energy into chemical energy. Other than chlorophylls, novel chlorin-type macrocycles (Fig. 1.3) have recently been isolated and reported to have special functions. For example, bonellin has a role in sex differentiation in the worm Bonellia viridis4, and factor 1 is now implicated in the biosynthetic pathway of siroheme and vitamin B12.5 C O 2 H H H COjH C0 2 H C 0 2 H COjH O0 2 H COjH Bonellin Factor 1 Figure 13: Examples of Recently Isolated CUorin-type Macrocycles A specific group of polypyrrolic pigments are the tetrahydroporphyrins, two types of which are bacteriochlorins and isobacteriochlorins. Bacteriochlorins feature reduced opposite 5 pyrrole rings and it is typical of them to absorb light at longer wavelengths than chlorophyll a.6 Examples are Bacteriochlorophylls a and b (Fig. 1.2), the magnesium complexes which comprise the main pigments of the photosynthetic apparatus of purple and green bacteria. Isobacteriochlorins are tetrahydroporphyrins, in which two adjacent pyrrole rings are reduced. These are widely distributed in plants and algae. One isobacteriochlorin is siroheme, the iron complex of sirohydrochlorin. This molecule is the prosthetic group of a number of sulfite and nitrite reductases which catalyse the six-electron reduction of sulfite to sulfide and nitrite to ammonia respectively.7 Heme di, an isobacteriochlorin, whose structure was elucidated a few years ago has two oxo groups attached to the saturated pyrrole rings.8 A final example of such macrocycles is vitamin B 1 2 , the cobalt complex of a corrin. It is the prosthetic group of a range of enzymes which carry out various rearrangement reactions and trans-methylations.9 The above examples illustrate the wide range of functions performed by the metalated porphyrins and their related macrocycles in nature and account for the continued interest of researchers in the isolation, structural determination and total synthesis of such molecules. Interest in this area resulted in studies aimed at the isolation of metalated porphyrins and led, 50 years ago, to the discovery of metalloporphyrin derivatives in petroleum and oil shale. The discovery of these petroporphyrins, in turn, gave birth to the field known today as "organic  geochemistry".10 Metalloporphyrin research has also expanded to include the search for potential new semiconductors,11 superconductors,12 catalysts in synthesis,13 and sensors in analytical chemistry.14 Non-metalated porphyrins and chlorins have also been the subject of recent research. In the last decade, some have been found to be effective photosensitizers, a property which has made them useful in a particular approach to the treatment of cancer . This new technique, referred to as photodynamic therapy (PDT) exploits two facts which pertain to porphyrins: (1) when injected in a living system, porphyrins localize preferentially in tumorous tissue and (2) they are effective photosensitizers for singlet oxygen production. It is their photosensitivity, 6 the ability to absorb light of a particular wavelength in a selected area which causes the photo-degradation of the cancerous tumor.15 Medical and industrial applications aside, these molecules serve as good models for the study of theoretical concepts such as aromaticity and diamagnetic ring currents. Interest in the basic skeleton of the tetrapyrrole macrocycles has led to the synthesis of a series of novel porphyrin analogs, such as pentaphyrin,16 sapphyrin,17 porphycene18, and enlarged porphyrin having a (26)-annulene structure.19 Porphycene Enlarged Porphyrin Figure 1.4: Porphyrin Analogs The exceptional and fascinating properties of the porphyrins and related compounds make them a worthwhile focus of further investigation. It is not unreasonably optimistic to believe that an interdisciplinary approach with contributions from photochemistry, photophysics, photomedicine and photobiology, will reveal new reactivities with useful applications for mankind in all spheres of life. 7 1.2 Structural Features H H H H H H Pdiphin H H Chlorin BacteriocMorin Figure L5: The Electron Delocalization Pathways in Some of the Macrocycles Porphyrins are planar, highly conjugated systems, each of which is endowed with a 22 it-electron system. Within a porphyrin there exists an internal delocalized conjugation pathway which utilizes 18 of the 22 TC-electrons. It is this feature which provides the aromatic character of the molecule and which expresses itself in the form of a diamagnetic ring current in the 1 H -NMR spectrum. The shielded inner NH protons appear at relatively high field (8 = -2 to -5 ppm). The outer methine protons, deshielded by the aromatic ring current, appear at 8 = 10 ppm. The remaining two double bonds are cross-conjugated and can be reduced as in the chlorins or the bacteriocMorins, (Fig. 1.5) without affecting the aromatic re-electron system. The nature and position of the substituents on the porphyrins dictate their physical and chemical properties. Porphyrins can complex with a variety of metals with the concomitant loss of the inner NH protons. In the metalation process, the key bonding factor is the donation of a electrons from the four nitrogen atoms to the central metal ion. By neutralizing the positive charge on the metal ion, the porphinato-ligand stabilizes the high formal oxidation states of the central metal ion. Such a metal chelation process changes the reactivity of the porphyrin 8 macrocycle by changing the electron density at the periphery. Clearly, metalated porphyrins vary considerably in reactivities and therefore present a challenge to the scientist seeking to understand the biological significance of their important redox-, photo-, and coordination chemistry.20 1.3 Nomenclature Figure 1.6: Fischer Numbering Scheme Two systems for naming porphyrins are in current use. The old Fischer system which has been applied to most early work is illustrated in Fig. 1.6. The four pyrrole rings are given the letter designations A-D, and the linking methine bridge carbon atoms are given the designations a-5. The peripheral pyrrole carbon atoms of the macrocyle are numbered 1-8. A large number of trivial names (see Table 1.1) were introduced to differentiate porphyrins based on the particular substituents at the periphery of the molecule. 9 Table 1.1 Trivial Names of Common Free Base Porphyrins Side Chain Name 1 2 3 4 5 6 7 8 Etioporphyrin Ul Me Et Me Et Me Et Et Me Uroporphyrin I A P A P A P A P Deuteroporphyrin IX Me H Me H Me P P Me Mesoporphyrin LX Me Et Me Et Me P P Me Protoporphyrin IX Me V Me V Me P P Me Hematoporphyrin IX Me HE Me HE Me P P Me Abbreviations: Me = - C H 3 ; Et = - C H 2 - C H 3 ; P = -CH 2 CH 2 COOH; A = -CH 2COOH; V = -CH=CH2; HE = - C H O H - C H 3 . The use of trivial names together with an isomer numbering system forms the backbone of Fischer's system of nomenclature. In this system, the porphyrin which carries one methyl and one ethyl group on each pyrrolic ring ( also referred as p positions) is named " Etioporphyrin " and the four possible isomers are labeled type I, U, Ul, and IV. vEt Me / \ Me Et .Et Et / \ Me Me ,Et Me / \ Me Et y Et Et Me Me Et Me ^Me E t ^ Me Me ^ E t Et^ Me Me x Et Et Me Me Type I TvpeU Typem TypelV 10 The more recent IUPAC-IUB numbering system21 for tetrapyrroles and their relatives which was recommended in 1960 operates as follows: the carbon skeleton is numbered from 1 to 20, the P positions are numbered 2,3,7,8,12,13,17,18, the four methine positions are numbered 5,10,15 and 20, and the nitrogen atoms are numbered 21 through 24. 23,73-Tetrahydroporphyrin Monotertzoporphyrin (Isobacteriochlorin) Figure 1.7: IUPAC-IUB Numbering System for Tetrapyrroles 11 The same numbering system is applied to hydrogenated porphyrins (chlorins, isobacteriochlorins, bacteriochlorins) and corrins (Fig. 1.7). Furthermore, the rUPAC-IUB system provides a way to name compounds with an extra ring or rings fused to the porphyrin nucleus, as in the case of benzoporphyrins. In these compounds, the additional positions are numbered as shown in Fig. 1.7. Despite the advantages of the rUPAC-IUB system, the trivial names remain important for two reasons. Firstly, they relate directly to the early work in the field and secondly, they are shorter than the systematic IUPAC names. The persistence of the trivial names has resulted in a hybrid semi-systematic approach in which the important Fischer names have been retained while the less important trivial names have been avoided.22 1.4 Electronic Spectra and Light Absorption Properties Unlike most organic molecules, porphyrins and related macrocycles absorb light strongly in the visible region. The colors associated with porphyrins (red) and chlorophylls (green) indicate the existence of low-lying electronically excited states of the molecules which lead to Tt-Tt* transitions. Typically, the electronic spectra of porphyrins are characterized by several intense absorption bands in the 400-700 nm region. A special feature of the spectra is a very intense band, the Soret band, in the 400 nm region. The molar extinction coefficient of this band is usually between 150,000 to 400,000 L mol'1 cm 1 . Besides the Soret band, there are four additional bands in the 500-700 nm region. The complex nature of the electronic spectra of porphyrins has been interpreted in terms of a four-orbital model (Piatt, 1956).23 According to Piatt, the low intensity absorption bands above 500 nm belong to a low-energy Q state, in which the transition dipoles nearly cancel each other out. The intense Soret band belongs to a strongly allowed excited state in which the transition dipoles add. The position and relative intensities of the four visible bands are influenced by the substitution pattern on the 12 rings. These effects are of great diagnostic value in the identification of structural types. The observed electronic spectra of porphyrins fall into four main classes. sn 500 550 600 650 500 550 600 650 U l a u e l e n g t h (nm) Figure 1.8: The Four Classes of Absorption Spectra for Porphyrins (a) Etio-type spectrum. The etio-type of spectrum is characterized by a IV > HI > II > I order of the band intensities (Fig. 1.8). This spectrum is observed in porphyrins in which six or more peripheral positions are occupied by ethyl, methyl, acetic or propionic side-chains. Most naturally occurring porphyrins such as uro-, meso- and deuteroporphyrin exhibit this type of spectrum. 13 (b) Rhodo-type spectrum. Porphyrins which display this type of spectra are those which have a single electron-withdrawing group (e.g. formyl, acetyl, or carboxyl) at one of the P positions. This situation causes band UI to become more intense than band IV, and is termed a "Rhodofying effect" (Fig. 1.8). The strongly electron-withdrawing group produces a bathochromic shift of all the bands, which distinguishes this spectrum from the etio-type spectrum. Rhodo-type spectra are exhibited by porphyrins which have a benzene ring fused to one pyrrolic ring (e.g. monobenzoporphyrin). A weak rhodofying effect is displayed by porphyrins with vinyl group substituents. An interesting feature to note is that when two rhodofying groups are on adjacent pyrrole rings, the rhodo-effect is cancelled out and an etio-type spectrum results (e.g. 3,8-diformyl deuteroporphyrin). However, the bathochromic shift of the absorption bands, being additive, still takes place. (c) Oxorhodo-type spectrum. An oxorhodo-type spectrum is characterized by a band intensity ratio of HI > II > IV > I (Fig 1.8). This spectrum is observed when two rhodofying groups are present on diagonally opposite pyrrole rings. The oxorhodo spectrum is considered to be the result of an additive enhancement of the rhodofying effects of two electron-withdrawing groups. (d) Phyllo-type spectrum. This spectrum displays a IV>U>III>I band pattern (Fig. 1.8). It derived its name from phylloporphyrin, and is observed when one meso-carbon is substituted by an alkyl group or when four or more unsubstituted P - positions are available in the molecule. The usefulness of electronic spectra becomes more apparent upon the realization that the spectra reveal evidence of the metalation, protonation and hydrogenation of the porphyrins. In the metalated porphyrins, the four visible bands are simplified to two, normally referred to as 14 the a and P bands (Fig. 1.9). This simplification is due to an increase in the symmetry of the conjugated rnacrocycle from E>2h to D4JJ. The intensities and absorption maxima of the a and p bands are dependent on the type of metal ion and on the particular porphyrin ligand present. Acidification of the poq>hyrin to its acid di-cation also leads to simplification of the spectrum. a A • 500 550 600 650 W a v e l e n g t h (nm) Figure 1.9: Typical Visible Spectrum of Metalated Porphyrins The positions of the absorption maxima are also influenced by changes in the hydrogenation level of porphyrins which in turn affects the extent of the conjugated chromophore. For example, hydroporphyrins (chlorins), having one double bond reduced in the macrocycle, exhibit different electronic spectra from porphyrins. A high intensity band near 660 nm (e «• 70,000 L mol'1 cm 1) and a Soret band of about three times this intensity, near 400 nm, are typical of such compounds (Hg.1.10). Another feature of non-metalated chlorins is a double band (e m 15,000 L mol'1 cm 1) in the 500 nm region which disappears upon metalation.24 The metalatdon of chlorins has a less dramatic effect on the spectra than has the metalation of a porphyrin. In the case of the hydroporphyrin, ring reduction has already lifted 15 the degeneration of the square ring and therefore, there is no increase in symmetry and consequently, little change in the spectrum. Further reduction of the ring system to a tetrahydroporphyrin (bacteriochlorin) brings a bathochromic shift of the red band. A narrow band at about 750 nm and a split Soret band are characteristic of the electronic spectra of bacteriochlorins.25 Upon metalation, the intensity of the red band shows a small increase while that of the Soret band undergoes a small decrease (Fig. 1.10). Chlorin MetaUochlorin — BattcriochJorin —. MMDoteferiochlarin 400 600 l l i t v t l i n g t h (nm) 800 400 600 800 W a v e l e n g t h (nm) Figure 1.10: Visible Spectra of Non-Metalated and Metalated Chlorins and Bacteriochlorins 1.5 Synthetic Aspects of Porphyrins Progress in the synthesis of porphyrins in the last two decades has been remarkable due to methods developed during early in the 1960's. The porphyrin macrocycle can be synthesized via three fundamental approaches. The simplest method is the one-pot synthesis from monopyrroles. In the second method, dipyrrolic intermediates (dipyrromethanes, 16 dipyrromethenes and dipyrroketones) are synthesized and then coupled in a [2+2] fashion. In the third method, linear tetrapyrroles, obtained by condensing two dipyrrolic intermediates or one tripyrrolic and one monopyrrolic intermediate, are cyclized to give the porphyrins.26.27 1.5.1 Porphyrins from Monopyrroles The direct tetramerization of monopyrroles produces symmetrical porphyrins which are of little synthetic value, given that the porphyrins found in nature are generally unsymmetrical. On the other hand, because they are easily prepared, the octaalkyl and meso-tetraphenylporphyrins have been widely used as models for naturally occurring porphyrins. The octaalkylporphyrins are normally prepared by reacting the corresponding 2,3-dialkylpyrrole with formaldehyde in acetic acid and pyridine.28 OEP (octaemylporphyrin) has also been prepared via an alternative approach which involves using pyrroles carrying a - C H 2 O H or - C H 2 N R 2 at one of the a positions. 2 9 Synthesis of 5,10,15,20-tetraphenylporphyrin (TPP) also involves reacting equimolar quantities of pyrrole and benzaldehyde in refluxing propionic acid.30 Extension of this method has led to the synthesis of many functionalized tetraphenylporphyrins.31'32 Unsymmetrical tetra-phenylporphyrins have also been prepared via the use of an insoluble cross-linked polystyrene support.33 1.5.2 Porphyrins from Dipyrromethanes Syntheses of new porphyrins from dipyrromethanes in the recent years have relied heavily on the MacDonald method.34 The reaction involves a mild acid-catalyzed condensation of a dipyrromethane dialdehyde 1 with a di-unsubstituted dipyrromethane 2 or its dicarboxylic analog with h/droiodic acid to give the porphyrin 3, as shown in Scheme 1. The main disadvantage of MacDonald's method is that it demands that one of the two dipyrromethanes be 17 symmetrical, if a single product is to be produced. Extension of MacDonald's method has led to the preparation of centrosymmetrically substituted porphyrins by self condensation of a 5-formyl-5'-unsubstituted dipyrromethane under mildly acidic conditions.35 Scheme 1: Synthesis of Porphyrins via MacDonald's Route 18 1.5.3 Porphyrins from Dipyrromethenes The use of dipyrromethenes in porphyrin syntheses owes its popularity to Hans Fischer. In his classical synthesis, a 5-bromo-5'-methyldipyrromethene 4 was self-condensed in organic acid melts at temperatures ranging from 160 to 200 "C to give the porphyrin 5 (Scheme 2).36 5 Scheme 2: Synthesis of Porphyrins via Dipyrromethenes Condensation of a S.S'-d^methyldipyrromethene 6 with a 5,5'-dibromodipyrromethene 7 under the same conditions also yields a porphyrin 8. In fact, Fischer made use of this approach to synthesize deuteroporphyrin-LX, an intermediate which he used in his classical synthesis and structure verification of hemin.37 19 Recent improvement of yields from this method, makes use of 5-bromo-5'-methyldipyrromethene perbromides or 5-bromo-5'-bromomethyldipyrromethene hydro-bromides.38 This method again suffers from inherent symmetry restrictions. To obtain a single porphyrin through condensation using the Fischer's method, one dipyrromethene unit has to be symmetrical if two such units are condensed. 1.5.4 Porphyrins from Dipyrroketones In this method, a 5,5'-diformyldipyrroketone 9 is condensed with a 5,5'-di-unsubstituted dipyrromethane 10 to give an oxophlorin 11 (Scheme 3). The latter is then converted into the corresponding porphyrin via the meso-acetoxyporphyrin 12, which upon hydrogenation and reoxidation gives the meso-unsubstituted porphyrin 13.39 R 2 Q R2 Scheme 3: Synthesis of Porphyrins from Dipyrroketones 20 1.5.5 Porphyrins from Open-chain Tetrapyrroles The synthesis of porphyrins via open-chain tetrapyrroles has received significant attention in the last ten years. Several specialized syntheses of porphyrins have been achieved via open-chains.26 The chain is normally constructed by the condensation of two dipyrrolic intermediates or by the addition of two monopyrroles to a preformed dipyrromethane. Examples of some open-chain tetrapyrroles that are encountered in literature include bilanes, a-oxobilanes, b-oxobilanes, a-bilenes, b-bilenes, and a,c-biladienes. The a,c-biladiene route has dorninated the other methods in the past. In one approach, it is prepared from the condensation of two equivalents of a 2-formyl-5-methyl pyrrole 14 with a dipyrromethane-5,5'-dicarboxylic acid 15 to give l,19-dimethyl-a,c-biladiene 16 which is cyclized to the porphyrin 17 using a copper (H) salt (Scheme 4).40 Scheme 4 : Synthesis of Porphyrins from l,19-Dimethyl-a,c-biladienes The latest development in this method has removed the symmetry restriction introduced by the simultaneous condensation of two equivalents of the formylpyrrole with the 21 dipyrromethane(Scheme 5). The dipyrromethane 18, which is normally derived from its benzyl ester analog after hydrogenolysis, is tailored in such a way that only one formylpyrrole unit 19 adds to give a tripyrrene 20. The latter is then reacted with a different formylpyrrole 21 to give the unsymmetrical l,19-dimethyl-a,c-biladiene 22, which upon cyclization using a copper(U) salt, gives the porphyrin 23 after copper removal.41 Scheme 5: Synthesis of Porphyrins via a Tripyrrene Intermediate In an alternative approach, the a,c-biladiene is prepared from the condensation of two dipyrromethenes 24 and 25 to give a l-bromo-19-methyl-a,c-biladiene 26 (Scheme 6). Cyclization of 26 in a pyridme-dimethylsulfoxide system over several days normally gives the porphyrin 27 in very good yield (Johnson Method). This two-stage version of Fischer's [2+2] method is very versatile for the synthesis of unsymmetrically substituted pcnphyrins.42 A 22 minor disadvantage of the method is the difficulty encountered in the bromination of some 5-methyl-5'-unsubstituted dipyrromethenes to give the required 5-bromo-5'-bromomethyl dipyrromethenes. 2 6 2 7 1 Scheme 6: Johnson's Porphyrin Synthesis via a,c-Biladienes 1.6 Synthetic Aspects of Reduced Porphyrins During the past decade the subject of synthesis of reduced porphyrins has been vigorously developed. The recent discovery of novel chlorins and isobacteriochlorins, coupled with their biological roles, has made their synthesis attractive. From a synthetic viewpoint, reduced porphyrins could be derived from porphyrins. Therefore all attempted syntheses of such molecules prior to 1979 started from the readily available porphyrins as precursors. A survey of such synthetic routes indicates that three main approaches have been developed over 2 3 the years to tackle the problem. The first route uses a reduction method of porphyrins, the second an oxidation of porphyrins, the third a contraction of the Tt-system of the porphyrins via an intermolecular or intramolecular addition. 1.6.1 Reduced Porphyrins from the Reduction of Porphyrins Many reducing agents have been used in the past to reduce the peripheral double bonds of the porphyrins to yield reduced porphyrins. Treatment of octaemylporphyrin (OEP) with sodium and alcohol brings -about the reduction of one or more double bonds to give chlorins, isobacteriochlorins and other products.43 With BH 3 , the reduction stops at the chlorin stage, giving a 5:1 mixture of cis and trans-octaethylchlorin.44 Raney nickel reduction of TPP in ether has been reported to produce tetraphenylbacteriochlorin (TPBC). On the other hand, if dioxane is used as the solvent, tetraphenyhsobacteriochlorin (TPiBC) was reported to be the product (Scheme 7).45 In all the above methods the reported yields are very low and the products were always obtained after painstaking chromatography. Ph TPiBC Scheme 7: Raney Nickel Reduction of TPP 24 Besides chemical reductions, photoreductions of porphyrins and metalloporphyrins in the presence of hydrogen atom donors such as ascorbic acid also lead to chlorins and other reduced products. Clean photoreduction of tin porphyrins in the presence of stannous chloride to yield the corresponding chlorin and subsequently the isobacteriochlorin has also been reported.46 Sn(OEP) Sn(OEC) Sn(OEiBC) Scheme 8: Photoreduction of Symmetrically Substituted Metalloporphyrins More recently, site-specific photoreduction of unsymmetrically substituted metalloporphyrins has been achieved.47 For example, zinc(II) vinylporphyrin 28 upon photoreduction in the presence of ascorbic acid and diazabicyclo[2.2.2]octane leads to the vinylchlorin 29 in which the double bond in the pyrrole ring carrying the vinyl group is regioselectively reduced (Scheme 9). Scheme 9: Site-Specific Photoreduction of Unsymmetrically Substituted Metalloporphyrins 25 When a stronger electron-withdrawing methoxycarbonyl group is introduced at some other site in an analogous molecule, photoreduction takes place preferentially on the ring carrying the stronger electron-withdrawing group as shown in compound 30. As an extension of this approach the metallochlorin 31 has also served as the precursor to an isobacteriochlorin (Scheme 10).48 From a mechanistic point of view, it is believed that, like in the metalloporphyrins, the photoreduction produces the cis isobacteriochlorin 32. Subsequent migration of the double bond of the vinyl group produces a different isobacteriochlorin 33 bearing an ethylidene system found in bacteriochlorophylls b and g. 1.6.2 Reduced Porphyrins from the Oxidation of Porphyrins Chemical oxidation of metal free porphyrins and metaUoporphyrins has also been employed in the synthesis of dihydro- and tetrahydroporphyrins analogs. The acidic hydrogen peroxide oxidation with OEP has been investigated by several groups. Fischer, the first to 26 report the green-colored chlorin from OEP, formulated the structure of the major product as an epoxide across a pyrrolic double bond.49 Re-investigation of this reaction thirty-nine years later established the resulting structure as a keto-chlorin originating from a P,P'-dihydroxy intermediate via a pinacol rearrangement (Scheme 11).50 Scheme 11: The Pinacol Rearrangement In The H2O2 Oxidation Chang, at Michigan State University, has relied heavily on this type of reaction to produce synthetic models of sirohydrochlorin and bonellin.51 With OEP, he reported the formation of the oxochlorin(19%), three dioxoisobacteriochlorins(11.3%), and two cUoxobacteric»chlorins (9%) as the major products.52 The isobacteriochlorin which possesses the oxo groups at positions 3 and 8 was then subjected to a methylation step, followed by a reduction step to give a synthetic analog of sirohydrochlorin (Scheme 12). 27 Scheme 12: Hydrogen Peroxide Oxidation of OEP In an attempt to elucidate the structure of heme di, Chang carried out the same reaction on mesoporphyrin.53 Indeed, when mesoporphyrin was reacted with H2O2/H2SO4, nine different compounds were isolated. Separation of the isomeric dioxoisobacteriochlorins gave a component 34 which was Subsequently converted to 35 as shown in Scheme 13. The visible spectrum of the latter matched with that of natural heme di. Based on this observation and 28 other spectroscopic evidence, Chang concluded that heme dj most likely is dioxoisobacteriochlorin. H 2 0 2 / H + C02Me C02Me Mesoporphyrin C02Me C02Me C02Me 34 z 35 C02Me C02Me C02Me Scheme 13: Synthesis of a Model Compound for Heme di An alternative route to P.p'-dioxohydroporphyrin involves treatment of the porphyrin 36 with O S O 4 . 5 4 It has been observed that the Os0 4 oxidation is controlled to a large extent by steric factors (Scheme 14). Also a large excess of Os0 4 leads to formation of tetrahydroxybacteriocMorin exclusively. This regioselective oxidation of the dihydroxychlorin is an indication of a preferred diagonal ic-electron delocalization pathway existing in 29 porphyrins. Oxidation therefore leaves this pathway untouched and as a consequence the two isolated and opposite pyrrolic p\p'-double bonds became oxidized. Scheme 14: Osmium Tetroxide Oxidation of Porphyrins 1.6.3 Reduced Porphyrins from the Contraction of the Porphyrin rc-System via Inter- or Intramolecular Additions It has been known for many years in the porphyrin area that protoporphyrin LX dimethyl ester (Pp LX DME) reacts with singlet oxygen in the presence of light to give photoprotoporphyrin dimethyl esters 37 and 38 as major products (Scheme 15). This intermolecular cycloaddition occurs on either ring A or B but not on both rings.55 In this reaction the porphyrin acts as the precursor and as the sensitizer responsible for the generation of the singlet oxygen. 30 Scheme 15: Reaction of Pp LX DME with Singlet O2 In 1973, it was reported that [4+2] cycloaddition of protoporphyrin LX dimethyl ester with acetylenedicarboxylate and tetracyanoethene gave both chlorins and isobacteriochlorins.56 A re-examination of these reactions some years later revealed that the isobacteriochlorin chromophore was not produced in the process. With tetracyanoethene, the reaction mixture contained [2+2], [4+2] and mixed-typed adducts.57 On the other hand, dimethyl acetylenedicarboxylate gave a [4+2] mono-adduct 39 and its isomer on ring A as the sole products. Isomerization of the double bond and elirnination of the angular methyl group provided a monobenzoporphyrin 40 in low yield (Scheme 16).58 Scheme 16: Reaction of Pp LX DME with Dimethyl Acetylenedicarboxylate More recently, some studies on [4+2] cycloaddition of protoporphyrin LX and protoporphyrin LT with p-nitrosobenzene revealed that the primary products were the mono Diels-Alder adducts. For example, when protoporphyrin II dimethyl ester was the diene, the mono-adduct 41 was isolated as the major product. Treatment of the resulting adduct with an excess of the nitro dienophile gave 42 as a result of an interesting conversion of the unreacted vinyl group to a formyl group.59 PpUDME Scheme 17: Reaction of Pp U DME with p-Nitrosobenzene 1 32 Intramolecular addition at the peripheral porphyrin double bonds has also been employed to produce reduced porphyrins. Woodward's synthesis of chlorin-e6 from 43 provided one of the first examples of such an addition (Scheme 18).60 The driving force of this reaction is the lowering of the steric strain in the resulting chlorin 44. Scheme 18: The Intramolecular Cyclization Step in Woodward's Chlorin e6 Synthesis Another example of an intramolecular addition involves the formation of a spiro-derivative 46 from the cyclization of the p^acetylaniinoethyl substituent of 45 in the presence of phosphoryl chloride and pyridine.61 NHAc POC1, Pyridine 45 46 Scheme 19: Formation of a Chlorin via an Intramolecular Addition. 1.6.4. Reduced Porphyrins via a Stepwise Approach The conventional methodology used to obtain reduced porphyrins from porphyrins is clearly impractical for the total synthesis of natural pigments such as bonellin, factor 1 and sirohydrochlorin. Clearly, there was a need to develop a stepwise route for the synthesis of such molecules. Historically, the first breakthrough in the area was accomplished by a joint effort of the Eschenmoser (Zurich) and Battersby (Cambridge) groups in 1979 whereby an octamethylisobacteriochlorin was put together in a rational fashion.62 Conceptually, the construction of the macrocycle involves the order in which the four rings would be assembled together before the final cyclization step. Figure 1.11. shows the two possible options available if the molecule is to be dissected into two reasonably equal halves. S Figure 1.11: Illustration of the Dissection of an IsobacteriocUorin into Two Halves In their classical approach, the Eschenmoser and Battersby groups tackled the challenge by disconnecting the macrocycle on the east-west line, thus dividing the molecule into a northern and a southern fragment (Scheme 20). Following this strategy the two fragments 47 and 48 were coupled in the presence of palladium diacetate and l,5-diazabicyclo[5.4.0]undec-5-ene (DBU) to give a palladium complex 49. The palladium was removed by potassium 34 cyanide, and the cyclization was effected with zinc perchlorate via the intermediate 50. Treatment of the zinc complex with acid afforded the required metal-free isobacteriochlorin 51. Scheme 20: The First Stepwise Synthesis of an Isobacteriochlorin Two years later, two groups independendy reported their outlines of new routes to C-methylated chlorins. Battersby et al. at Cambridge suggested a dissection of the macrocycle along the north-south line to give an eastern and a western fragment.63 The two key intermediates 52 and 53 (Scheme 21) were condensed using standard porphyrin chemistry to give a linear tetrapyrrole 54 which was cyclized in hot acetonitrile in the presence of copper (U) 35 acetate to give the C-methylated chlorin 56 as its copper complex via the imino-ether intermediate 55. Demetalation of the resulting complex with trifluoroacetic acid saturated with hydrogen sulfide gave the chlorin 57. Scheme 21: Synthesis of C-Methylated Chlorin via an Irrrino-Ether Intermediate In the same report Battersby et al. employed two different intermediates 58 and 59 in a second route to give another linear tetrapyrrolic component 60. Cyclization followed by 36 demetalation under the same conditions described above gave the required chlorin 62 (Scheme 22). Scheme 22: Synthesis of C-Methylated Chlorin via l-Bromo-19-Methyl Intermediate On the other hand, Montforts at Frankfurt implemented a sulfur ring contraction step to prepare his key tricyclic intermediate 66 in the synthesis of a model octamethylchlorin.64 The tricyclic component was obtained by coupling 63 and 64 with N-bromosuccinimide as brorninating agent. After sulfur contraction, 66 was condensed with a bromopyrrolaldehyde to give the tetracyclic chain 67. Cyclization of the latter with potassium tert-butoxide in the presence of zinc acetate gave the chlorin 69 after demetalation (Scheme 23). 37 Scheme 23: Synthesis of a C-Methylated Chlorin via a Sulfur Contraction Step It soon became clear that such approaches have great potential and that modification of the substituents around the periphery of the macrocycles might lead to total synthesis of natural pigments. So far, the above routes depend on rather vigorous conditions for the final cyclization step. In fact these routes ran into difficulties when the natural acetate and propionate groups were introduced. In view of the sensitive nature of the natural acetate and propionate side-chains, Battersby and coworkers engineered a new cyclization step fully compatible with these moderately reactive side-chains.65 The new approach involves an 18 Tt-electron photochemical cyclization of an open-chain tetrapyrrole normally referred to as a secosystem. In accordance with Woodward-Hoffmann rules the cyclization of the seco-system involving 18-7C electrons can proceed via: (a) a photochemical route by an antarafacial process (b) a thermal route by a suprafacial process 38 In their preliminary studies, they found that the ring closure went spontaneously when the seco-system was irradiated with light On the other hand, no reaction took place in the dark. This observation is a result of the much more favorable antarafacial process whereby good overlap of the rc-system is possible in the transition state with minimum twisting of the seco-system. In the process, methanol is lost to give the required macrocycle. Interestingly, the thermally-allowed 16 7C-ring closure to give a dehydrocorrin did not take place in the cyclization step (Scheme 24). Scheme 24: The Photochemical Cyclization of a Seco-system The photochemical route opened the way to the synthesis of bonellin dimethyl ester (1983,1988),66-67 sirohydrochlorin octamethyl ester (1985),68 and factor 1 (1988)69 in the given order. The bonellin macrocycle was divided along the north-south line giving rise to a western and an eastern block. The western half (Scheme 25, 74) with a reduced pyrrolic ring 39 (D) was prepared from the nitropyrrole 73. The eastern half (a dipyrromethene, 75) was prepared separately from readily available pyrroles. Condensation of the western block 74 with the eastern block 75 under acidic conditions generated a seco-system 76 which gave the racemate mixture of bonellin dimethyl ester 77 after methanolysis of the nitrile with methanolic hydrogen chloride. Scheme 25: Synthesis of (±) Bonellin Dimethyl Ester 40 The budding blocks involved in the synthesis of sirohydrochlorin octamethyl ester are outlined in Scheme 26. The crucial precursor was the imide 78 which has been synthesized via several routes with high enantioselectivity.70 Scheme 26: Building Blocks for Sirohydrochlorin Octamethyl Ester Its conversion into the two isomeric monotMoimides provided the precursors which contained the four chiral centers of the molecule. Coupling of the individual monothioimide to their respective pyrrolic partners gave an eastern and a western fragment which in turn were condensed to give the seco-system. Irradiation of the latter with light gave sirohydrochlorin octamethyl ester. 41 C h a p t e r 2 Results and Discussion 42 2.1 Synthetic Challenge and Objective The striking biological properties of porphyrins, chlorins, bacteriochlorins, isobacteriochlorins and corrins attracted the attention of organic chemists during the 1950's, followed by a rapid and worldwide expansion of activity in the 1960's and 1970's. Among those macrocycles, porphyrins being planar with no chiral centers at the periphery have enjoyed the most development from a synthetic point of view. Today, various methods for porphyrin synthesis are available and the construction of any imaginable porphyrin is feasible. As a result of this property a wide range of porphyrins have been synthesized in the past with different substituents at the periphery to mimic various biological functions. On the other hand, progress in the field of chlorins, bacteriochlorins and isobacteriochlorins has been very slow and in most cases these compounds have proved to be elusive targets prior to 1979. The main reason for this lack of activity was probably the presence of chiral centers in the molecule and their lack of implications in biological systems at that time. However, the recent isolation and elucidation of the structures of bonellin, factor 1 and sirohydrochlorin have made their total synthesis important to prove their structures unambiguously. This challenge was taken up by Battersby et al. at Cambridge and within the last five years they have completed the total synthesis of bonellin, sirohydrochlorin and factor 1 (Chapter 1). With the implication of factor 1 and sirohydrochlorin as intermediates in vitamin B12 biosynthesis, there was a need for larger amounts of material to carry out extensive chemical, spectroscopic and biological studies on these molecules. Clearly, Battersby's approach is too lengthy and does not present itself as the best way to provide large amount of products. Obviously, a shorter approach to such molecules or models of these molecules is welcome in the field to study and understand their properties. Interest in benzoporphyrins stems from the fact that such compounds have been isolated from petroleum and related deposits.71 They exhibit a rhodo-type absorption pattern in the visible region of their electronic spectrum. The presence of a benzene ring condensed to the 43 periphery of the molecule makes the origin of such a molecule intriguing since no porphyrin with this structural feature has been found in plants or animals. Also, chemical synthesis of the molecule became necessary to confirm such a structural feature of the molecule. Clezy and co-workers became the first in the porphyrin field to provide chemical evidence in support of this structure by synthesizing via two routes a porphyrin of this class that had electronic resemblance to the isolated material.72'73 In one of their approaches, Clezy and co-workers introduced a cyclohexanone ring at the pyrrolic level as the entry to the benzene nucleus. Further transformations of this six-membered ring after porphyrin formation led to unstable intermediates which eventually gave the benzoporphyrin in low yield (Scheme 27). Scheme 27: Clezy's First Route to Benzoporphyrin Baker and co-workers have advanced a hypothesis to explain the origin of the benzene ring in these porphyrins.74 They claimed that the system is generated by way of a Diels-Alder addition of a vinyl porphyrin to a quinone such as plastoquinone which occurs in substantial 44 amounts in photosynthetic organisms (Scheme 28). Preliminary work in our laboratory to test Baker's hypothesis provided encouraging results (Chapter 1, Scheme 16) and it therefore seemed a worthwhile exercise to continue synthetic studies in this area with an aim of providing a rational route to benzoporphyrins, thus facilitating a more detailed study of the naturally occurring petroporphyrins. Benzoporphyrin Scheme 28: Baker's Hypothesis on Benzoporphyrin Formation The recent developments in the field of photodynamic therapy using porphyrins as photosensitizers have given rise to considerable research in this area.75-76 Although significant progress has been achieved in the design and evaluation of a number of promising porphyrin related macrocycles (some of which will be discussed later), the problem of how to optimize 45 the properties of the sensitizer still remains largely unresolved The sensitizer of choice for the last decade is a mixture of porphyrins referred to as hematoporphyrin derivatives (HpD and later as Photofrin II™) whose exact chemical composition is still not known. This has prompted many research groups all over the world to seek other potential photosensitizers. Recent efforts from our laboratory have produced a chlorin type compound from a Diels Alder reaction on protoporphyrin LX dimethyl ester. This compound has an advantage over HpD in that it absorbs at longer wavelengths (650-700 nm). Preliminary in vitro studies have already indicated that the compound can develop into a promising photosensitizer.77'78 However, consideration of tissue and light characteristics suggest that the ideal photosensitizer for treatment of more deep-seated tumors should absorb at wavelengths >700 nm. 7 9 There is therefore a need to synthesize new macrocycles which will absorb in the far visible red or the near infrared range. As a continuation of this work, it was reasoned that since natural bacteriochlorins such as bacteriochlorophyll a or b absorb at light at longer wavelengths (>740 nm) than chlorins, they could be the right sensitizers in the future for photodynamic therapy. Any synthetic approach to the problem should also take into consideration the fact that the synthetic plans need to be short and can be scaled up. When one inspects the biologically important porphyrins and related macrocycles, the vinyl group immediately stands out as a common feature in most of them. Chemical manipulations of such a group in porphyrins are well documented.80 In most cases, the chemistry involves the use of the vinylic double bond as an olefinic substrate which undergoes electrophUic additions. The vinyl group has also been shown to undergo [4+2] cycloadditions with activated dienophiles.56'57 From a synthetic view point, it was felt that the Diels-Alder reaction might present itself as a powerful tool in the synthesis of porphyrin-related macrocycles. To explore this type of chemistry, different types of vinylporphyrins were synthesized as precursors for specific target molecules. The chemistry described in this thesis outlines the synthetic routes leading to vinylporphyrins, chlorins, benzoporphyrins and bacteriochlorins. 46 2.2 Synthetic Plan One of the most desirable features of a synthetic design is that it is applicable to more than one compound within any class of structurally related types. With this in mind, vinylporphyrins were synthesized and later transformed into chlorins, benzoporphyrins and bacteriochlorins. The success of the present work therefore depended on the development of a viable synthetic route to vinylporphyrins. Being aware of the sensitive nature of the vinyl group, its generation was delayed until the final stage of the synthesis. In planning the synthetic approaches from monopyrroles, the vinyl group precursor, an acetate ester, was introduced at the pyrrolic level to be transformed later via a hydroxyethyl group to the desired vinyl group at the porphyrin level. < Chlorins Benzoporphyrins Divinylporphyrins • Bacteriochlorins Inspection of the structure of factor 1 (a chlorin) revealed the presence of acetic and propionic acid side-chains at the chiral centers on ring A (Scheme 29). In this study, compound 79 (without any acetic and propionic acid side-chains on rings B, C, and D) was considered as the synthetic model of factor 1. 4 7 5 C02H C02H 79 Factor 1 Scheme 29: Strategy for a Factor 1 Model It was reasoned that the acetic and propionic side-chains on ring A could be generated from compound 80 after a ring opening. A retrosynthetic analysis of 80 suggested, as starting material, the monovinylporphyrin 81. The key step would be an intermolecular Diels-Alder reaction involving 81 and an olefinic dienophile (Scheme 30). 49 Upon perusal of the literature, it was noted that 1,2-disubstituted vinyl sulfones have been reported to be effective in controlling regiochemistry in Diels-Alder reactions and therefore were considered for this w o r k . 8 1 - 8 3 However, at the start of this work, the regiochemistry of cycloaddition of 1,2-disubstituted vinyl sulfones as dienophiles with an unsymmetrical diene such as monovinylporphyrin 81 had not been investigated to any great extent. Since in this plan, obtaining the right regiochemistry is of upmost importance, exploration of the regiochemistry of the proposed Diels-Alder step was considered essential. Towards this end, several (^substituted vinyl sulfones were prepared and their chemistry with monovinylporphyrin 81 were studied. For example, i f (E)-|3-phenylsulfonylacrylonitJile is the dienophile, regioisomer 82 would be the desired isomer in order to warrant continuation of the proposed transformations for a synthesis of a model compound of factor 1 (Scheme 30). Subsequent hydrogenation of the chlorin-type adduct 82 should afford the compound 83. If an oxidative desulfonation could be carried out on 83, it should be possible to obtain compound 84. Methanolysis of 84 would give 87 which could be ring opened by a retro-Claisen type reaction with base (methoxide) to give 79 with the acetate and propionate side-chains. Compound 87 could also be obtained by a desulfonation reaction on 83 followed by the epoxidation of the product 85 and further transformations. The plan therefore called for the synthesis of ester porphyrin 88 (Scheme 31). As the retrosynthetic analysis of Scheme 31 shows, it should be possible to obtain the porphyrin 88 from an a,c-biladiene intermediate via dipyrromethanes or dipyrromethenes. Both methods were investigated and the one giving higher overall yield was subsequently used to make the ester porphyrin on a gram scale. In the first method, the l,19-dimethyl-a,c-biladiene 89 was derived from the condensation of two equivalents of the formylpyrrole 92 with the dipyrromethane 91 after a debenzylation step. In the second method, the l-bromo-19-methyl-a,c-biladiene 90 was derived from the condensation of the 5-memyl-5'-unsubstituted dipyrromethene 93 with the S-brorrK^S'-bromomethyldlpyrromethene 94. Scheme 31: Retrosynthetic Analysis of Monovinylporphyrin 81 51 Fragmentation analysis of dipyrromethane 91, as shown in Scheme 32, suggested pyrroles 95 and 96 as the starting materials. Scheme 32: Retrosynthetic Analysis of Dipyrromethane 91 It became apparent by the same type of analysis that pyrroles 97 and 98 were the intermediates for dipyrromethene 93. It is however less obvious that the dipyrromethene 94 could be obtained in a one-step head to tail condensation of two molecules of the pyrrole 99 involving deesterification, decarboxylation and bromination.(Scheme 33). Scheme 33: Retrosynthetic Analysis of the Dipyrromethenes 52 The second target molecule from a monovinylporphyrin was a benzoporphyrin molecule. As mentioned before, recent work in our laboratory showed that activated dienophiles underwent [4+2] cycloaddition reactions with the vinyl and cross-conjugated p\P'-double bond of protoporphyrin IX dimethyl ester to give chlorin type adducts as stable products. Such chlorin adducts underwent aromatization with loss of the angular methyl group in the presence of a base and excess dienophile to give benzoporphyrins in poor yields.58 The process by which the methyl group was lost with concomitant aromatization is unclear. To investigate this reaction further, attention was turned to developing a more facile route for benzoporphyrins. From the outset, it was envisioned that the replacement of the methyl group adjacent to the vinyl group by a hydrogen atom would provide a more direct entry to such molecules. The plan therefore called for the synthesis of a P-unsubstituted-P'-monovinylporphyrin 101 to serve as the diene. Examination of literature revealed, somewhat surprisingly, only one previous report on this type of compound. The task of preparing a P-unsubstituted-P'-monovinylporphyrin therefore became more challenging than expected. Djerassi and co-workers, in the only reported synthesis of this type of compound, employed dipyrromethanes as intermediates with an acetyl function as the vinyl precursor.84 In their synthesis the acetyl group was responsible for the structurally similar by-products formed at the dipyrromethane and porphyrin stages and that accounted for the low yields observed. The basic strategy for the assault on the benzoporphyrin 100 is summarized in the antithetic format of Scheme 34. Thus the functional group interconversions and disconnections on 101 led to dipyrromethenes 94 and 104 via the a,c-biladiene 103. Subsequent disconnection of 104 led to the crucial pyrrolic precursors 105 and 97. Dipyrromethene 94 was obtained from the pyrrole 99 as mentioned before. 53 Scheme 34: Retrosynthetic Analysis of Benzoporphyrin 100 The third target molecule was the bacteriochlorin macrocycle. It is interesting to note that there has been very litde synthetic activity in this area. A literature survey (Chapter 1) revealed no rational route to such a molecule. The synthetic approach to be investigated here was based on two Diels-Alder cycloadditions on an A,C-(hvmylrx)rphyrin precursor (Scheme 35). 54 108 Scheme 35: Retrosynthetic Analysis of the Bacteriochlorin (Fischer's Approach) Only one previous report of Diels-Alder reactions on an A,C-divinylporphyrin has appeared in the literature. This involves the work of Jackson and co-workers59 whereby the bacteriochlorin-type product formed was too unstable to be isolated. In view of this, the present study of the Diels-Alder reactions on an A,C-divmylporphyrin, if successful, might present itself as a new entry into the bacteriochlorin chromophore. In particular, this work was 55 carried out with the aim of investigating the properties of such molecules as potential photosensitizers. Following the chemistry developed for the monovinylporphyrins, the A,C-divinylporphyrin 106 (Scheme 35) was prepared via dipyrromethene as intermediates. In order to make the synthetic route to the desired A,C-divinylporphyrin simple and efficient, a two-fold rotation axis perpendicular to the porphyrin plane (assuming equivalent nitrogen atoms and a planar porphyrin) was introduced in the macrocycle. A retrosynthetic analysis revealed that the di-ester porphyrin 108 could be prepared from the self-condensation of dipyrromethene 93 in a head-to-tail fashion. In fact, this process was the backbone of Fischer's classical porphyrin synthesis. As a first attempt, this method was explored due to its simplicity. A second route based on our approach towards the monovinylporphyrins was also undertaken at the same time. The latter not only seemed to be quite appealing, but it also seemed to be more challenging. Once a good route to the A,C-divinylporphyrin was made available, its chemistry with olefinic and acetylenic dienophiles was investigated. 56 2.3 Monopyrrolic Starting Materials Five basic pyrroles 99,109,110, 111, and 112 were required for the synthesis of the vinylporphyrins in the present work. They are all well known compounds and their preparations from acychc precursors are normally carried out on a large-scale using a modified version of the Knorr synthesis.85 Figure 2.1: Monopyrrolic Starting Precursors and Intermediates 57 The synthesis of pyrrole 99 in this work was based on the modified version introduced by Johnson and co-workers (Scheme 36).86 Tert-butylacetoacetate 113 was nitrosated with sodium nitrite to give the oxime 114 which was reacted without isolation with 2,4-pentanedione 115 in glacial acetic acid in the presence of zinc dust. Scheme 36: Synthesis of Pyrrole 99 58 The pyrrole 118, isolated in 76 % yield, was then treated with diborane, a procedure introduced by Whitlock and Hanauer87 to give the 2-(tert-butyloxycarbonyl)-4-ethyl-3,5-dirriethylpyrrole (99) in 80 % yield. The diborane was generated in situ by the slow addition of boron trifluoride etherate to a cooled stirred suspension of sodium borohydride and pyrrole 118 in tetrahydrofuran. The progress of the reaction was monitored by t.l.c. and upon completion, the reaction was quenched first with glacial acetic acid and then with water to afford the product 4 BF3 + 3 NaBH4 ^ 2 + 3 NaBF4 Pyrrole 111 carrying an acetate ester (vinyl precursor) at the 4-position was prepared in 46 % yield by an extension of the above method as shown in Scheme 37. NOH HI 122 Scheme 37: Synthesis of Pyrrole 111 59 The required diketone 121 (which was made available by Dr. Wijesekera) was prepared in 70 % yield by treating pentanedione with methylchloroacetate in acetone in the presence of potassium iodide and potassium carbonate. The pyrroles 109,110 and 112 were available in the laboratory as a result of their use in other projects. 2.4 Synthesis of Pyrrole Precursors of Dipyrrolic Intermediates Even with the large number of general methods for pyrrole synthesis, there are still many cases in which a required pyrrole cannot be obtained directly from acyclic precursors. In such cases, pyrroles readily available from the Knorr synthesis or one of its modified methods are used as precursors for further manipulations. Most of the chemistry described in this section will deal with the transformation of a pyrrole via reactions at the 2-position (a) or 3-position ({3). The a-modifications are for coupling purposes only and are required with all the starting pyrroles while the P-modifications are to obtain different peripheral substituents in the final porphyrins. A consideration of the various possible structures of the intermediates obtained from electrophilic attack at the 2- and 3- positions suggests that the 2-position should be more reactive than the 3-position toward electrophilic substitution (Scheme 38). H H Scheme 38: Etectrophilic Substitution on the Pyrrole Ring 60 Initial efforts were directed towards the transformation of four of the starting pyrroles into the crucial pyrroles 92, 95, 96, 97, 98 and 105 (Fig. 2.2) to be used to prepare the dipyrrolic intermediates. Figure 2.2: Crucial Pyrroles Required in Dipyrrolic Synthesis Pyrrole 97, which is a key intermediate in the synthesis of dipyrromethenes, was prepared from the pyrrole 109 (Scheme 39). The greater reactivity of the a-substituent over the P was used in regioselectively modifying the a-methyl group in pyrrole 109 without affecting 61 the P-methyl group. Treatment of 109 in dichloromethane with two equivalents of sulfuryl chloride afforded the dichloromethylpyrrole 123. o Scheme 39: Transformation of Pyrrole 109 62 To minimize tricMorination, the reaction was carried out at near 0°C. The resulting dichloromethylpyrrole was hydrolysed by stirring overnight with water and the formylpyrrole 124 was obtained in 84 % yield after crystallization from dichloromethane-hexane. Further chemistry in this transformation required the formyl group to be protected. Several protecting groups have been employed in pyrrole chemistry in the past, but the cyanovinyl group has received the most attention.88 Treatment of the formylpyrrole 124 with methyl cyanoacrylate in methanol containing catalytic amounts of trieuylarnine gave the cyanoacrylate derivative as bright yellow crystals. This reaction turned out to be a good way of purifying the formylpyrrole as the impurities were left in the methanol solution. In fact, it was possible to maximize yields in the preparation of the aldehyde by adding methyl cyanoacrylate to the mother liquor of the aldehyde. The protected formylpyrrole was isolated in greater than 90% yield. Although, in principle, two isomers can be produced in the reaction, only one product was observed. This presumably resulted from the fact that the bulkier ester group prefers to be in a trans relationship to the pyrrole nucleus. Catalytic hydrogenation of pyrrole 125 in tetrahydrofuran containing triethylamine and 10% palladium/charcoal provided the required carboxypyrrole 126 in 99 % yield as yellow crystals. Conversion of the carboxypyrrole 126 to the a-free-pyrrole was then successfully achieved via a decarboxylative iodination and deiodination sequence.89 A decarboxylative iodination process was chosen over the thermal decarboxylation because the former is known to give higher yields and purer products when the carboxypyrrole ring bears electron-withdrawing groups. Dropwise addition of iodine monochloride to the carboxypyrrole suspended in glacial acetic acid in the presence of excess sodium acetate as buffer gave the a-iodopyrrole 127. The excess iodine was removed by the addition of aqueous sodium bisulfite and the product was crystallized by the slow addition of water to the reaction mixture. The iodopyrrole 127 obtained in 80 % yield was deiodinated with zinc and glacial acetic acid to give the a-free-pyrrole 128 in 95 % yield. The latter was then deprotected with KOH in methanol-water under an inert atmosphere at refluxing temperature. The resulting oil on cooling gave the pyrrole 97 as a mixture of dark brown and colorless crystals (85 % yield) and was used in this form in the preparation of the dipyrromethenes. Pyrrole 96, a key intermediate in the dipyrromethane synthesis (see retrosynthetic Schemes 31 and 32) was prepared from pyrrole 112 as shown in Scheme 40. This route entails a sequence of reactions devised in an earlier project by Dr. Wijesekera in our group.90 The overall sequence converts the a-methylpyrrole to an a-free-pyrrole. In the first step, pyrrole 112 was reacted with three equivalents of sulfuryl chloride and subsequently hydrolysed to the acid. This trichlorination reaction was carried out according to the method devised by Battersby and co-workers,91 whereby ether and dichloromethane were both used as solvents. The starting pyrrole 112 was dissolved in dichloromethane, diluted with anhydrous ether (just prior to reaction) and treated at room temperature with sulfuryl chloride in dichloromethane. Sulfuryl chloride was added rapidly to maintain a high concentration of free radical chains which ensured a complete conversion of the a-methyl group to the trichloromethyl stage. A slow addition of sulfuryl chloride may stop or slow down the free radical chains and the hindered and less reactive dichloromethyl pyrrole may not be able to compete as well for the oxidant as the solvent (ether). This could result in the formation of increased amounts of aldehyde by-product After stirring for an hour, at room temperature, the solution was concentrated in vacuo to give a pale yellow oil. The hydrolysis was carried out and on cooling, the a-carboxypyrrole 129 was precipitated as a tan solid. Further purification was performed by dissolving the solid in warm aqueous sodium bicarbonate-methanol and extracting the impurities with ether. Acidification of the aqueous layer with hydrochloric acid reprecipitated the a-carboxypyrrole as a white solid in 67 % yield. 64 Scheme 40: Transformation of Pyrrole 112 In the next step, the a-carboxypyrrole 129 was converted to the a-free-pyrrole 131 via a decarboxylative iodination and deiodination sequence. The reaction was carried out in a two-phase system of dichloroethane and water, using sodium bicarbonate to solubilize the carboxypyrrole in the aqueous phase. The iodination was carried out with excess iodine in the 65 presence of potassium iodide under refluxing conditions. In the two-phase system, the iodopyrrole 130 once formed moves into the organic phase from which it can be easily isolated after destroying the excess iodine with sodium bisulfite. The deiodination of 130 was carried out with potassium iodide and hydrochloric acid and the iodine liberated was destroyed with sodium bisulfite solution, giving the ethyl ester pyrrole 131. Finally the ethyl ester was converted to the corresponding benzyl ester analog 96 by a transesterification process.This was effected by a high temperature modification of the procedure developed by Kenner and co-workers92 using benzyl alcohol and sodium benzyloxide. The benzyl alcohol was freshly distilled beforehand from anhydrous potassium carbonate in order to remove most of the water and any benzoic acid. The ethyl ester pyrrole 131 was heated to reflux in benzyl alcohol and a solution of sodium in benzyl alcohol was added in 1 ml portions. A vigorous evolution of ethanol vapor with concomitant lowering of the refluxing temperature was indicative of efficient transesterification. After work-up, the resulting orange-brown oil was purified on a silica gel column to give the desired pyrrole as a pale yellow oil. On standing in the refrigerator for a few days, the compound crystallized out as a pale yellow solid in 70 % yield. A key intermediate for the synthesis of the a,c-biladiene 89 (via the dipyrromethane 91) was the a-formylpyrrole 92 (see Scheme 31). It was synthesized from pyrrole 109 via the transformations described in Scheme 41. Hydrogenolysis over palladized charcoal gave the carboxypyrrole 132 which was thermally decarboxylated to give the a-free-pyrrole 133. This reaction was carried out in boiling dimemylformatnide under a nitrogen atmosphere and was monitored by UV spectroscopy (the disappearance of the 280 nm absorption indicated the completion of the reaction). Vilsmeier-Haack formylation with phosphorus oxychloride in dimemylforniamide was then jxrformed on 133 without isolation and the resulting iniinium salt was hydrolysed in weakly alkaline solution (sodium bicarbonate added in small portions), followed by heating on a steam bath to complete the hydrolysis. On cooling, the desired pyrrole 92 was collected in approximately 90 % yield. 6 6 S c h e m e 41: Transformation of Pyrrole 109 Pyrroles 95 and 98 were crucial intermediates in both dipyrromethene and dipyrromethane syntheses since the acetate ester was the precursor to the vinyl group. Hydrogenation of 111 over 10% palladized charcoal in tetrahydrofuran at room temperature gave the a-carboxypyrrole 98 which was a key intermediate in the pyrromethene synthesis. Treatment of the pyrrole 111 with lead tetraacetate93 on the other hand gave the acetoxymethylpyrrole 95. Lead tetraacetate was added in portions to a solution of the foregoing pyrrole in glacial acetic acid. After stirring for three hours at room temperature the reaction went to completion (tic check). Water was added dropwise to the reaction niixture until the 67 acetoxymemylpyrrole precipitated out This pyrrole was subsequendy used in the synthesis of the pyrromethane 91. Dipyrromethane Synthesis Dipyrromethene Synthesis 98 Scheme 42: Transformation of Pyrrole 111. Pyrrole 105, a key intermeditate in the synthesis of the P-unsubstituted-P'-monovinylporphyrin 101 was prepared from the triester pyrrole 110. Transformation of pyrrole 110 to pyrrole 105 involved both a- and P- modifications. Triester pyrrole 110 was selected in view of the presence of a benzyloxycarbonyl group at the |J-position which could be manipulated to a p-free analog without affecting the other two ethyl ester groups. Hydrogenolysis of the pyrrole 110 over 10% palladized charcoal in terrahydrofuran at room temperature and atmospheric pressure gave the carboxypyrrole 134 in 88 % yield. Scheme 43: Transformation of Pyrrole 110 69 Iodination of the resulting carboxypyrrole via the two-phase iodination method described previously gave the iodopyrrole 135. This procedure has consistendy given yields > 85% for both a and (3 iodination. The deiodination was carried out with sodium iodide and concentrated hydrochloric acid to give the P-free pyrrole 136. Transesterification on the diethyl ester pyrrole 136 by the method used on the other pyrroles in this work gave the dibenzyl ester analog 137 in 85 % yield. The monomethyl-monobenzyl ester pyrrole 138 was then obtained by a selective base-catalysed exchange reaction (methoxide-methanol)92 at room temperature. Normally the rate of exchange depends on the structure of the pyrrolic ester involved. In this reaction the dibenzyl ester pyrrole was dissolved in dry methanol at room temperature, the solubility being enhanced by the addition of tetrahydrofuran. Sodium methoxide (sodium in anhydrous methanol, 0.2 equivalents) was added slowly to the mixture and the reaction was allowed to stir to completion (approximately 2 hours). However, if a larger amount of sodium methoxide was used, transesterification of the benzyl ester group at the a-position was also observed thus producing a dimethyl ester analog. The presence of the benzyl ester group at the a-position has an advantage over the ethyl or methyl ester, because it can be cleanly removed by catalytic hydrogenolysis to give the a-carboxypyrrole 139. Hydrogenolysis of 138 in tetrahydrofuran containing 10% palladized charcoal and a few drops of triethylamine gave the carboxypyrrole analog 139. Its coupling with an a-free-pyrrole to give dipyrromethene 104 went sluggishly (see dipyrromethene section).To overcome this problem, the a-carboxypyrrole was converted to its a-free analog 105. The nature of the product 105 hinted that a thermal or iodinative decarboxylation would not be a good idea. In both cases, the reaction conditions are too vigorous and would very likely lead to the oxidation of the desired a,|3-imsubstirated-pyrrole 105. This decarboxylation was achieved rather cleanly with trifluoroacetic acid at room temperature. The a-carboxypyrrole 139 was dissolved with stirring in freshly distilled trifluoroacetic acid at room temperature under a nitrogen atmosphere. The reaction was monitored by tic, and on completion, the trifluoroacetic acid was removed in vacuo and the residual oil was chromatographed on a silica gel column. The desired fractions 70 were combined and evaporated to a pale yellow oil. The ^ - N M R spectrum of 105 showed the presence of the a-proton as a broad singlet centered at 8 6.55 ppm and the presence of the p-proton as a singlet at 8 5.80 ppm. Further structural confirmation was obtained from its mass spectrum. The molecular ion ( M + 153) was observed with a fragment ion at m/z 94 due to the loss of a methoxycarbonyl group. In planning the synthetic route from the monopyrroles to an A,C-m"vmylporphyrin, it was foreseen that the acetate ester might cause problems in the bromination step of the dipyrromethene 93 (Scheme 35).91>94 To overcome this obstacle, the acetate ester on pyrrole 111 was reduced at the pyrrolic stage to the hydroxyethyl and protected as an acetoxy before its coupling to the formylpyrrole 97 (Scheme 44). CH3i 1.0 M Borane/THF Complex Ph 111 140 Ac20/Pyridine Scheme 44: P-Modifications of Pyrrole 111 71 This reduction was carried out with diborane to afford the hydroxyethylpyrrole as a white product 140 in 87 % yield. In the following step, the resulting hydroxyethylpyrrole was protected as its acetoxy derivative 141. The reaction was carried out by dissolving the foregoing pyrrole in pyridine and adding acetic anhydride slowly to the stirred solution (97% yield). Hydrogenolysis of the acetoxy pyrrole provided the crucial a-carboxypyrrole 142 needed for the cUvinylporphyrin synthesis. 2.5 Synthesis of Dipyrromethanes Various synthetic routes to unsyrnmetrically substituted dipyrromethanes are available but the most commonly used approach today is the condensation of an a-free-pyrrole with an a-acetoxymethylpyrrole.95 OCH3 Scheme 45: Synthesis of Dipyrromethane 91 72 The a-free-pyrrole 96 and a-acetoxymethylpyrrole 95 (Scheme 45) were dissolved in glacial acetic acid and heated at 120°C under a nitrogen atmosphere (no reaction was observed at lower temperature). Completion of reaction occurred within two hours. The crude product after work-up was purified chromatographically on a silica gel column to give the desired product 91 in 81 % yield. The characteristic feature in the ^ - N M R spectrum of 91 is the singlet at 6 3.85 ppm which was assigned to the methylene protons at the bridge. Further structural confirmation was derived from its mass spectrum. The mass spectrum of 91 showed the molecular ion (m/z 542) and daughter ions derived from the loss of one tropylium ion (m/z 451) and one benzyloxycarbonyl moeity (m/z 407). 2.6 Synthesis of Dipyrromethenes 2.6.1 Synthesis of 5-Methyl-5'-unsubstituted Dipyrromethenes The S-methyl-S'-unsubstituted dipyrromethenes required in the present work were prepared by acid-catalysed condensation of an a-carboxypyrrole with an a-formylpyrrole (Scheme 46). In a typical case, the a-formylpyrrole 97 (in slight excess) and the carboxypyrrole 111 dissolved in tetrahydrofuran were treated with 48 % aqueous hydrobromic acid. The solvent was evaporated off while replenishing with methanol at regular intervals. The mixture was cooled at 0*C and the dipyrromethene hydrobromide collected by filtration. Yields of dipyrromethenes between 83 to 86 % were obtained when the carboxypyrrole carried ester or acetoxy groups at the [^ -positions. 73 The possible mechanism of such condensations is outlined in Scheme 47. The formyl pyrrole under acidic condition, is protonated and reacts with pyrrole 111 to give the dipyrromethene with concomitant loss of carbon dioxide and water. 74 Scheme 47: Possible Mechanism of the Acid-catalysed Condensation in Synthesis of Dipyrromethenes In the case of the P-unsubstituted carboxypyrrole 139 (corresponding to 105), the condensation reaction was slow and the yield was poor. It is believed that a hydrogen atom at the (3-position instead of an alkyl group deactivated the neighbouring a-position towards electrophilic attack resulting in a low yield. To overcome this problem, the P-unsubstituted carboxypyrrole was decarboxylated to give 105 prior to condensation with the formylpyrrole 97 (Scheme 46). Removal of the carboxyl group (an electron-withdrawing group) had the effect of increasing the electron density of the ring thereby increasing the nucleophilicity and as a result the condensation went smoothly. The ^ - N M R spectrum of pyrromethene 104 was in agreement with its structure, the expected doublet centered at 8 7.7 ppm (J = 2.5 Hz) was due to the a-hydrogen, the singlet at 8 7.38 ppm was due to the meso hydrogen and the P-75. hydrogen resonated as a singlet at 5 6.4 ppm. The mass spectrum did not give the molecular ion peak but the expected peak at m/z 272 due to (M+-HBr) was present 2.6.2 Synthesis of 5-Bromo-5'-bromomethyldipyrromethenes The non-availability of some 5-bromo-5'-brorrx)methyl dipyrromethenes has limited the use of Johnson's regioselective method of porphyrin synthesis from l-bromo-19-methyl-a,c-biladienes. Generally, such dipyrromethenes are prepared by brominating 5-methyl-5'-unsubstituted dipyrromethenes. In the traditional bromination procedure, the dipyrromethene was treated with an excess of molecular bromine in hot anhydrous formic or acetic acid.96 The result of such a reaction was never predictable. Over-oxidation or under-oxidation were often accompanied by partial self-condensation leading to porphyrin synthesis. In 1976, Battersby and co-workers at Cambridge91 reported a modification of the bromination conditions whereby the bromination was carried out with a large excess of bromine in boiling 1,2-dichloroethane (an aprotic solvent instead of acetic or formic acid). They were able to show that rapid substitution at the a-free position by bromine took place and that much slower attack on the a-methyl group occurred. By employing trifluoroacetic acid as a co-solvent (up to 25% in dichloromethane or 1,2-dichloroethane), a milder and more effective bromination procedure was developed in our laboratory.94 A wide range of alkyl-substituted dipyrromethenes were successfully brominated at the a-methyl position, the reaction being carried out at room temperature. However, the reaction still failed for dipyrromethenes bearing electron-withdrawing groups attached directly to the ring at positions 3', 4, or 4'. Using the new reaction conditions, the bromination operation was performed on the dipyrromethenes prepared in the previous section. The dipyrromethenes were dissolved in 1,2-dichloroethane/trifluoroacetic acid (3:1) with excess bromine (3.0 equivalents) at room temperature. The reaction mixture gradually changed color over 2-3 days, from an initial yellow-brown to a deep orange with a pink tinge 76 which is the characteristic color of the desired product. With only alkyl groups on the dipyrromethene (for example in 143), the bromination was completed in two days giving the (ubrominated dipyrromethene 94 in 87 % yield. However, when an acetate group was present at the 4-position (e.g. in 93), bromination at the 5-methyl group was extremely slow (the 1H-NMR spectrum of the mixture revealed the presence of the 5-methyl group at 8 2.72 ppm). When this reaction mixture was allowed to stir for two weeks, only 50 % of the material became brominated at the 5-methyl position (8 of -CH2Br at 4.84 ppm). This reluctance of the dipyrrromethenes to brominate at the 5-methyl position is the reason why Johnson's regioselective method has not been exploited extensively by the porphyrin chemists. To overcome this practical limitation, the acetate group at the 4-position was reduced to a hydroxyl group at the pyrrolic stage and was protected as its acetoxy analog. Subsequent bromination of the resulting dipyrromethene 144 produced the 5-bromo-5'-bromomethyl-2,2'-dipyrromethene hydrobromide 145 in good yield. In the work-up procedure, the reaction mixture was evaporated to remove most of the excess of bromine, 1,2-dichloroethane and trifluoroacetic acid. The residue was then dissolved in dichloromethane and treated with an excess of cyclohexene to destroy any perbromide. Addition of ethyl acetate and concentration of the mixture in vacuo gave the crystalline product in good purity. Dipyrromethene 94, a key dipyrrolic intermediate in the monovinylporphyrin syntheses with an unsymmetrical arrangement of the P-substituents, was prepared via a different approach from pyrrole 99. CH 3C02H/Br 2 r B 99 B N' H N H Br Br" 94 J In this reaction, pyrrole 99 undergoes a head-to-tail coupling to give a mixture of brominated dipyrromethenes 143 and 94 in yields ranging from 70 to 80 % based on the perbromide. Facile removal of the t-butyl ester group under the acidic reaction conditions leads 78 to the formation the a-carboxypyrrole which is brorninatively decarboxylated during the initial stages. Subsequent slow radical bromination at the a-methyl gives a very reactive pyrryl carbinyl cation source, which condenses with itself or its precursor pyrrole as fast as it is formed. The reaction mixture therefore provided both 143 and 94 and was later converted completely to 94 by treating the mixture with bromine and trifluoroacetic acid in dichloroethane. 2.7 Synthesis of Vinylporphyrins 2.7.1 Synthesis of the f3-Methyl-f3'-monovinylporphyrin The initial objective in this work was to synthesize a model porphyrin which had only alkyl substituents except at the position carrying the vinyl precursor group. Two approaches were considered from the retrosynthetic analysis (Scheme 3). Both routes had the merit of commencing from readily available monopyrroles and they both involved an a,c-biladiene intermediate for cyclization to the porphyrin. In the first approach (Scheme 48), the dipyrromethane 91 was catalytically debenzylated to its di-acid analog and was condensed with two equivalents of the 2-formyl-5-methyl pyrrole (92) to give the l,19-dimethyl-a,c-biladiene 89. In this reaction, the di-acid was stirred in trifluoroacetic acid at room temperature for 15 minutes under nitrogen to effect the decarboxylation. The formylpyrrole in dry methanol was added, followed by HBr in acetic acid and the mixture stirred at room temperature until precipitation of a red solid occurred (61 % yield). The cyclization step was based on the cupric-catalysed cyclization of a,c-biladienes which was introduced by Johnson and co-workers97 and improved by Clezy and co-workers.98 79 88 89 Scheme 48: Synthesis of Ester Porphyrin 88 from Dipyrromethane 91 The resulting a,c-biladiene was added to a solution of copper (II) chloride in (limemylformamide which had previously been heated to 145 C, the mixture was stirred for 4 minutes and poured into water. The porphyrin copper (U) chelate formed was demetalated using 2 % sulfuric acid in trifluoroacetic acid, the acetate group re-esterified with 5 % sulfuric 80 acid in methanol and the desired ester porphyrin 88 isolated in 23 % yield following chromatographic purification The mass spectrum of 88 showed the molecular ion (m/z 522) and a fragment derived from the loss of the methoxycarbonyl moiety (m/z 463). The iH-NMR of the compound was in agreement with the structure of the expected porphyrin. In the second approach (Scheme 49), the a,c-biladiene 90 was generated by coupling the 5-bromo-5'-bromomethyl-dipyrromethene 94 with the dipyrromethene 93 using stannic chloride as a template. In this reaction, the two dipyrromethenes were dissolved in dry dichloromethane, treated with stannic chloride and allowed to stand for 2 hours at room temperature. The reaction rnixture was then quenched with aqueous hydrobromic acid (48%) and the organic phase was separated and washed with water. Methanol and hydrobromic acid were added to the organic phase and the dichloromethane was removed under reduced pressure at 25 C. The dihydrobrornide salt precipitated out when cooled and it was filtered and washed with ethyl acetate. The l-bromo-19-methyl-a,c-biladiene dihydrobrornide 90 was kept at room temperature in dimethylsulfoxide and pyridine for a few days to give the ester porphyrin in 80 % yield. It is important to point out here that the a,c-biladiene must be totally dissolved in dimethylsulfoxide before the addition of pyridine. It has been observed that undissolved starting material became useless resin on the addition of pyridine, thus lowering the yield of the required porphyrin. The reaction was normally allowed to stand in the dark for 5 to 7 days. The porphyrin usually forms a crystalline scum on the surface of the solution. The initial greenish-brown color became pinkish-purple when the oxidation to porphyrin was complete. Scheme 49: Synthesis of Ester Porphyrin 88 via Dipyrromethenes 82 In practice, both these routes were successful. At the pyrrolic and dipyrrolic stages, the yields in both routes were comparable. However, the second route (via Johnson's method) provided the ester porphyrin 88 in higher yield at the cyclization step and therefore was the method of choice for the present work. The generation of a vinyl group during a porphyrin synthesis is an operation that porphyrin chemists have faced in the past in their attempts to synthesize protoporphyrin LX and its isomers. In general, the vinyl group has been introduced by an elimination from one of the four substituent types, namely the 2-aminoethylgroup,99 the 1-hydroxyethyl group,100 the 2-hydroxyethyl group101 and the 2-chloroethyl group.102 The earlier methods such as the Hofmann elimination of a 2-aminoethyl group and the reduction of an acetyl group followed by dehydration were not high yield reactions. In pursuit of a different substituent which could be carried out through pyrrolic to porphyrin stage, Jackson and co-workers101 introduced the use of 2-hydroxyethyl group and its acetyl derivative. In the present work, a 2-hydroxyethyl group was derived from the acetate group at the porphyrin stage. Trial experiments showed that a metalation step prior to reduction was necessary for a high yield. The metalation was performed in near quantitative yield by treating the ester porphyrin with a saturated solution of zinc acetate in methanol. The progress of the reaction was monitored by UV-vis spectroscopy and its completion was indicated by the conversion of a 4-band to a 2-band spectrum. Reduction of the metalated ester porphyrin 88 with lithium aluminum hydride provided the metalated 2-hydroxyethylporphyrin. The latter was demetalated in the work-up procedure to give the 2-hydroxyemylporphyrin 146 in high yield. Since the hydroxyl group is not a very good leaving group, its conversion to a better leaving group was investigated. Treatment of the 2-hydroxyemylporphyrin with an excess of tosyl chloride in pyridine at room temperature gave the tosylate 147 . Also, with time a by-product started to form according to tic analysis. Work-up of the reaction mixture followed by flash chromatography gave the by-product and tosylate in this order of elution with dichloromethane as solvent. Attempts to drive the reaction to completion with more tosyl chloride for a longer time were unsuccessful and the best yield for this reaction was found to be 62.0 %. The tosylate was characterized by its ^ - N M R and mass spectra. The by-product gave a molecular ion peak (m/z) at 476 suggesting that it was a product resulting from the loss of the tosylate group to generate the final desired vinylporphyrin 81 (5 % yield). In fact, this observation was later confirmed by its ^ - N M R spectrum. Since the tosylate formation was slow and it was not possible to drive the reaction to completion, another option was considered. 147 R = OTs 148 R = C1 Scheme 50: Transformation of Ester Porphyrin to Vinylporphyrin 84 Towards this end, a chlorination step with thionyl chloride as reagent was chosen. This reaction has extensively been used to convert hydroxyethylpyrroles to their chloro analogs by several porphyrin groups in the past.89-103 The hydroxyethylporphyrin was therefore treated with thionyl chloride in chloroform and cUmethylfc«mamide containing potassium carbonate at room temperature. Analysis of the reaction mixture by tic after 3 hours indicated no starting material was left. However, there was one surprising observation. The chloroethylporphyrin 148 (R f= 0.65, CH2CI2) was formed with a by-product (Rf = 0.35, CH 2C1 2). Using spectroscopic data, this by-product was characterized as the ethyl ether porphyrin. A plausible explanation for its formation is the presence of ethanol as a stabilizer in chloroform. The formation of the ether was indeed prevented by replacing the chloroform with dichloromethane as solvent. The product was pure 2-cMoroethylporphyrin and the yields for different batches were always > 85.0 % as compared to the 62.0 % for the tosylate reaction. The last step in the present synthetic sequence involves the generation of the vinyl group. This transformation, a dehydrochlorination, has been achieved by earlier workers on the porphyrin zinc chelate with potassium t-butoxide as the base.101 More recently, 1,5-diazabicyclo[5.4.0]undec-5-ene (DBU) and l,5-diazabicyclo[4.3.0]non-5-ene (DBN) have been used to carry out this transformation on the metal-free porphyrin in an attempt to avoid a metalation and demetalation step.89 Clezy and co-workers, on the other hand, found that refluxing the chloroethylporphyrin in pyridine with aqueous sodium hydroxide as base provided the vinylporphyrin in high yield.1 0 4 Under these conditions, 2-cUoroethylporphyrin 147 was converted to the desired vinylporphyrin 81 in yields ranging from 85 to 90 % after chromatography. On some occassions, a small amount of the starting material was isolated as the first fraction. Such material was recycled in the next dehydrochlorination batch. The special features of the 1 H-NMR spectrum of monovinylporphyrin are the resonances due to the vinyl protons, which can be analysed as an ABX system (Fig. 2.3). The doublet of doublets centered at 8 6.15 ppm (J = 1.5, 11.5 Hz) was assigned to H a resulting from coupling to H x and to H 0 . The doublet of doublets centered at 8 6.36 ppm (J = 1.5,17.5 85 Hz) was assigned to Hb resulting from coupling with H x (a trans-relationship) and to H a . The doublet of doublets centered at 8 8.32 ppm (J = 11.5,17.5 Hz) was assigned to H x . JL 111111 j 111111111111111111 j 1111 Mjl IT111111111111111111" / • • ' • i . 4 • i . a i . o 7.1 rtm H, H . N H i _ • 11111111.1 I I 11 -2 PPM -A 7 1111111 1111 111 I|<111 1 11111 I •» I I » I I • : • t .4 t .a • ' • w i H b H ° \ ILL JU 1 I I I I I I I I ^ I I I I I I I I I J I I I I I I I I I j l l l l l ' l l l j l l ' l l l ' ' ' j ' l l l l ' l ' ' j l  1 P Pm Figure 23: The ^ - N M R Spectrum of Monovinylrorphyrin 81 86 2.7.2 Synthesis of the P-Unsubstituted-P'-monovinylporphyrin From the outset, the starting material for the benzoporphyrin program was envisioned to be the P-unsubstituted-P'-monovmylporphyrin 101. The crucial monopyrrolic precursor 105 was prepared in high yield from the pyrrole 110 via the intermediates 134 through 139 using the transformations described in the section 2.4.2 (Scheme 51). Condensation of 105 with the 5-unsubstirated-2-formylpyrrole 97 in the presence of aqueous hydrobromic acid gave the 5'-unsubstituted-5-memyl-2,2l-dipyrromethene 104 in 75% yield. Coupling of 104 with 5'-bromomethyl-5-bromo-2,2'-dipyrromethene 94 using anhydrous stannic chloride as the catalyst produced the corresponding l-bromo-19-methyl-a,c-biladiene 103 in 90 % yield. The reaction time of the condensation was reduced to 60 minutes to prevent any reaction occurring at the P-H position in dipyrromethene 104. Interestingly, when the a,c-biladiene was analyzed, no by-product resulting from the condensation of 94 with 104 at the P-position was observed. This may be due to the greater steric bulk of the methoxycarbonylmethyl group or the reduction of electron density in the pyrrolic ring by the methoxycarbonylmethyl group, thus discouraging condensation at the P-H position. The notable feature in the mass spectrum of the a,c-biladiene 103 was the absence of the parent peak but a base peak at m/z 508 which resulted from cyclisation of 103 in the mass spectrometer giving the resulting porphyrin 102. The transformation of the a,c-biladiene 103 to the ester porphyrin 102 was carried out in (iimethylsulfoxide-pyridine with a product yield of 83%. Metalation of 102 with zinc, reduction of the acetate substituent (LiAUiO, followed by demetalation (trifluoroacetic acid) gave the hydroxyemylporphyrin 149 in 95 % yield. The latter was subsequendy converted to the chloroethyl derivative 150 in 93 % yield, using thionyl chloride. Treatment of 150 with sodium hydroxide in pyridine-water produced the desired mono-vinyl porphyrin 101 in 88 % yield. Scheme 51: Synthetic Route to the P-Unsubstituted-P'-monovinylporphyrin 101 88 The salient features of the ^ -NMR spectrum of 101 are the resonances due to the vinyl protons which can be analyzed as an ABX system. The doublet centered at 8 6.40 ppm (J = 12.0 Hz) was assigned to H a resulting from the coupling to H x , the doublet centered at 8 6.63 ppm (J =17.0 Hz) was attributed to Hb resulting from coupling to H x , and the doublet of doublets centered at 5 8.48 ppm (J = 12.0 and 17.0 Hz) is due to H x . A coupling between H a and Hb was not observed. The P-H proton, being aromatic in character (on the pyrrole carrying the vinyl group) was assigned to the singlet at 8 9.44 ppm which integrated to one proton. Its electronic spectrum as shown in Fig.2.4 exhibited a rhodo-type pattern (^ max CH2CI2 = 402, 504, 544, 572, 630 nm) and this observation differed from that reported by Djerassi et al. who synthesized the only previously reported P-unsubstituted-P'-monovinylporphyrin in their attempt to study magnetic circular dichroism of demethylmonosubstituted porphyrins with different substituents.84 1-CH 0.0 H 1 1 1 1 1 1 1 1 300 350 400 450 500 550 600 650 700 Wavelength (nm) Figure 2.4: The Electronic Spectrum of Monovinylporphyrin 101 89 2.7.3 Synthesis of the A,C-Divinylporphyrin As mentioned earlier, attention was then focused on the synthesis of the third target molecule (a bacteriochlorin) from an A,C-m'vinylporphyrin. To keep the number of pyrrolic intermediates involved in this program to a minimum, a symmetrical A,C-bUvmylporphyrin 106 was considered as the required vinyl precursor. Because of the symmetry of the molecule, the self-condensation of the dipyrromethene 93 was considered as a convenient route to the macrocycle (Scheme 52). In this first approach, the dipyrromethene 93 was refluxed in anhydrous formic acid with two equivalents of bromine for 2.5 hours. To naaximize the yield of this reaction, the condenser was removed and the reaction mixture was boiled to dryness. Some hydrolysis of the acetic ester side-chain was unavoidable because of extended heating. To repair the ester groups, the product was treated with methanol containing trimethylorthoformate and sulfuric acid. Work-up of the reaction mixture followed by chromatography on silica gel gave the di-ester porphyrin 108 in 24 % yield. Clearly, the function of the bromine in this case was two-fold. First, it brought about bromination at the cc-free position and second, it oxidized the cc-methyl group to an a-bromomethyl group. The fact that the yield was improved only when the solvent was boiled off suggested that oxygen was involved in the last stage probably through the oxidation of a porphodimethene to the porphyrin. The structure of the di-ester porphyrin was confirmed by mass spectroscopy (m/z 566) and its iH-NMR data. The latter indicated the high degree of symmetry of the molecule with one singlet for the two methyl ester protons, two singlets for the two types of ring methyl groups and two singlets for the four meso protons. 90 Scheme 52: Synthesis of A,C-I^vmylporphyrin 106 via Fischer's Approach Metalation with zinc acetate followed by reduction with LLAIH4 provided the bis(2-hy(hx)xyemyl)rx)rphyrin 151 in 64 % yield. This low yield was due to the low solubility of the 91 di-ester porphyrin in tetrahydrofuran in the reduction step. Chlorination of the latter with thionyl chloride went smoothly to give the bis(2-chloroethyl)porphyrin 152 in 85 % yield. Dehydrochlorination with 10% aqueous sodium hydroxide in pyridine provided the desired A,C-divinylporphyrin 106 in 84 % yield after purification by chromatography. The structure of 106 was confirmed by its -fl-NMR spectrum. Both vinyl groups in this symmetrical porphyrin were magnetically equivalent and they were analysed as an ABX system as before. The doublet of doublets at 8 6.15 ppm was assigned to H a . The doublet of doublets centered at 8 6.34 ppm was assigned to H D . The doublet of doublets centered at 8 8.32 ppm was assigned to H x . The coupling data found here agreed with the information already gained for the other two vinylporphyrins. At this point, it was felt that the low 24 % yield of di-ester porphyrin obtained by employing Fischer's method was unsatisfactory and should be improved upon by pursuing an alternative approach. Results from the synthesis of the previous monovinylporphyrins have already attested to the efficiency of Johnson's method. As before, the key intermediate in this method was a 5-bromo-5'-bromomethyl-2,2'-dipyrromethene hydrobromide. Accordingly, the bromination of 93 to give the dibrorninated derivative was carried out using the conditions developed in our group. This bromination step turned out to be a slow and was incomplete even after 2 weeks. To circumvent this rather annoying problem, the ester group was reduced and acetylated at the pyrrolic stage. The resulting dipyrromethene 144 (corresponding to 93) was then subjected to the same bromination conditions. In this case, the reaction went to completion to give 145 in a yield of 81 % (Scheme 53). The two dipyrromethenes 144 and 145 reacted smoothly in the presence of stannic chloride to give the l-bromo-19-methyl-a,c-biladiene dihydrobrornide as its tin complex. Treatment with methanolic hydrogen bromide gave the a,c-biladiene dihydrobrornide 153 in 80 % yield. 92 Scheme 53: Synthesis of A,C-Divinylporphyrin 106 via Johnson's method. Cyclization to the porphyrin in pyridine-dimethylsulfoxide gave the bis(2-acetoxyemyl)porphyrin 154 in 85% yield. In some batches, dc analysis indicated the presence of three separable porphyrins. The most polar product had both the acetyl groups removed, the 93 one of intermediate polarity had only one removed and the least polar product (the major component) was the desired bis(2-acetoxyethyl)porphyrin. Chromatography of a small amount of this reaction mixture on silica gel gave a pure sample of the bis(2-acetoxyethyl)porphyrin for characterization. In the conversion of the acetoxyethyl side-chains to the vinyl groups, the acetoxyl protecting groups were first removed with methanolic sulfuric acid (98% yield) and the resulting bis(2-hydroxyethyl)porphyrin 151 was transformed to the bis(2-chloroethyl)porphyrin (85 % yield) by treatment with thionyl chloride at room temperature. The final step involving the dehydrohalogenation was carried out as in the monovinylporphyrin case. It was characterized by a slighdy rhodo-type spectrum (ratio of bands LTI / IV > 1). This observation was used as a tool to monitor the dehydrohalogenation reaction since the dichloro-and ^vinylporphyrins ran with the same Rf on tic in different developing systems. l . O - i O co D CD O 0 . 2 0 . 4 -0 . 8 -0 . 6 -0 . 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 W a v e l e n g t h ( n m ) Figure 2.5: The Electronic Spectrum of A,C-Divinylporphyrin 106 94 2.8 Synthesis of Chlorin Derivatives From the retrosynthetic perspective, it was anticipated that the acetic and propionic side-chains of factor 1 could be derived from a retro-claisen reaction on chlorin 87 (Scheme 30). Therefore, it was envisioned that a Diels-Alder reaction between the p-methyl-P'-monovinylporphyrin 81 and an a,P-unsaturated sulfone would provide a rapid entry into the required carbon skeleton of the molecule. The choice of a,P-unsaturated sulfones was based on the knowledge that (a) phenyl vinyl sulfone can serve as a convenient ethylene equivalent in [4+2] additions, (b) oc,P-unsaturated sulfones undergo [4+2] addition with moderately reactive dienes showing high regiochemical control, (c) the phenylsulfonyl group can be removed reductively, oxidatively or by the use of a base. With this as background, P-phenylsulfonylacrylate and P-phenylsulphonylacrylonitrile were prepared and their reactions with monovinylporphyrin 81 were investigated. The preparation of a,P-unsaturated sulfones have been the subject of several patents because of their ability to exhibit antibacterial and antifungal activities. A survey of the literature indicated the following methods as routes towards their preparations: (1) reaction of sulfinic acids or their salts with a,P-dihalogenated carbonyl precursors.105 (2) reaction of sulfinic acids or their salts with cc-halogenated carbonyl precursors.106 (3) addition of sulfinic acids or their salts to alkyl propiolates.107 (4) oxidation of the corresponding P-arylthio analogs.108 (5) nucleophilic substitution of p-chlorovinyl ketones.109 (6) sulfonylmercuriation of a,p-unsaturated carbonyl precursors.110 (7) Iodosulfonyktion-Dehychioiodination reaction of ot,P-unsaturated carbonyl precursors.111 95 2.8.1 Synthesis of p-PhenylsuIfonylacrylate Ester Preparation of P-phenylsulfonylacrylate in the present work followed the method 1. Methyl 2,3-dichloro-propionate (155), instead of methyl 2,3-dibromo-propionate, was treated with the sodium salt of the benzene sulfinic acid in the presence of sodium acetate in aqueous methanol. Tic monitoring of the reaction indicated only one product from the reaction mixture. Work-up of the reaction gave 156 as white crystals in 81% yield. CH3O2C Hp C H 2 CH—C0 2 Me CH3C02-Na+ C 1 Q C6H5S02-Na+ H S02Ph 155 a 156 2 Scheme 54: Synthesis of (E)-P-Phenylsulfonylacrylate Ester The 1 H-NMR spectrum of 156 showed a singlet at 8 3.80 ppm, assignable to the methyl ester protons. The doublets at 8 6.85 ppm and 7.38 ppm with a coupling constant of close to 16.0 Hz indicated unambiguously a trans-relationship between the two olefinic protons. The aromatic protons appeared as a multiplet in the 8 7.60 to 8.00 ppm region. These data are in total agreement with the reported values.112 2.8.2 Synthesis of p-Phenylsulfonylacrylonitrile P-phenylsulfonylacrylonitrile (158) was prepared according to the method reported by Mikhailova and co-workers as shown in Scheme 55.1 1 3 They have shown that in the presence of an equimolar amount of boric or acetic acid the sodium salt of benzene sulfinic acid reacts 96 with a-chloroacrylonitrile (157) to give (E)-|3-phenylsulfonylacrylonitrile exclusively in moderate yield. When this reaction was repeated with a 20 % excess of a-chloroacrylonitrile in the presence of catalytic amounts of hydroquinone, an additional minor product was obtained. Work-up of the reaction mixture was modified as follows. The mixture was cooled, filtered and washed with water to give a white solid. Tic of this first crop indicated the presence of only one compound (E-isomer). The filtrate was extracted with dichloromethane and the organic phase was evaporated to dryness to provide a second crop. Tic of this second crop revealed the major compound (E-isomer) and the minor compound. Further separation on a silica gel column with dichloromethane as solvent provided the two pure products. H C N C6H5S02-Na+ N C C H C H S Q 2 P h H3BO3 H . C I E:Z ratio = 85:15 157 158 S c h e m e 55: Synthesis of (E)-and (Z)-Phenylsulfonylacrylonitrile The mass spectra of both compounds gave a molecular ion peak at m/z 193 suggesting that they are geometrical isomers. The ^ - N M R spectrum of the major product revealed two doublets with coupling constants of 15.6 Hz while the minor product revealed two doublets with coupling constants of 10.2 Hz (Table 2.1). The infrared spectra of the two products provided further confirmation of geometrical isomerism between them. The (E)-isomer exhibited a band at 944 cm - 1 while the (Z)-isomer exhibited a band at 657 cm - 1 for the C-H out of plane bending mode. The E : Z ratio of the reaction was ctetennined from the yields to be 85:15 and the overall yield was calculated as 80 %. It is interesting to note that the (Z)-isomer has never been reported in literature. It is therefore worthwhile to examine its reactivity in Diels-Alder reactions and compare its properties to the (E)-isomer. 97 Table 2.1: Chemical Shifts 8 (ppm) and Coupling Constants J (Hz) in CDC13 for (E)-and (Z)-P-Phenylsulfonylacrylonitrile (E) (Z) Compound 8 (Ha) 8 (Hp) J (Ho,Hp) (E)-isomer 6.55 7.25 15.6 (Z)-isomer 6.75 7.77 10.2 2.8.3 Diels-Alder Reaction of (E)-|3-PhenylsulfonyIacrylate with Mono-vinylporphyrin 81 Monovinylporphyrin 81 was reacted with excess (E)-P-phenylsulfonylacrylate in degassed toluene at HO'C for 3 days in a sealed tube. Tic of the crude reaction mixture revealed the formation of two products. Rash chromatographic separation on a silica gel column with dichloromethane as eluant gave the excess dienophile in the forerunning fractions followed by the two products which were not separable. Further purification and separation of the crude products on a chromatotron (1mm silica gel plate) with dichloromethane as eluant gave two products. The faster moving product with an Rf = 0.42 was named DAEST1. It was followed by a second product (Rf = 0.40) named DAEST2. Each compound gave a mass spectrum with a molecular ion peak at m/z 702 and each exhibited an electronic spectrum 98 characteristic of a chlorin macrocycle. After work-up the two products were obtained in a 40 % overall yield of an essentially 1:1 mixture. When the reaction was stopped after one day, only traces of the cycloadducts (UV-vis analysis of mixture) were observed. Longer reaction times (up to 1 week at 110'C) led to lower yield of the products due to the decomposition of the cycloadducts. In principle, 4 isomers could be expected from such a Diels-Alder reaction, each of these isomers being racemic (only one enantiomer is depicted in Fig. 2.6) because the dienophile can approach from either face of the diene. Obviously, the stereochemistry and regiochemistry of this reaction is not a simple problem to solve. No information on such compounds is present in the literature for comparison purposes and therefore a vigorous detailed spectroscopic analysis of the two cycloadducts were necessary in an attempt to assign their stereochemistries. The JH-NMR spectrum of isomer DAEST1 (Fig. 2.7) exhibited multiplets at 8 3.05 and 3.32 ppm due to the diastereotopic H D and He. A partially obscured one proton multiplet at 8 4.00 ppm was assigned to Hd- The doublet at 8 4.81 ppm (J = 7.0 Hz) was unambiguously assigned as He (proton at C-21). The doublet of doublets at 8 7.41 ppm (J = 2.0, 8.0 Hz) was assigned to H a (olefinic proton at C-24). The assignment of the chemical shifts of the protons in the cyclohexene ring was supported by a series of decoupling experiments. Irradiation of the signal at 8 3.05 ppm (H D or He) simplified the signal at 8 3.32 ppm (the geminal proton) and that at 8 4.00 ppm (Hd). In the same experiment, the signal at 7.41 ppm was also reduced to a doublet. Similarly, when the signal at 8 3.32 ppm was irradiated, the doublet of doublets at 8 7.41 ppm was reduced to a doublet (J = 8.0 Hz), the multiplet at 8 3.05 ppm was reduced to a doublet of doublets (J = 8.0,12.0 Hz) and the signal at 8 4.00 ppm was simplified. Irradiation of the multiplet at 8 4.00 ppm simplified the multiplets at 8 3.05 and 3.32 ppm while the doublet at 8 4.81 ppm was reduced to a singlet Irradiation of the signal at 8 7.41 ppm reduced both multiplets at 8 3.05 and 3.32 ppm to doublets of doublets. 99 159-A 159-B 159-C 159-D * All compounds are racemic; only one enantiomer is portrayed. Figure 2.6: Diels-Alder Reaction of 81 with (E)-pVPhenylsulfonylacrylate Figure 2.7: The -H-NMR Spectrum of Cycloadduct DAEST1 101 The most striking feature of this ^ - N M R spectrum (Fig. 2.7) was the upfield shift of the methyl ester protons to 8 2.54 ppm compared to its normal resonating position at around 8 3.6 ppm. This observation suggested that the methyl ester protons are very well shielded by the chlorin rc-system and this information was useful as an extra piece of evidence for the assignment of the stereochemistry of the adduct. Difference nuclear Overhauser effect (DNOE) experiments were also performed to deduce the stereochemistry of the adduct. The data obtained from the DNOE experiments on DAEST1 are summarized in Table 2.2. Table 2.2: Results of Difference n.O.e Experiments on DAEST1 Irradiated proton(s) Observed (+) n.O.e 8 (ppm) 8 (ppm) 2.05 (ang.-CH3) 4.81 (He); 3.32(Hb) 3.05 (He) 3.32(Hb); 4.00 (Hd); 7.41 (Ha) 3.32 (Hb) 2.05(ang.-CH3); 3.05 (He); 4.81 (He) 4.81 (He) 2.05 (ang.-CH3) 7.41 (Ha) 3.05 (He) Irradiation of the angular methyl group at 8 2.05 ppm led to a large n.O.e at 8 4.81 ppm (He) and a small n.O.e at 8 3.32 ppm (Hb). This suggested that Hb resonates more downfield that He. Irradiation of He at 8 3.05 ppm led to a n.O.e at 8 3.32 ppm (Hb, the geminal proton to He) and at 8 4.00 ppm (Hd, the proton attached to C-22). Irradiation of signal at 8 3.32 ppm (Hb) led to a n.O.e at 8 2.05 (ang.-CH3), 3.05 (He), and 4.81 ppm (He, the doublet at C-21). Irradiation of He at 8 4.81 ppm led to a n.O.e at 2.05 ppm (ang.-CH3). Irradiation of Ha at 8 7.41 ppm led to a n.O.e at 8 3.05 ppm (He). These data are consistent with the structures 159-B and 159-D (see Fig. 2.6). These two stereoisomers possess exactly the same proton 102 arrangement in space and differ only in the position of the ester and phenylsulfonyl groups. However, only structure 159-D permits the methyl group of the ester to reside under the chlorin rc-system thus causing its resonance (singlet) to occur relatively upfield at 8 2.54 ppm (a shielding effect). On the basis of these considerations, it was concluded that the methyl ester was attached to C-21 and the phenylsulfonyl was attached to C-22. Figure 2.8: Information on DAEST1 from Difference n.O.e Experiments A study of Dreiding models suggested that the only likely conformations for 159-D were the distorted half-chair and the distorted boat Both conformations allow the methyl ester group to sit under the 7C-system of the macrocycle. However, examination of the coupling constants between H a and Hb (J = 2.0 Hz) and between H a and He (J = 8.0 Hz) favored the boat conformation in which the dihedral angle between H a and Hb is close to 115* and that between and He is close to 0 \ 1 1 4 In the distorted half-chair, those two coupling constants are expected to be approximately equal as judged by their dihedral angles in the model. The unusual low field resonance of He (a proton attached to a carbon carrying an ester) was also 103 accounted for by the model study. In the proposed boat conformation, this proton lies nearly in the same plane as the chlorin macrocycle (towards the adjacent meso proton). Therefore, it experiences a deshielding effect (just like a meso proton), thus explaining the observed anomaly. One would assume that a proton attached to a carbon carrying the phenylsulfonyl group would resonate at lower field than a carbon carrying an ester group.115 Under such circumstances, the low field doublet at 8 4.81 ppm would have been assigned to the carbon carrying the phenylsulfonyl and the assignment of the regio- and stereochemistries would have been reversed. Obviously, in the present case, the anisotropy effects associated with the macrocycle 7t-system outweighed the effect due to electronegativities. Being aware of the fact that the anisotropic contribution to the observed ^ - N M R spectra of porphyrins or chlorins is relatively large compared to the usual observed range of chemical shifts (ca. 14.0 ppm), it was hoped that the 1 3 C-NMR spectroscopy may help to confirm the above assignments. The 1 3C-NMR spectra of these macrocycles have an observed chemical shift range ca. 200 ppm and are only affected by a few percent in the position of the peak as a result of the anisotropic effect.116 Three different 1 3 C-NMR experiments were run on DAEST1 in 10 % TFA-CDCI3 to clarify and remove any ambiguity regarding the regiochemistry. A broad-band decoupled spectrum , assisted with an attached proton test (APT) spectrum, enabled assignments of the carbons in the molecule (Fig. 2.9). The peaks resonating in the region below 8 30.00 ppm were assigned to the carbons of the methyl and methylene groups of the molecule. The peaks in the range 8 90.0 to 105.0 ppm were assigned to the meso carbons.while those in the range 8 130.0 to 160.0 ppm were assigned to the a and P skeletal carbon atoms and to the carbon atoms of the phenyl group. The singlet at ca. 8 173.0 ppm was assigned to the carbonyl carbon of the methyl ester. The only other carbon atoms left to be assigned were observed in the 8 30.0 to 70.0 ppm region which turned out to be the area of interest. 105 Based on the hybridization and the electronegativities of the substituents, these peaks were assigned as follows. The quarternary C-2 carbon (a sp3 carbon) and the methyl ester carbon were assigned to resonances at 8 52.57 and 60.22 ppm respectively. The remaining two carbon atoms bearing the phenylsulfonyl and methyl ester groups were assigned to the signals at 8 52.57 ppm (this position is responsible for two different carbon resonances, see APT experiment) and 48.85 ppm respectively. In the next 1 3 C-NMR experiment, selective irradiation of an unambiguously assigned proton was carried out to permit a direct correlation of a specific proton resonance to a specific carbon resonance. Therefore, when the isolated doublet (He) at 8 4.81 ppm was selectively irradiated, the peak at 8 48.85 ppm showed a n.O.e and appeared as a singlet while the peak at 8 52.57 ppm appeared as a triplet without any n.O.e. In addition, when the proton H4 at 8 4.00 ppm was selectively irradiated the peak at 8 48.85 ppm appeared as a doublet and that at 8 52.57 ppm appeared as a singlet with a n.O.e. These observations confirmed that proton He and the methyl ester group are indeed attached to C-21 thus supporting the regiochemistry assigned to DAEST1 based on the iH-NMR data. DAEST2 was analyzed in the same way as DAEST1. The proton spectrum of DAEST2 (Fig. 2.10) exhibited a complex multiplet at 8 3.34 ppm which integrated to two protons, assignable to the two diastereotopic methylene protons (Hb and He). A doublet at 8 3.66 ppm (J = 12.0 Hz) was unambiguously assigned to He, while the multiplet at 8 4.74 ppm was assigned to The olefinic proton H a appeared at 8 7.07 ppm as a triplet. The assignment of the chemical shifts of these protons was also supported by a series of decoupling experiments. Thus, irradiation of signal at 8 3.34 ppm (Hb and He) led to the collapse of the triplet at 8 7.07 ppm to a singlet In the same decoupling experiment, the multiplet at 8 4.74 ppm was reduced to a doublet (J = 12.0 Hz). This observed coupling constant is indicative of a trans-relationship between H4 and He and that the dihedral angle between these two protons is close to 180°. 107 When the doublet at 8 3.65 ppm (He) was irradiated, the multiplet at 8 4.74 ppm collapsed to a doublet of doublets (J = 6.0, 10.0 Hz). Similarly, when the multiplet at 8 4.74 ppm (Hd) was decoupled the doublet at 8 3.65 ppm collapsed to a singlet while the multiplet at 8 3.34 ppm underwent simplification. Difference nuclear Overhauser effect experiments were also used in this case to deduce the stereo- and regiochemistry of the adduct. Irradiation of the angular methyl group at 8 1.98 ppm led to an n.O.e at 8 4.74 ppm (Hd). This information suggested that Hd has a 1,3 diaxial relationship with the angular methyl group. A look at the Fig. 2.6 indicated that only two of the four possible structures provide this kind of arrangement, namely structures 159-A and 159-C. Table 2.3. reports all the difference n.O.e data obtained from irradiating the various protons in the six-membered ring. Table 2.3: Results of Difference n.O.e Experiments on DAEST2 Irradiated proton(s) 8 (ppm) Observed (+) n.O.e 8 (ppm) 1.98 (ang.-CH3) 4.74 (Hd) 3.34 (Hb and He) 1.98 (ang.-CH3); 3.66 (He); 7.07 (Ha) 3.66 (He) 3.34 (He) 4.74 (Hd) 1.98 (ang.-CH3) 7.07 (Ha) 3.34 (Hb) These observed data are consistent with the structure 159-C only. In structure 159-C, both the phenylsulfonyl and the methyl ester groups reside in equatorial positions. The methyl 108 ester group in this position is not at all affected by the anisotropy of the cMorin 7t-system. The observation of a n.0.e between H4 and the angular methyl group coupled with an observed n.O.e between He and He suggest a trans-1,3-diaxial relationship between these pairs of protons. Taking into account these facts, the structure 159-C was assigned the half chair conformation (Fig. 2.11). Figure 2.11: Information on DAEST2 from Difference n.O.e experiments The 1 3 C-NMR spectrum of DAEST2 was quite similar to that of DAEST1. When the isolated multiplet at 8 4.74 ppm (Hd) was selectively decoupled, the new 1 3 C - N M R spectrum showed a n.O.e at 53.18 ppm (carbon atom to which the phenylsulfonyl group was assigned) This observation confirmed that Hd was indeed attached to the same carbon atom (C-22) as the phenylsulfonyl group and that the assigned stereochemistry provided by ^ - N M R analysis based on coupling constants, chemical shifts and difference n.O.e experiments were correct. From these observations it was concluded that the Diels-Alder reaction with (E)-|3-phenylsulfonylacrylate ester went with high regioselectivity. However, there was essentially no corresponding "endo" stereoselectivity (relative to phenyl sulfonyl group). 109 2.8.4 Diels-Alder Reaction of (E)-|3-Phenylsulfonylacrylonitrile with Mono-vinylporphyrin 81 The Diels-Alder reaction of the monovmylrx>rphyrin 81 with (E)-p%phenylsulfonyl acrylonitrile was also carried out in toluene at 110°C for 3 days in a sealed tube. The crude product was purified by flash chromatrography (silica gel, dichloromethane) to give a fraction that had a strong visible absorption in the 656 nm region. This fraction was further purified on the chromatotron to give the only chlorin product 160 in a 60% yield. The reaction therefore went in a regio- and stereo-controlled manner. The obvious question to ask is which group really controlled the direction of addition. To answer this question, the !H-NMR (Fig. 2.13) and 1 3 C-NMR (Fig. 2.15) spectra of the product were analysed as in the previous case. The olefinic proton (Ha) gave a doublet of doublets at 8 7.15 ppm (J = 4.5, 6.0 Hz), the two methylene protons (Hb, He) appeared as multiplets at 8 3.40 and 3.60 ppm, the methine proton (Hd) at C-22 appeared as a multiplet at 8 4.38 ppm, and the methine proton (He) at C-21 was assigned to the doublet at 8 3.87 ppm (J = 11.0 Hz). All of these assignments were confirmed by decoupling experiments. The irradiation of each proton simplified the spectrum in the manner expected. Irradiation of the signal at 8 7.15 ppm collapsed the multiplets at 8 3.40 and 3.60 ppm into doublet of doublets. These doublets had different coupling constant values. The doublets at 8 3.40 ppm (He) had 10.0 and 18.0 Hz as coupling constants while the doublets at 8 3.60 ppm had 3.0 and 18.0 Hz as values. Irradiation of the signal at 8 4.38 ppm (Hd) collapsed the doublet at 8 3.87 (He) ppm into a singlet, while the multiplets at 8 3.40 and 3.60 ppm were again reduced to doublet of doublets respectively. The magnitude of the coupling constant between the two methine protons (Hd and He), which is 11.0 Hz, suggests that these two protons are in an antiperiplanar arrangement. 160-A 160-B 160-C 160-D * All compounds are racemic; only one enanuomer is portrayed. Figure 2.12: Diels-Alder Reaction of 81 with (E>P-Phenylsulfonylacrylomrrile Figure 2.13: The 'H-NMR Spectrum of Cycloadduct 160 112 Difference n.O.e. experiments on the different protons provided more evidence for the stereochemistry of the adduct. Table 2.4: Results of Difference n.O.e Experiments on 160 Irradiated proton(s) Observed (+) n.O.e 8 (ppm>; 8 (ppm) 2.08 (ang.-CH3) 4.38 (Hd) 3.87 (He) 3.40 (He) 4.38 (Hd) 2.08 (ang.-CH3) 7.15 (Ha) 3.60 (Hb) Thus, irradiation of the angular methyl group at 8 2.08 ppm led to a large n.O.e at 8 4.38 ppm (Hd). This information narrowed down the structure of the adduct to stereoisomers 160-A and 160-C only (Fig.2.12). Figure 2.14: Information on 160 from Difference n.O.e Experiments 113 The attached proton test spectrum of the adduct 160 is shown in Fig. 2.15. The carbon atoms carrying the phenylsulfonyl and the cyano groups were assigned at 58.6 ppm and 35.5 ppm respectively. When the isolated proton at 4.38 ppm (Hd) was selectively irradiated, the signal at 58.6 ppm appeared as a singlet with a n.O.e. On the other hand, the carbon to which the cyano group was attached appeared as a doublet without any n.O.e. This result confirmed that proton Hd is attached to the same carbon atom as the phenylsulfonyl group and therefore assigns the adduct to structure 160-C. In addition, all of the coupling constants measured suggested that the proposed stereostructure conforms to the observed data. The behavior of (E^-|3-phenylsulfonylacrylonitrile relative to (E)-j3-phenylsulfonyl acrylate is very interesting. The comparison of these two dienophiles leads to the following conclusions: 1. The (E)-P-phenylsulfonylacrylonitrile is a more reactive dienophile under the present condition with this diene (higher yield). The greater dienophilic reactivity of (E)-J3-phenylsulfonylacrylonitrile over (E)-|3-phenylsulfonyl acrylate can be rationalized in terms of the electronegativity of the group at the a-position to the phenylsulfone group. (Cyano > carbonyl ester). 2. The (E)-p^phenylsulfonylacrylonitrile undergoes the Diels-Alder reaction regio- and stereoselectively, while (E)-p-phenylsulfonylacrylate undergoes the Diels-Alder only regio-selectively. The high endo-selectivity (relative to the sulfonyl group) observed in the case of the acrylonitrile suggests that the phenylsulfonyl group exhibits complete control over the cyano group in directing the stereoselectivity. This suggests that the phenylsulfonyl group exhibits a greater tendency for secondary orbital overlap than the cyano group. On the other hand, the phenylsulfonyl group in the used acrylate experiences competition from the carbonyl group for secondary orbital interactions and this explains the lack of stereoselectivity. J L ! T CM I U CM u r rj T - I T rr :• r r 11 t r i i y r i r i p i i T p i i 1 ] i • i : > i : • > • i i I T I i-ri I ' : i 1 8 0 1 6 0 1 4 0 1 2 0 1 0 0 BO : i r r i I n i I i i i i T I i r ! r rr I n r r n r n p r n n 6 0 4 0 2 0 P P M 6 Figure 2.15: The Attached Proton Test Spectrum of 160-C 115 2.8.5 Diels-Alder Reaction of (Z)-(3-PhenylsuIfonylacrylonitrile with Mono-vinylporphyrin 81 The Diels-Alder reaction of (Z)-pVphenylsulfonylacrylorntrile with monovinylporphyrin 81 was carried out under exactly the same conditions as with its geometrical isomer. Purification of the crude reaction mixture by flash chromatography on a silica gel column, followed by further purification on the chromatotron (1mm silica gel plate), gave a fraction which absorbed strongly at 656 nm as the major product 161 in 40% yield. The excess dienophile recovered from chromatography was shown to be still the (Z)-isomer, suggesting that no cis-trans isomerization was promoted at 110°C. The assignment of the stereochemistry was again based on the !H-NMR and 1 3C-NMR data. The ]H-NMR spectrum of the compound displayed the diagnostic doublet of doublets at 8 7.02 ppm (J = 3.0, 7.0 Hz) for H a . Signals at 8 2.85 and 3.25 ppm are characteristic for the diastereotopic methylene protons (Hb, He) of the adduct. The multiplet at 8 3.50 ppm was assigned to Hd- The doublet at 8 4.18 ppm (J = 9.0 Hz) was assigned to He, the proton attached to C-2 1.All of these assignments were confirmed by decoupling experiments. Irradiation of each proton simplified the spectrum in the expected manner. For example, when the doublet at 8 4.18 ppm (He) was irradiated, the multiplet at 8 3.50 ppm was simplified, thus confirming that Hd was assigned correctly. Irradiation of the multiplet at 8 2.85 ppm simplified the doublet of doublets at 8 7.02 ppm into a doublet (J = 3.0 Hz) and the multiplet at 8 3.25 ppm was reduced to a doublet of doublets. 116 161-A 161-B 161-C 161-D * All compounds are racemic; only one enantiomer is portrayed. Figure 2.16: Diels-Alder Reaction of 81 with (Z)-p-Phenylsulfonylacrylonitrile Figure 2.17: The !H-NMR Spectrum of Cycloadduct 161 118 Further information to help in the assignment of the stereochemistry was provided by difference n.O.e experiments. Table 2.5 displays the results observed when the different protons in the cyclohexene ring of the cycloadduct were irradiated. No enhancement was observed between the angular methyl group and He (the doublet). This observation immediately excluded structures 161-A and 161-C from Fig. 2.16. The two remaining structures 161-B and 161-D differ only in the position of the cyano and phenylsulfonyl groups. Table 2.5: Results of Difference n.O.e Experiments on 161 Irradiated proton(s) 8 (ppm) Observed (+) n.O.e 8 (ppm) . 2.08 (ang.-CH3) 3.25 (Hb) 2.85 (He) 3.25 (Hb) 3.25 (Hb) 2.85 (He); 2.08 (ang.-CH3) 3.50 (Hd) 4.18 (He) 4.18 (He) 3.50 (Hd) 7.02 (Ha) 2.85 (He) To choose between these two structures, the isolated doublet at 8 4.18 ppm (He) was selectively irradiated in a 1 3 C experiment The observation of a singlet at 34.64 ppm (assigned as the carbon carrying the cyano group) with an n.O.e. in the resulting 1 3 C spectrum, suggested that He is attached to the carbon carrying the cyano group. Taking all of these data into account, 161 was assigned to structure 161-D. Figure 2.18: The Attached Proton Test Spectrum of 161-D 120 Figure 2.19: Information on 161-D from Difference n.O.e experiments The most direct information regarding the conformation of 161-D was provided by the measured vicinal coupling constants. Molecular models indicate that according to the Karplus relationship, J (Ha, Hb) would be larger than J (Ha, He) in a distorted boat conformation. The observed values of J (H a , Hb) = 7.0 Hz, J (Ha, He) = 3.0 Hz, and J (Hd, He) = 9.0 Hz are consistent with a distorted boat conformation for the adduct With (E)- and (Z)-|3-phenylsulfonylacrylonitrile, the regiochernistries of the cyclo-adducts were also confirmed via a chemical approach. Both adducts were heated separately in benzene containing DBU and DDQ. The aromatization of both adducts provided a benzoporphyrin in around 20% yield in each case. After purification by chromatography on a silica gel column, !H-NMR spectroscopy indicated that the two resulting benzoporphyrins were identical. The notable feature of the spectrum was the presence of two doublets at 8 8.40, 9.60 ppm and a triplet at 8 8.13 ppm suggesting the benzoporphyrin 162 has the cyano group 121 at C-21. The findings here were in agreement with the proposed structures assigned to these adducts based on !H-NMR and 1 3 C data. 162 In conclusion, the reaction of monovinylporphyrin 81 with both (E)-and (Z)-(P)-phenylsulfonylacrylonitriles proceeded with high regiochemical and stereochemical control. In both cases, the sole isomer had the angular methyl and cyano groups vicinal to each other. However, the stereoselectivity of the reaction with the (Z)-isomer turned out to be the exact opposite (with respect to the phenylsulfonyl group) of the reaction with the (E)-isomer. This observation suggests interesting aspects concerning endo-exo selectivity (with respect to the phenylsulfonyl group). With the (E)-isomer, the adduct arose from a transition state in which the phenylsulfonyl group of the dienophile and the diene are endo-oriented (Fig. 2.20). On the other hand, with the (Z)-isomer, the adduct arose from a transition state in which the phenylsulfonyl group of the dienophile and the diene are exo-oriented (Fig. 2.20). From these results, it appears that steric effects involved in the transition states may be playing a bigger role than electronic effects. For example, with the (Z)-isomer, the dienophile prefers to approach the diene with both the cyano and phenylsulfonyl groups away from the diene,thus, experiencing lesser steric interactions. The higher reactivity of the (E)-isomer over the (Z)-isomer most likely is the result of the secondary orbital interactions between the phenylsulfonyl group and the 7t-system of the diene during the transition state. Thus, the 122 present investigation showed that both P-phenylsulfonylacrylonitrile and |3-phenylsulfonylacrylate undergo cycloaddition regioselectively and therefore can find more o synthetic utility in the preparation of cyclohexene derivatives regioselectively. It is interesting to note that there are no systematic studies reported on the effect of the geometry of an unsymmetrically disubstituted dienophile with an unsymmetrical diene. It was therefore not possible to compare the present observation with related examples in the literature. However, there are two reports that need mention in relation to this study. Gandolfi and co-workers have studied the 1,3-dipolar cycloadditions of cyclic nitrones to (E)- and (Z)-p-nitrostyrenes and have also found a reversal (with respect to one substituent only) of the stereoselectivity in their system.117 With the (E)-isomer, their reaction gave a major product resulting from an endo transition state with respect to the nitro groups. On the other hand, with the (Z)-isomer, they obtained a cycloadduct resulting from an exo transition state relative to the nitro group. In a different recent communication, Parsons and co-workers reported they could reverse the regiochemistry of the cycloadduct by changing the geometry of the P-phenylsulfonylacrylate esters as dienophiles.118 H H Figure 2.20: Transition States in Cycloadditions with (E)- and (Z)-p-Phenylsulfonylacrylonitrile 123 2.8.6 Further Chemistry on the Chlorin Derivatives The cycloadducts obtained from the described Diels-Alder reactions gave the correct carbon skeleton to pursue the plan as described in Scheme 30. In the next step, 160-C was catalytically hydrogenated in tetrahydrofuran containing palladium over carbon. The progress of the reaction was monitored by UV-vis spectroscopy. Its completion was indicated by the disappearance of the 522 nm band and a shift of the 656 nm band (Fig.2.21). Work-up of the reaction mixture gave the desired product 163 in 85.0 % yield. In addition a polar by-product without the exo-double bond was formed in the reaction. Examination of the proton and mass spectra of this by-product revealed that an angular hydroxyl group has been introduced at the newly created chiral center. To avoid forming this by-product, a brief bubbling of an inert gas such as argon through the tetrahydrofuran to be used as the solvent was found necessary. By-product 124 1,0 n 0,0 500 600 700 Wavelength (nm) Figure 2.21: The Electronic Spectrum of 160-C and 163 126 The !H-NMR spectrum of the hydrogenated adduct 163 (Fig. 2.22) displayed a triplet at 5 4.65 ppm, assignable to the angular proton (Ha) at C-3. The diastereotopic protons (Hb, He) at C-24 appeared upfield at 8 2.75 and 3.30 ppm respectively. Signals due to protons Hd and He at C-23 were well separated at 8 2.27 and 2.65 ppm. The doublet at 8 2.75 ppm was unambiguously assigned to Hg while the multiplet at 8 3.72 ppm was assigned to Hf. All these assignments were confirmed by decoupling and difference n.O.e experiments. Thus, irradiation of the signal at 8 2.65 ppm (Hd, see n.O.e data) converted the multiplet (ddd) at 8 3.72 ppm into a triplet with a coupling constant of 10.0 Hz. This suggested that J (Hf, H g ) ~ J (He, Hf) « 10.0 Hz and implied that protons He, Hf and Hg are in a trans-relationship with each other. In the same experiment, the multiplets at 8 2.27, 2.75 and 3.30 ppm collapsed to simpler patterns. Irradiation of the triplet due to Ha at 8 4.65 ppm simplified the signals at 8 2.75 and 3.30 ppm only. Irradiations of the other signals in the cyclohexane ring simplified the spectra in the manner expected. The stereochemistry at the newly created chiral center was established by difference n.O.e experiments. The difference n.O,e data are shown in Table 2.6. Table 2.6: Results of Difference n.O.e experiments on 163 Irradiated proton(s) 8 (ppm) Observed (+) n.O.e 8 (ppm) 2.27 (He) 2.65 (Hd), 2.75 (Hg) 2.52 (ang.-CH3) 3.72 (Hf), 4.65 (Ha) 2.65 (Hd) 2.27 (He); 3.72 (Hf) 2.75 (Hb,H g) 3.30 (He), 3.72 (Hf),4.65 (Ha) 3.30 (He) 2.75 (Hb) 3.72 (Hf) 2.52 (ang.-CH3), 2.75 (Hb) 4.65 (Ha) 2.52(ang.-CH3), 2.75(Hb) 127 Figure 2.23: Jjiformation from Difference n.0.e experiments Thus, the observation of a n.0.e between the angular methyl group at 8 2.52 ppm and Ha at 8 4.65 ppm suggested a cis-relationship between the methyl group and Ha. This led to the conclusion that the hydrogenation step went with a high degree of selectivity and that the preferred mode of hydrogen addition was cis to the angular methyl group. The stereochemistry of the product was therefore opposite to that required in the plan (Scheme 30). At this stage, it was felt that this stereochemical problem could be tackled later in the plan and therefore attention was turned to testing the feasibility of the subsequent proposed reactions in the attempt to synthesize a model compound for factor 1. In the next step, an oxidative desulfonation reaction was considered. Conditions for this reaction were provided by the work of Little and co-workers.119'120 According to them, it is possible to convert an a-sulfonyl carbanion to a ketone upon treatment with a complex of molybdenum peroxide, pyridine and HMPA (MoO5.Py.HMPA) in tetrahydrofuran at -78°C (Scheme 56). 128 S c h e m e 56: An Oxidative Desulfonation Reaction Attempted oxidative desulfonation on 159-C and 163 (Scheme 57) were unsuccessful under a variety of conditions and therefore an alternative route was investigated. Scheme 57: Attempted Oxidative Desulfonation on 159-C and 163 As a next step, a desulfonation reaction using DBU as base was carried out to introduce a double bond in the molecule (Scheme 58). In fact, this reaction proceeded rapidly and smoothly at room temperature to give the desired product 168 almost quantitatively. 129 The mass spectrum of 168 showed the expected molecular ion at m/z 529 and a daughter ion at 514 derived from the loss of a methyl group. The ^ - N M R spectrum (Fig. 2.24) of the desulfonated adduct exhibited a triplet at 8 6.80 ppm (J = 5.0 Hz) due to the olefinic proton (Hf). Another triplet at 8 4.95 ppm (J = 5.0 Hz) was assigned to the angular proton (Ha) due to its coupling with Hb and He- The multiplet at 8 2.50 ppm was assigned to the two ally lie protons at C-23. Irradiation of the C-23 protons (He, Hd) signal gave rise to a singlet replacing the triplet at 8 6.80 ppm. In the same experiment, the multiplet at 8 2.98 ppm collapsed to a doublet of doublets (J =5.0, 15.0 Hz) while the multiplet at 8 2.71 ppm also collapsed to a doublet of doublets (J = 5.0, 15.0 Hz). Dreiding model studies suggested that the only likely conformations for this compound were the distorted half-chair and the distorted boat. Examination of the coupling constants between Ha, Hb, He, Hd, He and Hf favored the half chair conformation in which the dihedral angles between these protons agree with the observed coupling constants. Figure 2.24: The 'H-NMR Spectrum of the ^sulfonated Adduct 168 131 The next step undertaken was the epoxidation of the double bond of the desulfonated adduct to give the epoxide 169. Initial attempts to accomplish this transformation with tert-butyl hydroperoxide in the presence of M o ( C O ) 6 1 2 1 , 1 2 2 in refluxing dichloroethane were unsuccessful. Under these conditions, the starting material was always recovered from the reaction mixture. Since a large number of epoxidations have been reported to occur with p e r a c i d s , 1 2 3 ' 1 2 4 adduct 168 was treated with m-chloroperbenzoic acid at 0°C in dichloromethane in the presence of sodium diphosphate. After 4 hours, a dc analysis indicated no trace of starting material. Purification of the reaction mixture after work-up gave a major product with lower Rf than the starting material followed by a bluish by-product which did not exhibit a Soret band. The mass spectrum of the major product showed a peak at m/z 545 suggesting that the product indeed was oxidized. However, the ^ - N M R spectrum of the compound was disappointing. The olefinic triplet at 8 6.8 ppm was intact but the triplet at 8 4.95 ppm had vanished. These observations suggested that the angular hydrogen has been oxidized to a hydroxyl group to give 170 leaving the double bond intact. A l l attempts to force this reaction under various conditions were fruidess. Scheme 59: Attempted Epoxidation on Adduct 168 132 In light of these results, it was decided that an alternative approach was required to convert the adducts from the Diels-Alder reaction to intermediate 87 prior to the ring opening step. In conclusion, it has to be admitted that the present plan was blocked by some unforeseen problems and was therefore abandoned. Nonetheless, some interesting observations were made in the course of this work regarding the regio- and stereochemistry of the monovinylporphyrin 81 with different 1,2-disubstituted vinyl sulfone dienophiles. 2.9 Synthesis of Benzoporphyrin Derivatives Baker and co-workers have suggested that the benzoporphyrin system in oil shales may have originated from a sedimentary Diels-Alder reaction involving a vinylporphyrin with a natural dienophile. In their classical hypothesis, they have proposed an intermediate that eventually undergoes aromatization to give the benzene ring of the benzoporphyrin.74 Preliminary studies in our laboratory using protoporphyrin LX dimethyl ester as the diene have shown that this hypothesis has some significance. However, this reaction turned out to be of low yield because the aromatization involving the loss of a methyl group was not a facile step.58 Utilization of protoporphyrin LX as the diene was therefore viewed as the stumbling block since the carbon adjacent to the vinyl group carries the methyl group to be ejected. The key retrosynthetic disconnection of a benzoporphyrin as shown in Scheme 34 suggested a R -unsubstituted-p'-monovinylporphyrin 101 as the desired precursor. The first step in this program was to prepare this key precursor. The successful synthesis of 101 has been described in the vinylporphyrin section and here, its transformations to the benzoporphyrin derivatives are described. When the porphyrin 101 was heated in degassed toluene solution in a sealed tube at 110 °C with a 50 fold molar excess of dimethyl acetylenedicarboxylate, it underwent the 133 desired Diels-Alder reaction to provide a major product in 24 hours. Isolation and purification of this product by chromatography afforded the cycloadduct 171 in 80 % yield (Scheme 60). ROjC ROjC R 0 2 C — G = C — C 0 2 R Toluene 110'C 171 R= CH 3 172 R=C2H5 Scheme 60: Diels-Alder Reaction of 101 with Esters of Acetylenedicarboxylate The mass spectrum of 171 showed a molecular ion at m/z 602 and peaks at 587 and 572 suggesting successive loss of methyl groups from the parent molecule. High resolution mass spectroscopy gave an accurate molecular mass at 602.2895 (calculated value for C37H38N4O4 = 602.2893). The electronic spectrum of 171 exhibited a rhodo-type spectrum (Fig. 2.25). The ^ - N M R spectrum (Fig. 2.26) showed the four meso protons as singlets at 8 9.91, 10.03, 10.14 and 10.40 ppm, each corresponding to one proton. The three ring methyl singlets appeared at 8 3.51, 3.55 and 3.70 ppm and the two methyl ester singlets at 8 4.20 and 4.64 ppm, each corresponding to three protons. Furthermore, methylene envelopes were observed as quartets between 8 3.95 and 4.20 ppm , having an integration for six protons belonging to the ethyl groups and methyl envelopes as triplets between 8 1.80 and 2.00 ppm, having an integration for nine protons belonging to the ethyl groups. Finally, the two aromatic protons on the exocyclic benzene ring characteristic of the benzoporphyrins were seen as doublets at 8 8.71 and 9.32 ppm (J = 8.0 Hz) as expected. 400 500 600 700 W a v e l e n g t h ( n m ) Figure 2.25: The Electronic Spectrum of the Benzoporphyrin 171 CHjOzC CH302C 2*-H 2 3 - H in 10 JJK_JJL 3 2 J ppm Figure 2.26: The 'H-NMR Spectrum of the Benzoporphyrin 171 136 Thus, as anticipated, the Diels-Alder reaction went efficientiy and produced exclusively one product. However, it was interesting to note that the cycloadduct (a chlorin) was not isolated as a product. Presumably, the cycloadduct , once formed in situ, is an unstable compound because of its propensity towards aromatization. This newly observed "dehydrogenative" Diels-Alder reaction in the present work therefore, provides an efficient approach to the benzoporphyrin chromophore. However, it would be interesting to investigate the pathway that the reaction follows in the conversion of the cycloadduct to the benzoporphyrin system. To probe into the mechanism of this dehydrogenative Diels-Alder reaction, this reaction was carried out with acetylenic dienophiles having increasing steric bulk introduced on the ester groups. It was hoped that an intermediate prior to the dehydrogenation process could be isolated and that ^-NMR spectroscopy could be used to resolve the problem. When, the reaction was carried out with diethyl acetylenedicarboxylate under similar conditions, the only product 172 was obtained in 75% yield after purification as above. The electronic spectrum of 172 exhibited a typical rhodo-type pattern as in the previous example. The mass spectrum conformed with the expected molecular ion at m/z 630 for the corresponding benzoporphyrin. The 1H-NMR spectrum clearly revealed the presence of the two aromatic protons on the exocyclic benzene ring as doublets at 8.73 and 9.40 ppm. However, when di-tert-butyl acetylenedicarboxylate was used under the same conditions, two porphyrin products 173 and 174 (in 3:1 ratio) were isolated in 60% overall yield (Scheme 61). The minor component 173, with a rhodo-type spectrum (slighdy higher Rf) was assigned the benzoporphyrin structure on the basis of electronic, mass spectral and NMR data. The mass spectrum of this compound did not show the expected molecular ion at m/z 686. The most prominent peak at m/z 556 was attributed to the concomitant loss of an ethyl and a tert-butyloxycarbonyl group by the parent molecule. However, a (M+ 1) peak at m/z 687 in the fast atom bombardment mass spectrum helped to confirm the structure of 173. The 137 1 H-NMR spectrum was consistent with the proposed benzoporphyrin structure showing the two characteristic doublets at 8 8.58 and 9.44 ppm. The major component 174 of this reaction also exhibited a rhodo-type spectrum, but to a lesser extent. Its mass spectrum showed a molecular ion at m/z 688. High resolution mass spectroscopy gave an accurate molecular mass at 688.3996 for the formula C43H52N4O4 (calculated = 688.3988). Further evidence for a structure for 174 came from its *H-NMR spectrum. The absence of the two aromatic protons in the benzene ring of the benzoporphyrin molecule coupled with the appearance of two triplets at 8 3.33 and 4.25 ppm confirmed that 174 was a dihydro analog of 173. Thus, the introduction of the tert-butyl group has served two purposes. First, it has made the dienophile a less effective dehydrogenating agent and second, it has provided some insights as to the pathway en route to the benzoporphyrin system. Scheme 61: Diels-Alder Reaction of 101 with Di-tert-butyl Acetylenedicarboxylate B u l 0 2 C Figure 2.27: The JH-NMR Spectrum of the Compound 174 139 Three possible mechanisms for the formation of the benzoporphyrin system should be considered (Scheme 62). In path A, the hydrogen at C-2 undergoes a 1,3 hydrogen shift to C-22, thus rearranging the double bond between the two ester groups to give 176. In path B, the hydrogen at C-2 undergoes a 1,3 hydrogen shift to C-24 with isomerization of the double bond formed during the Diels-Alder reaction into the macrocycle to generate porphyrin 177. In path C, an isomerization of the double bond between the ester groups to give a more stable chlorin adduct 178 can take place. Scheme 62: The Possible Pathways for the Formation of the Benzoporphyrin 140 From the above results, it seems that path B is the most plausible mechanism for the present dehydrogenative Diels-Alder reaction. With di-tert-butyl acetylenedicarboxylate, it was possible to isolate a compound which was dehydrogenated with DDQ to give the required benzoporphyrin. The present results therefore suggests that the reaction of acetylenedicarboxylate esters with the vinylporphyrin 101 proceeds via a [4 + 2] cycloaddition reaction to give, initially, a chlorin type adduct 175. However, unlike the case of the methyl-vinyl analog (protoporphyrin LX), the chlorin 175 undergoes a rapid rearrangement to give the thermodynamically more stable porphyrin 177 (via path B) which is subsequently dehydrogenated to the benzoporphyrin. In conclusion, this newly observed dehydrogenative Diels-Alder reaction has been employed in an efficient way to synthesize benzoporphyrin derivatives in high yields. 2.10 Synthesis of Bacteriochlorin Derivatives 141 In vitro and in vivo studies over the past decade indicate that photodynamic therapy (PDT) is potentially an effective means of treating cancer.125-126 Suitable photosensitizers must not only absorb light in the visible or near infrared region but must also be compatible with the biological systems into which they are injected. Unfortunately, many dyes that have been tested have proved to be toxic. Recent efforts have centered on HpD and the purified product Photofrin H™ in hope of producing an effective, non-toxic photosensitizer. However, for quite some time, it has been realized that, in the long run, these are not ideal compounds for use in the exploitation of this technique. Both of these compounds are known to be mixtures of ester and ether-linked dimers and oligomers of hematoporphyrin. The composition of such complex mixtures has been observed to vary from preparation to preparation and with length of storage time.127 They absorb poorly in the red region of the light spectrum and, in clinical trials, skin photosensitivity in humans has been observed for several weeks following an injection.128 Clearly, the future promise of PDT for cancer treatment depends on the design of new compounds as sensitizers. Any endeavor in this direction should lead to the synthesis of new sensitizers with the following characteristics. They should be pure, stable, well characterized, non-cytotoxic to normal tissues, non-mutagenic and have the ability to absorb light in the far visible red or near infrared region. These sensitizers should localize preferentially in tumors and generate the photodynamic effect when illuminated with laser light of a particular wavelength. To date, most synthesized compounds absorb light around the 650 to 700 nm region. Light within this range can penetrate only about 10% of the subcutis.79 An examination of light penetration as a function of the wavelength of light reveals that the most effective photosensitizers should absorb in the range of 730 to 800 nm. The need for new sensitizers with the above requirements is understood and well-documented by researchers throughout the world. 1 2 9- 1 3 0 Recently, substantial progress has been achieved in the synthesis of new sensitizers to replace HpD and Photofrin H™. Many of the emerging contenders are other porphyrins, chlorins, porphycenes, phthalocyanines and naphthalocyanines. meso-tetra(m-hydroxyphenyl)rxxphyrin meso-tetra(p-sulfor^ tophenyl)porphyriri Figure 2.28: Synthetic Porphyrins Proposed as Potential Photosensitizers Porphyrin C , 1 3 1 hernatoporphyrin-di-hexyl-ether and hematoporphyrin-di-ethyl-ether,132 basically derivatives of hernatororphyrin, are all new porphyrins which have been introduced as possible photosensitizers (Fig.2.28). 143 Verdin ' COfS* O COjH o = c C 0 2 H R Monoaspartyl Chlorin t$ OpjCjHs COjH COjH Benzoporphyrin Derivative Pheophorbide a Figure 2.29: "Chlorins" Proposed as Potential Photosensitizers In addition, meso-tetra(m-hydroxyphenyl)porphyrin133 and meso-tetra(p-sulfonato-phenyl)porphyrin134 have been regarded as promising new sensitizers because they have been found to localize in the tumor better than does Photofrin II™. Unfortunately, these porphyrin-144 type sensitizers absorb light poorly above 650 nm and therefore, are limited to treatment of tumors on or near the skin surface. However, chlorins, which are simply reduced porphyrin, absorb light more strongly in the 650 to 700 nm region, and thus have an advantage over porphyrin-type sensitizers in terms of their ability to penetrate a tumor more deeply. Examples of chlorins (Fig.2.29) presendy employed are Chlorin e 6 , 1 3 5 monoaspartyl chlorin e 6 , 1 3 6 verdin,1 3 7 purpurin,138 benzoporphyrin derivative (BPD) 7 7 - 7 8 and Pheophorbide a. 1 3 9 Bacteriochlorins are also considered to show potential as photosensitizers for PDT as they have the ability to absorb light in the 730 to 800 nm range. Porphycene, a novel porphyrin-like compound synthesized at the Institute of Organic Chemistry in West Germany (Cologne), has also been shown to possess the necessary properties of a photosensitizer for PDT. 1 4 0 A 50-fold increase in absorption in the red region of the visible spectrum relative to porphyrin photosensitizers makes the porphycene macrocycle a good alternative to HpD or Photofrin IJ™. Figure 2.30: Synthetic Porphycene as a Photosensitizer Two other classes of compounds which are closely related to the porphyrin macrocycles, are the phthalocyanines and naphthalocyanines. They exhibit very similar properties as the porphyrins and have also been the focus of much research in the PDT area. 1 4 1" 1 4 4 Being tetraazatetrabenzoporphyrin-type molecules, phthalocyanines and naphthalocyanines absorb light strongly in the 650 nm and 750 nm region respectively. 145 However, the fact that the free base phthalocyanines are insoluble in water and organic solvents, makes them unsuitable for use in biological systems. Nevertheless, this problem has recently been overcome by the sulfonation or hydroxylation of the benzene rings in the molecule.145 Complexation of the free base of these compounds with certain metals (eg, Al, Zn, Si, Sn, Ce, Ga) to produce metalated phthalocyanines or naphthalocyanines has also been achieved by many researchers hoping to tune the properties of these molecules.146"148 In vitro and in vivo studies have already shown that some phthalocyanines exhibit photocytotoxicity and are presently regarded as potential substitutes for HpD and Photofrin 11^149,150 M = 2H R = H M = 2H R = S03" M = A1 M = Zn M = Sn M = A1 M = Zn M = Ce M = Ga R = H R = H R = H R = S03" R = S03" R = S03" R = S03" Phthalocyanines Naphthalocyanines Figure 2 J l : Phthalocyanines and Naphthalocyanines as Photosensitizers 146 2.10.1 Diels-Alder Reactions of the A,C-Divinylporphyrin 106 As has already been mentioned, the light absorption properties of bacteriochlorins has caused them to be regarded as prospective candidates as photosensitizers for PDT. This section describes the synthetic approach to the bacteriochlorin chromophore. The synthetic strategy here relied on two cycloaddition reactions on an A,C-divinylporphyrin system. The first step in this program was to prepare the A,C-divmylporphyrin from simple pyrroles. Two routes were investigated and both gave the desired A,C-divinylporphyrin 106 in moderate yields (described in section 2.7.3). Having solved the problem of preparing the precursor, attention was then turned to the ultimate goal of the project, the synthesis of the bacteriochlorin chromophore. As a first attempt, the reaction of A,C-divmylporphyrin 106 with N-phenylmaleimide was investigated. Heating a degassed solution of the A,C-divmylporphyrin in toluene with a 50 molar excess of N-phenylrmleimide at 110°C for three days, gave a mixture of the mono- and bis-adducts (Scheme 63). Column chromatography using silica gel to remove the excess dienophile, followed by further separation on the chromatotron using a silica gel plate gave a major product 179 in 45 % yield and a minor product 180 in 8% yield. The electronic spectrum of the minor component 180 (X m a x CH2CI2 = 404, 580 ,544 , 598, 656 nm) confirmed that this compound was indeed the mono-adduct (a chlorin). Its : H -NMR spectrum exhibited a singlet at 8 2.09 ppm assignable to the angular methyl group. The one-proton multiplet at 8 3.90 ppm was assigned to HQ" and the two-proton multiplet in the range of 8 3.45 to 3.55 ppm was assigned to Hb and He- The doublet at 8 4.67 ppm was assigned to He and the one-proton triplet at 8 7.42 ppm was assigned to H a . The assignment of these signals was based on a series of decoupling experiments. When the doublet due to He was irradiated, the one-proton multiplet pattern at 8 3.90 ppm was simplified. Similarly, the hidden multiplet pattern due to protons Hb and He at 8 ~ 3.50 ppm was reduced to a simpler pattern when H a was irradiated. 147 Bis-adduct Mono-adduct Scheme 63: Diels-Alder Reaction of 106 with N-Phenylmaleimide Many examples exist in literature whereby N-phenylmaleimide undergoes Diels-Alder reaction via an endo transition state giving the endo-adduct as exclusive product.1 5 2'1 5 3 Difference n.O.e experiments were performed to determine the stereochemistry of the adduct. The key protons for the assessment of the stereochemistry are the angular methyl protons, Hd and He. 148 Irradiation of the angular methyl group at 6 2.09 ppm led to positive enhancement at 8 4.67 ppm due to He (the doublet) which confirmed the endo selectivity of this addition and clearly demonstrated the importance of secondary orbital effects for the stereoselectivity of the Diels-Alder reaction (Fig. 2.32). When the doublet at 8 4.67 ppm was irradiated, positive enhancement was observed at 8 2.09 ppm (angular methyl) and 8 3.90 ppm (one-proton multiplet due to Hd). This observation suggested that the dienophile had retained its stereochemistry and provided extra information for the assignment of the stereochemistry of the mono-adduct 180. The conformation of this adduct was deduced from the coupling constant value between Hd and He- In a distorted boat conformation, a model suggested a dihedral angle close to 0" between these two protons while in a half-chair conformation the dihedral angle should be close to 90*. A measured value of 8.5 Hz favored the distorted boat conformation for 180 based on the Karplus relationship.154 approach endo Figure 2.32: The Endo-Transition State in the Cycloaddition with N-Phenylmaleimide 149 The electronic spectrum of the major product (X m i x C H 2 C I 2 = 388, 410,490, 526, 702, 738 nm) exhibited an intense absorption band at 738 nm, a characteristic of the bacteriochlorin chromophore. It was clear at this stage that the major product was a bis-adduct resulting from two successive Diels-Alder reactions on one A,C-divinylporphyrin molecule. Further structural confirmation was obtained from its mass spectrum. The molecular ion at m/z 820 was in agreement with mat expected for the bis-adduct. 0.0 H 1 1 1 1 1 1 1 1 1 r— 300 400 500 600 700 800 Wavelength (nm) Figure 233: The Electronic Spectrum of the Bis-adduct 179 150 Fig. 2.34 shows the ^ - N M R spectrum of the bis-adduct 179. Two singlets of different intensities at 8 2.00 and 8 2.05 ppm were assigned to the two angular methyl protons. The proton He appeared as a doublet at 8 4.54 ppm. Its neighbor Hd resonated at 8 3.8 ppm as a multiplet over the quartets due to the methylene protons in the ethyl groups at the periphery of the porphyrin macrocycle. Protons Hb and He were revealed as multiplets at 8 3.40 ppm. The assignment of the signal at 8 3.80 ppm as being due to Hd was proven by a decoupling experiment. Irradiation of the doublet at 8 4.54 ppm (He) simplified the signal at 8 3.8 ppm (Hd). Similarly, by decoupling the signal at 8 7.25 ppm (Ha), the multiplet due to Hb and He also underwent simplification. A closer examination of the ^ - N M R spectrum revealed that two compounds (diastereomers) were present as the products of the reaction. Their ratio was calculated as 3 : 2 by measuring the integrated area due to the two different angular methyl groups at 8 2.00 and 2.05 ppm respectively. In attempting to clarify the stereochemistry of the bis-adduct, several difference n.O.e experiments were performed. The normal (control) ^ - N M R and difference n.O.e spectra of the bis-adduct are shown in Fig. 2.35. Irradiation of the angular methyl group at 8 2.00 led to a positive n.O.e at 8 4.54 ppm (He) and 8 8.87 ppm (meso proton). This information is consistent with the structure in which He and the angular methyl group have a cis-relationship, thus confirming that the bis-adduct also resulted from endo cycloaddition. Irradiation of the other angular methyl group at 8 2.05 ppm (the one with less intensity), also led to positive n.O.e at 8 4.54 and 8 8.90 ppm. The results obtained from the selective irradiation of the two individual methyl peaks revealed two pieces of information. First, it made possible the assignment of the meso proton in the vicinity of the angular methyl group, and second, it confirmed the idea that two species (ratio 3 : 2) were obtained in this double Diels-Alder reaction. Figure 234: The 'H-NMR Spectrum of the Bis-adduct 179 152 (e) irradiated at 6 4.54 1 (cf) Irradiated at S 3J0 J (c) irradiated at 5 2.05 (b) irradiated at S 2.00 " 1 V ^ " (a) control i l l i i 8 7 6 5 4 p p m Figure 2.35: The Difference n.0.e Spectra on Bis-adduct 179 153 These data suggest that both additions on the A,C-divmylrx>rphyrin 106 resulted from an endo transition state, and that the two species resulted from two molecules of the dienophile adding from the same face of the diene or from the opposite faces of the diene. Further, difference n.O.e experiments on this bis-adduct provided more evidence that the above analysis of the ^ - N M R spectrum was correct For example, when the multiplet centered at 6 3.40 ppm (Hb and He) was irradiated, a positive n.0.e enhancement was observed at 8 3.80 ppm (Hd) and at 8 7.22 ppm (Ha). These data are consistent with the assignment of the signals of H a , Hb, He, and Hd based on decoupling experiments. Irradiation of the doublet at 8 4.54 ppm (He), conclusively proved that Hd is cis to He, thus revealing that the dienophile had retained its geometry after the reaction as expected. The formation of a mixture of stereoisomers can be explained in the following manner. After one Diels-Alder reaction has occurred, the resulting mono-adduct possesses two different reactive faces (Fig. 2.36). Depending on the mode of approach, the stereoisomeric mixture may result from the attack of the second dienophile molecule from the same face or from the opposite face of the diene (relative to first cycloaddition). The analysis of the problem was simplified in the case of N-phenylmaleimide because it undergoes [4+2] cycloaddition with high endo-preference. Since the chiral centers introduced in the mono-adduct are too far away from the next reactive site (the other vinyl group) to account for any selectivity arising in the bis-adduct, the facial stereoselectivity obtained in this reaction was explained in terms of simple steric effects and Van der Waals-London interactions.155-156 There is good reason, therefore, to believe that the more abundant species is the bis-adduct 17 9B, resulting from one dienophile molecule approaching from the top face and the other approaching from the bottom face, thus minimizing steric and Van der Waals-London effects (Fig. 2.36). It is interesting to note that under such conditions 179B is expected to exist as a meso compound while the other species 179A is expected to be racemic. 154 Mono-adduct + enantiomer N-phenylmaleimide Figure 2.36: Mode of Approach of a Second Molecule of the Dienophile N-phenylmaleimide, being a symmetrical dienophile, proved to be useful for the study of the stereochemistry induced in the adducts as a result of an endo transition state. The usual endo-selectivity observed is mainly due to the well-known secondary orbital interactions. To extend this study with regard to olefinic dienophiles, it was decided that an unsymmetrical olefinic dienophile was worth investigating. (E)-p-phenylsulfonylacrylonitrile, which underwent the Diels-Alder reaction with the monovinylporphyrin 88 regio- and stereoselectively was chosen for this purpose. 155 Heating the A,C-divinylporphyrin 106 in degassed toluene with a 50-fold molar excess of (E)-P-phenylacrylonitrile at 110'C for three days gave a mixture of compounds which exhibited a strong absorption at 734 nm. Purification of the reaction mixture with a combination of flash chromatography (silica gel) and chromatography on a chromatotron (silica plate) provided the mono-adduct and the bis-adduct in 5 % and 45 % yield respectively. Scheme 64: Diels-Alder Reaction of 106 with (E)-p*-Phenylsulfonylacrylonitrile The ^ - N M R spectrum of the bis-adduct 181 showed a doublet at 8 3.75 ppm which was assigned to He (Fig. 2.37). The proton Hd (adjacent to the sulfonyl group) resonated at 8 4.31 ppm as a multiplet The protons Hb and He appeared as a complex mixture of multiplets in the 8 3.25 to 3.60 range. The multiplet at 8 7.00 ppm was assigned to the proton Ha (two doublets of doublets superimposed on each other). The stereochemistry of the reaction was expected to be endo (phenyl group under diene system) and the positive enhancement that was observed at 8 4.31 (Hd) when the angular methyl group was irradiated confirmed this expectation. The last fact to be considered in the analysis of the stereochemical outcome was the face selectivity of the second dienophile when adding onto the mono-adduct to give the bis-adduct. ppm Figure 2.37: The !H-NMR Spectrum of Bis-adduct 181 O N 157 Indeed, further analysis of the ^ - N M R spectrum showed that the ratio of the two possible diastereomers were 1.3 :1. Again, on the basis of steric factors, it was postulated that the more abundant bis-adduct was the one formed when the two dienophile molecules approched from opposite faces. The success of the reaction of A,C-divinylporphyrin with olefinic dienophiles, and the urge to synthesize new compounds which would absorb light in the 700-800 nm range, provided the impetus to investigate the reactivity of the A,C-divinylporphyrin towards an acetylenic dienophile. Refluxing the A,C-divinylporphyrin with a 50-fold molar excess of diethyl acetylenedicarboxylate in toluene for three days gave a mixture with a strong absorption at 738 nm. Flash chromatrography on a silica gel column followed by further purification on the chromatotron (1 mm silica gel plate) gave the bis-adduct 182 in a 52 % yield. The electronic spectrum of the bis-adduct conformed to the characteristic features of a bacteriochlorin (X m a x CH 2C1 2 = 384, 406, 484, 538, 698, 738 nm). The mass spectrum of the compound showed the molecular ion as the base peak at m/z 814. 106 Diethyl acetylene-dicarboxylate Toluene /110*C 182 C O ^ H j CO2C2H5 Bis-adduct Scheme 65: Diels-Alder Reaction of 106 with Diethyl Acetylenedicarboxylate The stereostructure for the bis-adduct 182 followed direcdy from ^ -NMR decoupUng experiments and chemical shift data. Multiplets in the range of 8 4.30 to 4.40 ppm and 8 4.42 158 to 4.62 ppm were assigned to the diastereotopic methylene protons of the ethoxycarbonyl groups. These two multiplets were further differentiated on the basis of the decoupling experiments done on the methyl protons of the ethoxycarbonyl groups. The upfield multiplet (8 4.42 to 4.62 ppm) was assigned to the methylene protons attached to the methyl group at 8 1.08 ppm. Dreiding model of the bis-adduct indicated that this methyl is position above or below the plane of the bacteriochlorin macrocycle and is shielded to a great extent by the n cloud of electrons. This accounted for the upfield position of the particular methyl group. Fig. 2.38 gives some relevant ^ - N M R data on the chemical shifts of the protons on ring A. These data are in good agreement with those published on a related molecule derived from protoporphyrin IX diester in which only one Diels-Alder reaction took place.157 (3.62, 3.95) Figure 2.38: Relevant !H-NMR Chemical Shifts for Ring A of 182 When the above bis-adduct was stirred overnight with l,5-diazabicyclo[5.4.0]undec-5-ene (DBU) at room temperature, a drastic bathochromic shift was observed (X max CH2CI2 = 448,468, 558, 622,702,742, and 786 nm). 1.0 0-0 — i 1 1 1 1 1 1 r ~ 5 0 0 6 0 0 7 0 0 8 0 0 Wavelength (nm) Figure 2.39: The Electronic Spectrum of Bis-adduct 183. 160 The most significant changes in the ^ - N M R spectrum of the new adduct were the appearance of a new signal at 8 4.90 ppm for the new sp3 center generated at C-21 and C-121 and two new doublets at 5 7.28 ppm and 8 7.78 ppm for the new sp2 centers. This observation indicated an isomerization of the double bond in both six-membered rings thus extending the conjugation in the molecule (Fig.2.40). 0.62, iS5) Figure 2.40: Comparison of the Relevant ^ - N M R Data for Ring A between 182 and 183 161 Furthermore, the methyl protons of the ethoxycarbonyl groups (now attached to the new sp3 carbons) after rearrangement, showed an unusual upfield shift Examination of a Dreiding model (Fig. 2.41) and results from difference n.O.e experiments (Fig. 2.42) accounted for this information. The angular methyl groups at 8 1.74 and 1.78 ppm when irradiated gave a positive n.O.e at C-21 and C-121. This information led to the conclusion that the ethoxycarbonyl groups at C-21 and C-121 are trans to the angular methyl groups (occupying a pseudoaxial position), and are free to rotate over the porphyrin ring system (Fig. 2.41). The upfield shift of the methyl groups of the ethoxycarbonyl groups at C-21 and C-121 at 8 0.33 and 0.38 ppm are due to the shielding effect of the aromatic porphyrin system. Figure 2.41: Spacial Representation of Rearranged Bis-adduct 183 in Ring A (b) Irradiated at 6 1.78 i Mir - i 1 1 ' • • • "T ~r~ 9 8 7 6 5 4 ppm 3 2 I Figure 2.42: The Difference n.O.e Experiment on 183 163 2.10.2 Absorption and Fluorescence Data All the synthesized bacteriochlorin derivatives absorb further into the red region of the visible spectrum than HpD, synthetic chlorins and porphycenes. They also possess an exceptionally high extinction coefficient at the furthest band in the red region ( » 70,000 to 80,000 L moHcm 1 ). A split Soret, characteristic of the bacteriochlorin chromophore, was also observed in all the compounds. These spectroscopic data are listed in the Table 2.7. Table 2.7: Absorption Data of the Synthetic Bacteriochlorin Derivatives Compounds Solvent X m a x of Soret (nm) Farthest X max (nm) 179 CH 2C1 2 388,410 738 181 CH 2C1 2 384,412 734 182 CH 2C1 2 384,406 738 183 CH 2C1 2 448,468 786 Apart from the absorption properties, the emission properties of these new compounds were also studied by Dr. Walter Dandliker at Diatron Corporation (San Diego, California). The fluorescence emission spectrum of 181 is shown in Fig. 2.43. When 181 was excited at 735 nm, it produced an emission at 781 nm. A similar red shifted emission band was observed for 179 and 183 (Table 2.8). As expected, there was considerable overlap between the absorption and emission bands. ill 8 5 5 W A V E L E N G T H ( n m ) Figure 2.43: The Fluorescence Emission Spectrum of Bis-adduct 181 _ 165 Table 2.8: Fluorescence Data of the Synthetic Bacteriochlorin Derivatives Compounds Solvent Exciting X EX Emission X EM (nm) (nm) 179 toluene 733 783 181 toluene 735 781 183 toluene 770 790 2.10.3 Cytotoxicity of the Synthetic Bacteriochlorin Derivatives In vitro studies with the bacterichlorin derivatives were carried out in Dr. Julia Levy's laboratory (Microbiology Department, University of British Columbia) with either cell line P815 or MI-S as model systems. Various concentrations of the test compound were added to washed suspensions of cells from cultures of the target cells and the mixtures were then irradiated using 700 to 820 nm light for 30 minutes. Following irradiation, the cells were recovered from light exposure and assayed for viability by incubating them for 18 hours in tritium-labeled mymidine (thymidine incorporation is equated with viability). The cells were then harvested and radioactivity uptake was measured by a scintillation counter. Preliminary results indicated that all the synthetic bacteriochlorin derivatives were highly photocytotoxic. Compound 183 was actually found to more potent than 179, which in torn was better than 181. 166 2.10.4 The Bacteriochlorin Derivatives as Photosensitizers for PDT In conclusion, the Diels-Alder reactions of A,C-divinylporphyrin 106 with N-phenylmaleimide, (E)-P-phenylsulfonylacrylonitrile and diethyl acetylenedicarboxylate took place to give the bis[4+2]-cycloadducts as the stable compounds. These reactions therefore provided a good entry to the bacteriochlorin chromophore, the third target molecule. In addition, some interesting observations regarding the regio-, stereo- and faceselectivity were obtained regarding the products arising from the double Diels-Alder reaction. The present work has led to the synthesis of a class of compounds with potential as future drugs in the field of photodynamic therapy. These new compounds have been prepared in pure form and have been fully characterized. They absorb strongly in the 730 to 800 nm range, a fact which, according to current thinking renders them imminently desireable as photosensitizers for use against penetrating tumors. In addition, in vitro studies have already provided encouraging results. 2.11 Summary In the first part of this work, the monovinylporphyrin 81 was successfully prepared via Johnson's route and its reactions with 1,2-disubstituted vinyl sulfones were studied. The results indicated that 81 reacted with both (E)-P-phenylsulfonylacrylate and (E)-P-phenylsulfonylacrylonitrile regioselectively to give cycloadducts which had the carbon skeletons necessary to continue the proposed synthesis of a model compound of factor 1. A comparison of the reactivity of the two trans-olefinic dienophiles led to the conclusion that (E)-pV phenylsulfonylacrylonitrile was more useful than (E)-p^phenylsulfonylacrylate on the basis of reactivity and stereoselectivity. The reaction of the (Z)-isomer of (J-phenylsulfonyl-acrylonitrile was also studied to compare the stereochemical outcome of this Diels-Alder reaction with that of the (E)-isomer. The reaction observed was of opposite stereoselectivity to that observed in the previous case. The above fmdings constituted interesting results with 167 regard to the regio- and stereochemical courses of the Diels-Alder reaction of 1,2-disubstituted vinylsulfones. The role of the sulfonyl group deserves further study in order to understand more fully the details of its effect on the regio- and stereoselectivity observed here. Pursuit of the synthetic plan from the resulting cycloadducts was prevented due to unforseen problems. In the second part of this work, the P-unsubstituted-P'-vinylporphyrin 101 was synthesized and reacted with acetylenedicarboxylate esters to give monobenzoporphyrins in high yield (75-80%). The observed dehydrogenative Diels-Alder reaction provided the first recorded example of this type of reaction in vinylporphyrin chemistry. The use of a sterically hindered di-tert-butyl ester of acetylenedicarboxylic acid suggested an isomerization of the cycloadduct to an intermediate porphyrin which could subsequently dehydrogenate to give the benzoporphyrin molecule. In the third part of this work, the A,C-divinylporphrin 106 was synthesized via two routes and its Diels-Alder reaction with olefinic and acetylenic dienophiles was studied. The resulting bis-adducts, being bacteriochlorin derivatives, were isolated and fully characterized for the first time. The present approach led to a new route towards the synthesis of stable bacteriochlorins and provided interesting information regarding regio-, stereo-, and facial selectivity for the formation of the bis-adduct. Furthermore, these new compounds have been shown to be effective photosensitizers for PDT. In conclusion, the present work demonstrates the potential of vmylporphyrins as precursors in cycloaddition reactions in the synthesis of porphyrin related macrocycles. C h a p t e r 3 Experimental 169 3.1 General Methods This general section covers the techniques and instruments used for the analysis and the purification of the products. Melting Point Determinations Melting points were performed on a 6548-J17 microscope equipped with a Thomas model 40 hot stage melting apparatus; the values reported are uncorrected. Elemental Analysis Microanalyses were carried out at the microanalytical laboratory of Mr. P. Borda at the University of British Columbia using a Carlo Erba Elemental Analyzer 1106. Nuclear Magnetic Resonance Spectroscopy Proton nuclear magnetic resonance ('H-NMR) were obtained in deuterochloroform or deuterodimethylsulfoxide as solvents on a Varian XL-300 (300MHz) or a Bruker WH-400 (400 MHz) spectrometer. The chemical shifts are reported in parts per million (ppm) on the 8 scale with tetramethylsilane (8 = 0) or chloroform (8 = 7.27) as internal standards. Signal multiplicity, coupling constants and integration ratios are indicated in parentheses. The carbon-13 NMR ( 1 3 C-NMR) spectra were obtained in 10 % TFA / deuterochloroform with a Varian XL-300 (75MHz) instrument The chemical shifts were also reported on the 8 scale using tetramethylsilane (8 = 0) as internal standard. Attached Proton Test (APT) and Selective Irradiation experiments were also performed on the same instruments to help assign the signals. 170 Mass Spectroscopy Low resolution mass spectra (LRMS) were recorded on either a Varian MAT CH4B spectrometer or a Kratos-AEI MS-50 spectrometer. The peaks which were analytically useful are reported. High resolution mass spectra were recorded on a Kratos-AEI MS-50. Infrared Spectroscopy The infrared spectra were recorded on a BOMEM FT-IR Michaelson-100 connected to a IBM compatible microcomputer. Electronic Spectroscopy The electronic spectra were recorded on a diode array spectrophotometer (Hewlett Packard 8452A Model). Chromatography Flash chromatography was performed using silica gel 60,70-230 mesh, supplied by E. Merck Co. Preparative thin layer chromatography was performed on a chromatotron (model 7924T, Harrison Research, Palo Alto, CA, U.S.A). The rotors were coated with silica gel PF-254 containing CaSC«4. 1/2 H2O (E. Merck Co.); the layer thickness was 1 mm (prepared in the laboratory from a slurry formed of 50 g of silica gel and 100 mL of water). Solvent was delivered by a pump at a flow-rate of 4-6 mL/minute. Visualization of compounds was achieved with ultraviolet light at 254 and 366 nm. Reagents and Solvents All chemicals and solvents were reagent grade. The solvents were purified according to procedures given in the literature.171 171 Reaction Conditions The preparations and reactions of vinylporphyrins were carried out in a dark room or in flasks covered with duminum foil because of the sensitive nature of the vinyl group to light 3.2 Nomenclature Used for the Synthesized Compounds The nomenclatures used in this work will be consistent with the guidelines described in this section. The naming of some of the new compounds was not easy because they did not have precedence. To tackle this problem, it was necessary to extend the already existing nomenclature. The porphyrins All porphyrins reported in this thesis have been named according to the IUPAC numbering system given in section 1.3 of Chapter 1. For consistency and comparison purposes, the vinyl group has been assigned to position C-3 in the monovinylporphyrins and to positions C-3 and C-13 in the divinylporphyrins. In addition, the rings are labeled A to D and the side-chain positions are numbered as shown below when necessary. Cl 8-(2-CWoroemyl)-18-ethyl-2,7,12,17-tetramemyl-3,13-divmylpc)rphyrm 172 The Chlorin Derivatives The chlorin derivatives were considered as reduced benzoporphyrins and were named according to the nomenclature introduced by IUPAC for benzoporphyrins. In this system, the components of the ring fused to the porphyrin molecule are numbered as substituent groups of the lowest possible position of the porphyrin ring. This is illustrated by the example below. 2^22-Bis(methoxycarbonyl)-monobenzo[b]porphyrin The Bacteriochlorin Derivatives The bacteriochlorin derivatives, having no precedence in the literature, were also considered as reduced benzoporphyrins and the above nomenclature was extended to name such compounds. This is illustrated by the dibenzo|l5,l]porphyrin molecule. 21,22-Bis(methoxycarbonyl)-dibenzoI>,l]porphyrin 173 The Pyrroles H O 3-Emyl-2-formyl-4-memylpyrrole For the pyrroles, the system commonly used among porphyrin chemists has been adopted. In this system, the substituent with the lowest alphabet letter is assigned to the lowest The Dipyrroles The two types of dipyrrolic intermediates prepared in this work were named according to the the system commonly used by the porphyrin chemists. In this system, the two linking carbon atoms of the pyrrolic rings are assigned the smallest numbers as illustrated in the dipyrromethane and dipyrromethene below. number possible in the pyrrole molecule with the nitrogen numbered as 1. H H 33\4,4'4,5'-Hexameuiyl-2^ ,-drpyrromethane 5-Bromo-3,4'-dlemyl-3\4,5'-trimethyl-22'-drpyrromethene hydrobromide 174 The Tetrapyrroles The linear tetrapyrroles were numbered from one end of the molecule to the other as follows. b H c H 2Br" a,c-Biladiene dihydrobromide 3.3 Synthesis of Dienophiles (3-PhenylsulfonylacryIonitriIe. Sodium benzene sulfinate (13.7 g, 84 mmoles) and boric acid (5.17 g, 84 mmoles) were dissolved in water (120 mL). To this solution were added a-chloroacrylonitrile (8 mL, 100 mmoles) dissolved in dioxane (100 mL) and a catalytic amount of hydroquinone. The solution was stirred at room temperature overnight. The precipitated solid was cooled and collected to give a colorless first crop which was the (E)-isomer (6.82 g). The filtrate was extracted with dichloromethane and the organic phase was evaporated to dryness to provide a second crop. This second crop was chromatographed (silica gel, dichloromethane eluant) to give the (E)-isomer (4.20 g) as the first fraction and the (Z)-isomer (1.94 g) as the second fraction (total yield = 80%, ratio E:Z = 85:15). (E)-P-Phenylsulfonylacrylonitrile M.P.: 102-103 °C (Lit: 95-96 ° C ) 1 1 3 175 1H-NMR ( C D C I 3 ) 8: 6.55 (d, 1H, J = 15.6 Hz, olefinic H); 7.25 (d, 1H, J = 15.6 Hz, olefinic H); 7.20-8.00 (m, 5H, Ar-H) IR (CHCI3, cm-1): 2350 (ON), 1150, 1335 (S02), 944 (C-H) LRMS (m/z): 193 (M+), 141 (C6H5S02+), 77 (Q>H5+) ANAL: calcd for C 9 H 7 N O 2 S : C, 55.94; H, 3.65; N, 7.25 % found: C, 56.23; H, 3.84; N, 7.11 % (Z)-P-Phenylsulfonylacrylonitrile M.P.: 85-87 °C 1H-NMR (CDCI3) 8: 6.75 (d, 1H, J = 10.2 Hz, olefinic H); 7.77 (d, 1H, J = 10.2 Hz, olefinic H); 8.25-8.75 (m, 5H, Ar-H) IR (CHCI3, cm-1): 2351 (CsN), 1157,1329 (S02), 944 (C-H) LRMS (m/z): 193 (M+), 141 (C6H 5S0 2 +), 77 (C6H5+) ANAL: calcd for C9H7NO2S: C, 55.94; H, 3.65; N, 7.25 % found: C, 56.22; H, 3.73; N, 7.29 % 176 (E)-P-Phenylsulfonylacrylate A solution of sodium benzene sulfinate (16.90 g, 0.10 moles) in methanol (100 mL) and water (50 mL) was treated with sodium acetate (8.20 g, 0.10 moles). To the stirred solution was added methyl 2,3-dichloropropionate. The mixture was stirred at room temperature. A white precipitate slowly fell out of solution and after 2 hours, the reaction was complete (as shown by tic). The precipitate was filtered off, collected, redissolved in dichloromethane which was washed with water, dried over anhydrous magnesium sulfate, filtered and the filtrate evaporated to dryness to give a white solid. Recrystallization from pentane-ether gave the desired product (18.4 g, 81%). M . P . : 96-97 °C (Lit.: 97 ° C ) 1 1 2 ! H - N M R (CDCI3) 8: 3.80 (s, 3H, -OCH3); 6.85,7.38 (2d, 1H each, J = 16 Hz, 2 x olefinic H); 7.60-8.00 (m, 5H, Ar-H) IR (CHCI3, cm-1): 1732 (C=0), 1326,1148 (SO2) L R M S (m/z): 226 ( M + ) , 195 (M+ - O C H 3 ) , 77 ( C 6 H 5 + ) EXACT MASS: calcd for C10H10O4S: 226.0299 found by high resolution mass spectroscopy: 226.0298 177 3.4 Synthesis of Monopyrroles 3-Acetyl-5-tert-butyIoxycarbonyl-2,4-dimethylpyrrole (118) O A solution of sodium nitrite (420 g, 6.1 moles) in water (600 mL) was slowly added to a well stirred solution of tert-butylacetoacetate (113) (949.2 g, 6.0 moles) in acetic acid (1000 mL), keeping the reaction mixture temperature below 20 °C. The reaction mixture was then stirred at room temperature for 45 minutes. The oxime separated out as a pale yellowish-brown oil. A 12-L three-necked, round-bottomed flask equipped with a mechanical stirrer and thermometer was charged with sodium hydroxide (350 g, 8.8 moles) and acetic acid (3000 mL) and the mixture cooled to 0 °C. 2,4-Pentanedione (115) (700 g, 7.0 moles) was added to the mixture followed by an additional 500 mL of acetic acid. The mixture was stirred for 15 minutes and zinc powder (300 g, 4.6 moles) was added. The oxime was added dropwise via a dropping funnel, keeping the reaction mixture temperature below 65 °C. More zinc powder (600 g, 9.2 moles) was added in small portions during the oxime addition (3 hours). After the oxime addition, the solution was stirred for 15 minutes at 60 °C. Addition of water to the reaction mixture gave a precipitate which was filtered and washed with water. The solid was dissolved in dichloromethane on a steam bath and the 178 solution filtered through a pad of celite to remove the zinc powder and concentrated using a rotary evaporator. Addition of methanol to the solution gave the product as white crystals which were suction filtered and air dried (1080 g, 76%). M.P.: 148-149 °C (Lit: 148°C) 1 5 8 1H-NMR (CDC13) 8: 1.60 (s, 9H, -C02C(CH3)3); 2.45 (s, 3H, -CH 3); 2.50 (s, 3H, - C H 3 ) ; 2.60 (s, 3H, -C(=0)CH3); 8.85 (br s, 1H, NH) LRMS (m/z): 237 (M+), 166 (M+-OC(CH3)3 + 2H) EXACT MASS: calcd for C 1 3 H 1 9 N 0 3 : 237.1365 found by high resolution mass spectroscopy: 237.1366 ANAL: calcd for C13H19NO3: C, 65.80; H, 8.07; N, 5.90 % found: C, 65.55; H, 7.89; N, 5.76 % 2-Tert-butyloxycarbonyl-4-ethyl-3,5-dimethylpyrrole (99) O Boron trifluoride etherate (336 mL, 2.67 moles) was added dropwise to a stirred, ice-cooled mixture of 3-ac»tyl-5-tm-butyloxycarbonyl-2,4-dimethylpyrrole (118) (270 g, 1.14 moles) and sodium borohydride (76 g, 2 moles) in THF (1000 mL) and ethyl acetate (350 mL). After the addition was complete, the reaction mixture was stirred at room temperature until completion of reaction as shown by tic. The reaction mixture was quenched by dropwise 179 addition of acetic acid (500 mL) and the product extracted using dichloromethane. The dichloromethane extract was concentrated by rotary evaporation and addition of a 3:1 ethanol/water mixture gave the product (178 g, 80%) as white crystals. M.P.: 134-135 °C (Lit: 134-135 °C)^ iH-NMR (CDC13) 8: 1.05 (t, J = 7.5 Hz, 3H, -CH 2 C H -3); 1.55 (s, 9H, -C(CH3)3); 2.2 (s, 3H, -CH 3); 2.25 (s, 3H, -CH3); 2.38 (q, J = 7.5 Hz, 2H, -CH 2 CH 3 ) ; 8.45 (br s, 1H, NH) LRMS (m/z): 223 (M + ) , 167 (M+ - C(CH 3) 3 + H), 152 M+ - OC(CH3)3) EXACT MASS: calcd for C n H „ N O , : 223.1572 ANAL: calcd for Ci 3 H 2 iN02: C, 69.92; H, 9.48; N, 6.27 % found: C, 69.65; H, 9.29; N, 6.25 % 2-Benzyloxycarbonyl-4-methoxycarbonylmethyl-3,5-dimethylpyrrole (111) found by high resolution mass spectroscopy: 223.1573 CH3O A solution of sodium nitrite (100 g, 1.45 moles) in water (350 mL) was slowly added to a stirred solution of benzylacetoacetate (119) (260 g, 1.35 moles) in acetic acid (450 mL), 180 maintaining the reaction temperature below 10 °C. After standing in the refrigerator overnight, the solution was slowly added to a solution of methyl-3-acetyl-4-oxopentanoate (121) (220 g, 1.28 moles), zinc dust (260 g, 4.0 moles) and anhydrous sodium sulfate (260 g, 3.17 moles) in acetic acid (500 mL). When the addition was complete, the reaction mixture was heated on a steam bath for 1 hour and then poured in water. The solid was filtered off and the product extracted into dichloromethane. The dichloromethane was evaporated and replaced with methanol to give the product as fine white needles (180 g, 46%). . M.P.: 104-105 °C (Lit.: 93-94 ° C ) 1 0 1 1H-NMR (CDC13) 8: 2.21 (s, 3H, -CH3); 2.31 (s, 3H, -CH 3); 3.40 (s, 2H, -CH2C(=0)); 3.67 (s, 3H, -OCH3); 5.32 (s, 2H, -Q&CdHs); 7.39 (m, 5H, -C6H5); 9.18 (br s, 1H, NH) LRMS (m/z): 301 (M+), 242 (M+ - C O 2 C H 3 ) , 91 (C7H7+) ANAL: calcd for C17H1.9NO4: C, 67.76; H, 6.36; N, 4.65 % found: C, 67.91; H, 6.45; N, 4.71 % 2-BenzyloxycarbonyI-4-ethyl-5-formyl-3-methylpyrrole (124) 181 2-Benzyloxycarbonyl-4-ethyl-3,5-dimethylpyrrole (109) (117.3 g, 0.46 moles) was dissolved in dichloromethane (1500 mL) under a nitrogen atmosphere and cooled to 0 °C for 30 minutes. Sulfuryl chloride (77 mL, 0.96 moles) was added drop wise with stirring. After the addition, the reaction mixture was stirred at room temperature for 2 hours. Distilled water (1000 mL) was then added to the mixture which was stirred overnight The organic phase was separated, washed with water and evaporated to dryness under reduced pressure. The oil thus obtained was dissolved in a minimum of dichloromethane and hexane added to precipitate the product as tan crystals (104.9 g, 84%). M.P.: 86-87 °C (Lit: 86-87 ° C ) 1 6 0 1H-NMR (CDC13) 8: 1.20 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.31 (s, 3H, -CH 3); 2.75 (q, 2H, J = 7.5 Hz, -CH2CH3); 5.34 (s, 2H, -CH2C6H5); 7.39 (m, 5H, -CH?CfiHs); 9.5 (br s, 1H, NH); 9.78 (s, 1H, -CHO) LRMS (m/z): 271 (M+), 180 (M+ - C H 2 C 6 H 5 ) , 164 (M+ - OCH 2 C6H 5 ) , 91 (C7H7+) EXACT MASS: calcd for C16H17NO3: 271.1203 found by high resolution mass spectroscopy: 271.1215 ANAL: calcd for C 1 6 H 1 7 N O 3 : C, 70.83; H, 6.32; N, 5.16 % found: C, 70.88; H, 6.45; N, 5.11 % 182 2-Benzyloxycarbonyl-5-(2-cyano-2-methoxycarbonylvinyl)-4-ethyl-3-methylpyrrole (125) 2-Benzyloxycarbonyl-4-ethyl-5-formyl-3-methylpyrrole (124) (107.4 g, 0.40 moles) and methylcyanoacetate (75 g, 0.76 moles) were heated to boiling in methanol (150 mL) on a steam bath. Triethylamine (10 drops) was added to the mixture and heating continued for 20 minutes. Cooling of the solution resulted in precipitation of the product as a brilliant lemon-yellow, fluffy solid which was filtered, washed with cold methanol, then with hexane and air dried (138.0 g, 99%). An analytical sample was recrystallized from dichloromethane-methanol. M.P.: 124-126 °C (Lit. 122-123°C) 1 6 1 1H-NMR (CDC13) 8: 1.12 (t, 3H, J = 7.5 Hz, - C H 2 C H 3 ) ; 2.30 (s, 3H, -CH 3); 2.62 (q, 2H, J = 7.5 Hz, - C H 2 C H 3 ) ; 3.90 (s, 3H, - C O 2 C H 3 ) ; 5.39 (s, 2H, - C H 2 C 6 H 5 ) ; 7.39 (m, 5H, -C 6 H 5 ) ; 8.05 (s, 1H, -CH=); 10.26 (br s, 1H, NH) LRMS (m/z): 352 (M+), 91 (C 7 H 7 + ) EXACT MASS: calcd for C20H20N2O4: 352.1416 found by high resolution mass spectroscopy: 352.1420 183 ANAL: calcd for C20H20N2O4: C, 68.17; H, 5.72; N, 7.95 % found: C, 67.96; H, 5.93; N, 7.93 % 2-Carboxyl-5-(2-cyano-2-methoxycarbonylvinyl)-4-ethyI-3-methyIpyrrole (126) 2-Benzyloxycarbonyl-5-(2-cyano-2-memoxycarrx)nyl\dnyl)-4-em (125) (60 g, 0.17 moles) was suspended in 200 mL of THF with 0.1 mL of triethylamine. The mixture was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 5 g) until uptake of hydrogen was complete. The solution was filtered through celite and the solvent evaporated to give a light yellow oil which gave yellow crystals when methanol was slowly added (43.9 g, 99%). M.P.: 195 °C (d) [Lit 200°C (d)]1 6 2 1H-NMR (DMSO-d6) 8: 1.06 (t, 3H, J = 7.5 Hz, -CH2C£L3); 2.24 (s, 3H, -CH 3); 2.61 (q, 2 H , J = 7.5 Hz, - C H 2 C H 3 ) ; 3.85 (s, 3H, -CO2CH3); 8.02 (s, 1H, -CH=); 10.7 (br s, 1H, NH); 13.35 (br s, 1H, -CO2H) LRMS (m/z): 262 (M +), 247 (M+ - CH3), 244 (M+ - H 20) 184 EXACT MASS: calcd for C13H14N2O4: 262.0948 found by high resolution mass spectroscopy: 262.0952 5-(2-Cyano-2-methoxycarbonylvinyl)-4-ethyl-2-iodo-3-methylpyrrole (127) 2-C!arboxyl-5-(2-cyanc-2-memoxycarbonylvmyl)-4-emyl-3-memylpyrrole (126) (26.2 g, 0.10 moles) was suspended in acetic acid (400 mL) containing sodium acetate (30 g, 0.40 moles) and acetic anhydride (20 mL, 0.21 moles) in a 1-L erlenmeyer flask. The stirred mixture was heated to 90 °C on a stirrer-hot plate. After dissolution of the pyrrole, a solution of iodine monochloride (20 g, 0.12 moles) in acetic acid (50 mL) was added drop wise over 5 minutes. The mixture was stirred for an additional 10 minutes at 90 °C. The mixture was cooled and aqueous sodium bisulfite was added to destroy the excess iodine. Water (400 mL) was then added to precipitate the product as yellow crystals which were filtered, washed with water and air dried (27.5 g, 80%). M.P.: 163-164 °C (Lit 163-164'C)163 !H-NMR (CDCI3) 8: 1.14 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.05 (s, 3H, -CH 3); 2.65 (q, 2H, J = 7.5 Hz, -CH2CH3); 3.90 (s, 3H, -CO2CH3); 7.85 (s, 1H, -CH=); 9.65 (br s, 1H, NH) 185 LRMS (m/z): 344 (M+), 329 (M+ - CH 3), 312 (M+ - CH3OH), 285 (M+ - CO2CH3) EXACT MASS: calcd for C 1 2 H 1 3 N O 2 I : 344.0016 found by high resolution mass spectroscopy: 344.0020 5-(2-Cyano-2-methoxycarbonyIvinyI)-4-ethyl-3-methylpyrrole (128) 5-(2-Cyano-2-methoxycarbonylvinyl)-4-ethyl-2-iodo-3-methylpyrrole (127) (25 g, 72.7 mmoles) and zinc dust (25 g, 0.39 moles) were suspended in acetic acid (200 mL) in a 1-L erlenmeyer flask and the mixture stirred and heated at 95 °C for 1 hour. The mixture was cooled and the zinc dust removed by filtration. The zinc was washed several times with methanol and the washings added to the filtrate. Addition of water to the filtrate afforded the product as fine yellow needles which were collected by filtration, washed with 20% aqueous methanol and air dried (15.1 g, 95%). M.P.: 144 °C (Lit. 141-142'C)164 186 1H-NMR ( C D C I 3 ) 8: 1.15 (t, 3H, J = 7.5 Hz, - C H 2 C H 3 ) ; 2.07 (s, 3H, -CH 3); 2.63 (q, 2 H , J = 7.5 Hz, - C H 2 C H 3 ) ; 3.87 (s, 3H, - C O 2 C H 3 ) ; 7.00 (d, 1 H , J = 4 Hz, pyr-H); 8.00 (s, 1H, -CH=); 9.67 (br s, 1 H , NH) LRMS (m/z): 218 (M+), 203 (M+ - CH3), 187 (M+ - OCH3), 186 (M+ - CH3OH) EXACT MASS: calcd for C12H14N2O2: 218.1050 found by high resolution mass spectroscopy: 218.1054 4-Ethyl-5-formyI-3-methylpyrroIe (97) 5-(2-Cyano-2-methoxycarbonylvinyl)-4-ethyl-3-methylpyrrole (128) (15 g, 68.8 mmoles) was refluxed in water (100 mL) in the presence of potassium hydroxide (13.4 g, 240 mmoles) for 3 hours under a nitrogen atmosphere. The product oiled out of solution but crystallized as light tan needles upon standing in the refrigerator overnight. The crystals were filtered, washed with water and air dried (8.0 g, 85%). M.P.: 74-75 °C (Lit 74-75°C) 1 6 5 187 1H-NMR (CDCI3) 8: 1.22 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.05 (s, 3H, -CH 3); 2.75 (q, 2H, J = 7.5 Hz, -CH2CH3); 6.90 (d, 1H, J = 3.5 Hz, pyr-H); 9.60 (s, 1H, -CHO); 10.10 (br s, 1H, NH) LRMS (m/z): 137 (M+), 122 (M+ - CH 3), 120 (M+ - OH), 108 (M+ - C ^ ) EXACT MASS: calcd for CgHiiNO: 137.0837 found by high resolution mass spectroscopy: 137.0836 2rCarboxyI-5-ethoxycarbonyI-3-ethyl-4-methylpyrrole (129) 2-Ethoxycarbonyl-4-ethyl-3,5-dimethylpyrrole (112) (58.5 g, 0.30 moles) was dissolved in dichloromethane (300 mL) in a 3-L erlenmeyer flask equipped with a magnetic stirring bar, dropping funnel, Claisen head and nitrogen inlet. Anhydrous diethyl ether (500 mL) was added to the stirred solution and a solution of sulfuryl chloride (128.4 g, 0.95 moles) in dichloromethane (200 mL) was added rapidly over a period of 15 minutes. The solution darkened to a greenish yellow and was stirred for 60 more minutes. The solvents were evaporated under reduced pressure and a 20% water-acetone mixture (300 mL) was added to the resulting red oil. The solution was refluxed for 30 minutes. Upon cooling of the solution, the product crystallized out as a tan solid which was filtered off and 188 then redissolved in a mixture of methanol and saturated aqueous sodium bicarbonate by heating on a steam bath. After cooling, the solution was washed with ether. The aqueous layer was collected and cautiously acidified with concentrated hydrochloric acid. The product precipitated out as a colorless solid which was filtered, washed with water and air dried (45.3 g, 67%). 1H-NMR (DMSO-d6) 8: 1.05 (t, 3H, J = 7.4 Hz, -CH2CH3); 1.32 (t, 3H, J = 7.0 Hz, -OCH2CH3); 2.24 (s, 3H, -CH 3); 2.72 (q, 2H, J = 7.4 Hz, -CH2CH3); 4.29 (q, 2H, J = 7.0 Hz, -OCH2CH3); 11.18 (br s, 1H, NH); 12.62 (br s, 1H, -C0 2H) LRMS (m/z): 225 (M+), 210 (M+ - CH 3), 196 (M+ - C 2 H 5 ) , 180 (M+ - CO2H) 2-Ethoxycarbonyl-4-ethyl-5-iodo-3-methylpyrroIe (130) 2-Carboxyl-5-ethoxycarbonyl-3-ethyl-4-m (129) (22.5 g, 0.10 moles) and sodium bicarbonate (33.0 g, 0.39 moles) were suspended in a mixture of water (250 mL) and dichloroethane (150 mL) in a 1-L erlenmeyer flask and heated until all the solid dissolved. The mixture was cooled to room temperature and vigorously stirred while a solution of iodine (29.0 g, 0.11 moles) and potassium iodide (45.1 g, 0.27 moles) in water was rapidly added within 3 M.P.: 210-211 °C (Lit.: 211 ° C ) 1 6 6 189 minutes. The solution was refluxed for 30 minutes and the excess iodine destroyed by addition of a sodium bisulfite solution (the color of the solution changed from deep purple to pale yellow). The solution was cooled and dichloromethane (200 mL) added. The organic layer was separated and dried over anhydrous magnesium sulfate. After filtration, the solvent was removed using a rotary evaporator to give a light yellow solid. The crude product was dissolved in hot absolute ethanol (200 mL) and water was added until crystallization began. The solid was filtered, washed with 50% aqueous ethanol and air dried to give a pale yellow solid (20 g, 65%). Concentration of the mother liquor gave a second crop of crystals (6 g, 19%). M.P.: 114-115 °C (Lit.: 114-115 ° C ) 1 6 7 1H-NMR (CDC13) 5: 1.03 (t, 3H, J = 7.4 Hz, -CH2CH3); 1.33 (t, 3H, J = 7 Hz, -OCH2CH3); 2.27 (s, 3H, .CH 3 ); 2.37 (q, 2H, J = 7.5 Hz, -CH2CH3); 4.31 (q, 2H, J = 7 Hz, -OCH2CH3); 9.02 (br s, 1H, NH) LRMS (m/z): 307 (M+), 292 (M+ - CH 3), 278 (M+ - C 2 H 5 ) , 262 (M+ - OC 2 H 5 ) EXACT MASS: calcd for C10H14NO2I: 307.0068 found by high resolution mass spectroscopy: 307.0072 2-Ethoxycarbonyl-4-ethyl-3-methylpyrrole (131) 190 2-Ethoxycarbonyl-4-ethyl-5-iodo-3-methylpyrrole (130) (56.6 g, 0.184 moles) was dissolved in 95% ethanol (460 mL) by heating on a steam bath. The solution was removed from the steam bath and treated with potassium iodide (47.7 g, 0.288 moles), followed by concentrated hydrochloric acid (72 mL).The liberated iodine was destroyed by the addition of 50% hypophosphorous acid (45 mL). A further 45 mL of acid was added to the mixture which was heated for an additional 15 minutes to ensure completion of reaction (as shown by tic). The solution was cooled to room temperature and dichloromethane (450 mL) was added, followed by water (250 mL). The organic layer was separated and dried over anhydrous magnesium sulfate. Filtration and concentration of the organic phase provided a dark red oil. The crude product was purified by column chromatography (silica gel 60, dichloromethane eluant) to give the pale yellow a-free pyrrole (32.4 g, 97%). M.P.: 22-23 °C (Lit: 25 ° C ) 1 6 8 1 H-NMR (CDCI3) 6: 1.13 (t, 3H, J = 7.4 Hz, -CH2QI3); 1.31 (t 3H, J = 7.0 Hz, -OCH2CH3); 2.23 (s, 3H, -CH3); 2.40 (q, 2H, J = 7.4 Hz, -CH2CH3); 4.27 (q, 2H, J = 7.0 Hz, -OCH2CH3); 6.60 (d, 1H, J = 2.8 Hz, pyr-H); 8.87 (br s, 1H, NH) 191 LRMS (m/z): 181 (M+), 166 (M+ - CH 3), 152 (M+ - C 2 H 5 ) , 136 (M+ - OC 2 H 5 ) EXACT MASS: calcd for C10H15NO2: 181.1103 found by high resolution mass spectroscopy: 181.1105 2-Benzyloxycarbonyl-4-ethyl-3-methylpyrrole (96) 2-Ethoxycarbonyl-4-ethyl-3-methylpyrrole (131) (32.3 g, 0.178 moles) was dissolved under a nitrogen atmosphere in benzyl alcohol (200 mL, 0.190 moles). The solution was stirred vigorously and heated to reflux (209 °C). A concentrated solution of sodium benzyloxide (prepared from freshly cut sodium in benzyl alcohol) was added in 1 mL portions to the boiling mixture until-no more evolution of ethanol vapors were observed (a lowering of the reflux temperature was also indicative of completion of reaction). The hot solution was poured into a stirred mixture of methanol (800 mL) and acetic acid (40 mL) in a 2-L erlenmeyer flask. Addition of water to the mixture precipitated an orange-brown oil. The oil was extracted into dichloromethane and the dichloromethane evaporated to give a dark brown oil. The oil was purified by column chromatography (silica gel 60, dichloromethane eluant) to give the benzyl ester as a pale yellow oil (12.26 g, 70%). 192 M.P.: 26.0-27.0 °C (Lit.: 26.5-28 °C) 1H-NMR (CDC13) S: 1.15 (t, 3H, J = 7.0 Hz, -CH2CH3); 2.30 (s, 3H, -CH 3); 2.40 (q, 2H, J = 7.5 Hz, -CH2CH3); 5.30 (s, 2H, -OCH 2-); 6.65 (d, 1H, J = 2.8 Hz, pyr-H); 7.35 (m, 5H, -C6H5); 9.00 (br s, 1H, NH) LRMS (m/z): 243 (M+), 228 (M+ - CH 3), 152 (M+ - CH2C6H5), 136 (M+ - OCH2C6H5) ANAL: calcd for C15H17NO2: C, 74.05; H, 7.04; N, 5.76 % found: C, 74.15; H, 7.27; N, 5.65 % 2-Carboxyl-4-ethyI-3,5-dimethylpyrrole (132) 2-Benzyloxycarbonyl-4-emyl-3^-dimethylpyrrole (109) (5.14 g, 0.02 moles) in THF (75 mL) and triethylamine (5 drops) was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 0.51 g) until the uptake of hydrogen had ceased (about 3 hours). The catalyst was removed by filtration through a pad of celite and the filtrate evaporated to dryness to give a white solid. This solid was recrystallized from dichloromethane/n-hexane to give the product as white prisms (3.2 g, 96%). H II o H 193 M.P.: 132-133 °C 'H-NMR (DMS0-d6) 8: 0.98 (t, 3H, -CH2CH3); 2.09 (d, 3H, -CH 3); 2.20 (s, 3H, -CH 3); 2.25 (q, 2H, -CH2CH3); 9.21 (br s, 1H, NH) LRMS (m/z): 167 (M+), 122 (M+ - CO2H) EXACT MASS: calcd for C9H1JNO2: 167.0946 2-Carboxyl-4-ethyl-3,5-cUmemylpyrrole (132) (3.0 g, 18 mmoles) was thermally decarboxylated in DMF to give the a-free pyrrole. This was carried out in an erlenmeyer flask fitted with a Claisen adapter connected to a nitrogen gas supply. The reaction mixture was stirred at 153 °C and the progress of the reaction followed by U.V. spectroscopy. The disappearance of the peak at 280 nm indicated complete decarboxylation (45 minutes). The DMF was removed under reduced pressure to give a pale yellow oil. The oil was immediately dissolved in dichloromethane (11 mL) and added dropwise to the Vilsmeier complex prepared from phosphoryl chloride (2.3 mL, 25 mmoles) and DMF (2 found by high resolution mass spectroscopy: 167.0948 4-Ethyl-2-formyl-3,5-dimethylpyrro!e (92) H 194 mL). The dichloromethane was removed in vacuo at 40 ° C and the dark brown mixture was poured in a 600-mL beaker containing ice. Small portions of sodium bicarbonate were added cautiously to the solution until the solution reached pH=8. The mixture was then warmed at 60 °C for 1 hour until total hydrolysis of the Vilsmeier inline salt (as shown by dc). The solution was cooled, water added and the solution left to stand overnight to give a precipitate of brown crystals which were filtered, washed with water and air dried (2.7 g, 88.8%). M.P.: 105 °C (Lit. 105-106'C)17J 1 H - N M R (CDC13) 8: 1.06 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.25 (s, 3H, -CH 3); 2.26 (s, 3H, - C H 3 ) ; 2.40 (q, 2H, J = 7.5 Hz, -CH2CH3); 9.41 (s, 1H, - C H O ) ; 9.50 (br s, 1H, NH) LRMS (m/z): 151 (M+), 136 (M+ - CH 3), 122 (M+ - CHO) EXACT MASS: calcd for C Q H I 3 N O : 151.0997 found by high resolution mass spectroscopy: 151.0996 2-Acetoxymethyl-5-benzyloxycarbonyl-3-methoxycarbonylmethyl-4-methylpyrrole (95) 195 Lead tetraacetate (8.86 g, 20.0 mmoles) was added in portions over 2 hours to a stirred solution of 2-benzyloxycartonyl-4~methoxycarbonylm (111) (5.0 g, 16.6 mmoles) in acetic acid (100 mL) and acetic anhydride (2.0 mL). After stirring overnight at room temperature, 100 mL of water was added dropwise. The precipitated pyrrole was filtered, washed with water and air dried. The solid was recrystallized from ether-hexane to give white needles (5.07 g, 85%). M . P . : 130-131 ° C ' H - N M R ( C D C 1 3 ) 8: 2.05 (s, 3 H , - C H 3 ) ; 2 .30 (s, 3 H , - C ( = 0 ) C H 3 ) ; 3.50 (s, 2H, - C H 2 C ( = 0 ) - ) ; 3.67 (s, 3 H , -CC>2CH3); 5.06 (s, 2H, - C H 2 O C ( = 0 ) - ) ; 5.32 (s, 2H, -CH2C6H5); 7.39 (m, 5 H , -C6H5); 9.20 (br s, 1H, N H ) L R M S (m/z): 359 (M+), 344 ( M + - C H 3 ) , 300 (M+ - C 0 2 C H 3 ) , 268 (M+ - C H 2 C 6 H 5 ) , 91 (C7H7+) E X A C T M A S S : calcd for C I Q H 2 I N 0 6 : 359.1369 found by high resolution mass spectroscopy: 359.1372 2-Carboxyl-4-methoxycarbonylmethyl-3,5-dimethyIpyrrole (98) 196 2-Benzyloxycarbonyl-4-methoxycarbonylmethyl-3,5-di^ (111) (18.1 g, 0.06 moles) in THF (330 mL) and triethylamine (5 drops) was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 1.8 g) until uptake of hydrogen was complete (about 2 hours). The solution was filtered through a pad of celite and the filtrate was evaporated to dryness to give a pink oil. The oil was dissolved in THF and hexane added to give the product as a pale pink solid (12.66 g, 94%). M.P.: 137-138 °C 1H-NMR (CDCI3) 6: 2.16 (s, 3H, -CH 3); 2.18 (s, 3H, -CH 3); 3.42 (s, 2H, -CH2CO2CH3); 3.60 (s, 3H, -CO2CH3); 11.10 (s, 1H, NH); 11.90 (br s, 1H, -CO2H) LRMS (m/z): 211 (M+), 167 (M+ - CO2), 152 (M + - C02CH3) EXACT MASS: calcd for C 1 0 H 1 3 N O 4 : 211.0845 found by high resolution mass spectroscopy: 211.0846 197 4-Carboxyl-2-ethoxycarbonyl-3-ethoxycarbonyImethyl-5-methyIpyrrole (134) OEt 4-Benzyloxycarbonyl-2-emoxycarbonyl-3-emoxycarbonylrnemyl^  (110) (37.3 g, 0.10 moles) in THF (500 mL) containing triethylamine (5 drops) was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, I. 5 g) until uptake of hydrogen stopped. The solution was filtered through a pad of celite and the filtrate evaporated to dryness. The residue was dissolved in ethanol (300 mL) on a steam bath and the solution cooled with addition of water to induce crystallization. The product, a white powder, was collected by filtration, washed with water and dried in a vacuum desiccator at 0.5 mm pressure for 24 hours (25.0 g, 88%). M.P.: 244-246 °C 'H-NMR (DMSO-d6) 5: 1.25 (t, 3H, -OCH2CH3); 1.32 (t, 3H, -OCH2CH3); 4.10 (q, 2H, -OCH2CH3); 4.15 (s, 2H, -CH2CO2C2H5); 4.30 (q, 2H, -OCH2CH3); 8.00 (s, 1H, NH); II. 80 (s, 1H, -C02H) LRMS (m/z): 283 (M+), 265 (M+ - H 20), 237 (M+ - C 2H 5OH) 198 ANAL: calcd for C13H17NO6: C, 55.12; H, 6.05; N, 4.94 % found: C, 55.07; H, 6.10; N, 4.85 % 2-Ethoxycarbonyl-3-ethoxycarbonylmethyl-4-iodo-5-methyIpyrrole (135) OEt To a hot solution of sodium bicarbonate (35.0 g in 150 mL of water) was carefully added the carboxypyrrole 134 (23.0 g, 81.3 mmoles). The suspension was stirred and 1,2-dichloroethane (150 mL) was added to dissolve the pyrrole. A solution of iodine (25 g) and sodium iodide (30 g) in water (150 mL) was added carefully over 5 minutes. The reaction mixture was then refluxed until completion of reaction (as shown by tic, 15 minutes). The excess iodine was destroyed by cautious addition of aqueous sodium bisulfite (effervescence). The mixture was extracted with dichloromethane. The organic phase was filtered and the solvent removed under reduced pressure. The yellowish-brown oil was dissolved in warm ethanol and water added until the solution went turbid. On cooling, the iodopyrrole crystallized out as pale yellow crystals which were filtered, washed with 65% aqueous ethanol and air dried (25.6 g, 86%). M.P.: 110-111 °C 199 'H-NMR (CDCI3) 6: 1.30 (2t, overlapping, 6H, 2 x -OCH2CH3); 2.30 (s, 3H, -CH 3); 3.80 (s, 2H, -CH2CO2C2H5); 4.20 (q, 2H, -OCH2CH3); 4.30 (q, 2H, -OCH2CH3); 9.50 (br s, 1H, NH) LRMS (m/z): 365 (M+), 320 (M+ - OC2H5), 319 (M+ - C2H5OH), 292 (M+ - CO2C2H5) EXACT MASS: calcd for C12H16NO4I: 365.0123 found by high resolution mass spectroscopy: 365.0123 2-Ethoxycarbonyl-3-ethoxycarbonylmethyl-5-methylpyrrole (136) OEt The iodopyrrole 135 (21.3 g, 58 mmoles) was dissolved in boiling methanol (150 mL) in a 1-L erlenmeyer flask. A solution of sodium iodide (18 g in 20 mL of water) was added to the stirred solution, followed immediately by the addition of concentrated hydrochloric acid (10 mL). The solution turned dark brown with liberation of iodine. The mixture was refluxed for 10 minutes, then cooled to room temperature. Solid sodium bisulfite was added to destroy the iodine formed and water (150 mL) added to dissolve the excess bisulfite. The mixture was extracted with dichloromethane (3 portions of 100 mL). The combined organic phases were washed with saturated aqueous sodium bicarbonate (2 portions of 200 mL) and water (200 200 mL). The organic phase was dried over anhydrous magnesium sulfate, the mixture filtered and the filtrate concentrated on a rotary evaporator to give a pale yellow oil. The oil was dissolved in ether (30 mL) and n-hexane added dropwise to induce crystallization. The white crystals were collected by filtration, washed with n-hexane and air dried (11.0 g, 79%). M.P.: 80-81 °C 1H-NMR (CDC13) 8: 1.28 (t, 3H, -OCH2CH3); 1.35 (t, 3H, -OCH2CH3); 2.30 (s, 3H, -CH 3); 3.82 (s, 2H, -CH2CO2C2H5); 4.18 (q, 2H, -OCH2CH3); 4.30 (q, 2H, -OCH2CH3); 5.95 (s, 1H, pyr-H); 8.92 (br s, 1H, NH) LRMS (m/z): 239 (M+), 193 (M+ - C2H5OH), 166 (M+ - CO2C2H5) EXACT MASS: calcd for C12H17NO4: 239.1158 found by high resolution mass spectroscopy: 239.1155 2-Benzyloxycarbonyl-3-benzyloxycarbonyImethyI-5-rnethylpyrrole (137) 201 2-Ethoxycarbonyl-3-ethoxycarbonylmethyl-5-methylpyrrole (136) (6.0 g, 25.2 mmoles) was dissolved under a nitrogen atmosphere in benzyl alcohol (150 mL). The solution was stirred vigorously and heated to reflux. A concentrated solution of sodium benzyloxide (prepared from freshly cut sodium in benzyl alcohol) was added in 1 mL portions to the refluxing mixture until no more evolution of ethanol vapors took place (a lowering of the reflux temperature was also indicative of completion of reaction). The hot solution was poured into a stirred solution of methanol (100 mL) and acetic acid (10 mL). Water was then added to the chilled mixture until crystallization occurred. The solid was collected by filtration, washed with 50% aqueous methanol, then with water and air dried to give the desired dibenzyl ester pyrrole as colorless crystals (7.78 g, 85%). M.P.: 92-93 °C 1H-NMR (CDC13) 8: 2.24 (s, 3H, -CH 3); 2.86 (s, 2H, -CH2CO2CH2C6H5); 5.08 (s, 2H, - C H 2 C 6 H 5 ) ; 5.21 (s, 2H, -CH2C6H5); 5.92 (s, 1H, pyr-H); 7.32 (m, 10H, 2 x - C 6 H 5 ) ; 8.70 (br s, 1H, NH) LRMS (m/z): 363 (M+), 228 (M+ - C O 2 C H 2 C 6 H 5 ) , 91 (C7H7+) EXACT MASS: calcd for C22H21NO4: 363.1471 found by high resolution mass spectroscopy: 363.1470 202 2-Benzyloxycarbonyl-3-methoxycarbonylmethyl-5-methylpyrrole (138) OMe 2-Benzyloxycarbonyl-3-benzyloxycarbonylmethyl-5-methylpyrrole (137) (6.0 g, 17 mmoles) was suspended in dry methanol (25 mL) in a 100 mL round-bottomed flask. The suspension was stirred under an argon atmosphere and THF (20 mL) was added to dissolve the pyrrole. A solution of sodium methoxide (63.3 mg of sodium in 5 mL of methanol) was added slowly to the stirred mixture at room temperature. The mixture was stirred until completion of reaction (as shown by tic). The reaction was quenched with acetic acid (10 mL) and the solution then concentrated on a rotary evaporator to a pale yellowish-brown oil. This oil was dissolved in a minimum of methanol and water added to afford a precipitate of tan crystals. The solid was recrystallized from ether-hexane to give the product as colorless crystals (4.30 g, 91%). M.P.: 82-84 °C 'H-NMR (CDC13) 6: 2.25 (s, 3H, -CH3); 3.65 (s, 3H, -CO2CH3); 3.82 (s, 2H, -CH2C02CH3); 5.30 (s, 2H, -CH2C6H5); 5.95 (s, 1H, pyr-H); 7.40 (m, 5H, -C6H5); 8.90 (br s, 1H, NH) LRMS (m/z): 287 (M+), 196 (M+ - C 7 H 7 + ) , 153 (M+ - CO2CH2C6H5 + H), 91 (C7H7+) 203 ANAL: calcd for C16H17NO4: C, 66.89; H, 5.96; N, 4.88 % found: C, 66.69; H, 5.95; N, 4.84 % 2-CarboxyI-3-methoxycarbonylmethyl-5-methylpyrrole (139) OMe 2-Benzyloxycarbonyl-3-methoxycarbonylmethyl-5-methylpyrrole (138) (4.0 g, 14 mmoles) in THF (400 mL) and triethylamine (0.1 mL) was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 0.4 g) until uptake of hydrogen was complete (about 1 hour). The solution was filtered through a pad of celite and the filtrate evaporated to dryness to give a pale yellow oil which was crystallized from THF-hexane to give the desired product as a white powder (2.6 g, 95%). M.P.: 190 °C (d) 'H-NMR (DMSO-d6) 8: 2.10 (s, 3H, -CH 3); 3.60 (s, 3H, -OCH3); 3.76 (s, 2H, -CH2CO2CH3); 5.80 (s, 1H, pyr-H); 11.22 (br s, 1H, -CO2H); NH too broad to be observed. LRMS (m/z): 153 (M+ - CO2), 94 (M+ - CO2 - CO2CH3) 204 EXACT MASS: calcd for CgHnNC^ (M+ - CO2): 153.0790 found by high resolution mass spectroscopy: 153.0791 3-MethoxycarbonyImethyl-5-methylpyrrole (105) OMe The foregoing acid pyrrole 139 (2.50 g, 12.7 mmoles) was decarboxylated by stirring in trifluoroacetic acid (20 mL) under a stream of nitrogen for 30 minutes (reaction complete as shown by tic). The trifluoroacetic acid was evaporated in vacuo and the oil was dissolved in dichloromethane (50 mL). The organic phase was washed twice with aqueous sodium bicarbonate and dried over anhydrous magnesium sulfate. The solution was filtered and evaporation of the filtrate yielded a yellow oil. This oil was purified by column chromatography (silica gel 60, dichloromethane eluant) to give the product as a colorless oil (1.85 g, 95%) which could not be induced to crystallize. 'H-NMR (CDCI3) 8: 2.30 (s, 3H, -CH 3); 3.50 (s, 2 H , -CH2CO2CH3); 3.70 (s, 3H, -CO2CH3); 5.85 (s, 1H, 4-H); 6.56 (s, 1H, 2-H); 7.86 (br s, 1H, NH) LRMS (m/z): 153 (M+), 94 (M+ - CC>2CH3) EXACT MASS: calcd for CgHnNC^: 153.0790 found by high resolution mass spectroscopy: 153.0796 205 2-Benzyloxycarbonyl-4-(2-hydroxyethyl)-3,5-dimethylpyrroIe (140) 2-Benzyloxycarbonyl-4-methoxycarbonylmemyl-3,5- (111) (24 g, 80 mmoles) was dissolved in dry THF (75 mL) in a 500-mL, two-necked, round-bottomed flask equipped with a magnetic stirring bar, nitrogen inlet tube and rubber septum and the solution cooled to 0 °C. Borane-THF complex (88 mL of a 1.0 M solution, 88 mmoles) was syringed in the solution and the mixture stirred at 0 °C for 15 minutes , then at room temperature for 2 hours (reaction completed as shown by dc). The reaction was quenched by slow addition of acetic acid (75 mL), followed by water (200 mL). The product was extracted into dichloromethane and the organic layer separated and evaporated to a pale yellowish-orange oil. The oil was dissolved in ethanol and addition of water gave the desired pyrrole as colorless crystals (19 g, 87%). M.P.: 119-120 °C (Lit: 120.0-121.5 ° C ) 1 0 1 206 1H-NMR (CDCI3) 8: 1.58 (s, 1H, -OH); 2.21 (s, 3H, -CH 3); 2.30 (s, 3H, -CH 3); 2.65 (t, 2H, -CH2CH2OH); 3.68 (t, 2H, -CH2CH2OH); 5.30 (s, 2H, -CH2C 6H 5); 7.38 (m, 5H, - C6H5); 8.85 (br s, 1H, NH) LRMS (m/z): 273 (M+), 256 (M+ - OH), 182 (M+ - CH2C6H5), 91 (C 7 H 7 + ) ANAL: calcd for CioHioNOs: C, 70.13; H, 7.01; N, 5.13 % found: C, 70.39; H.7.20; N, 5.23 % 4-(2-Acetoxyethyl)-2-benzyloxycarbonyl-3,5-dimethyIpyrrole (141) 2-Benzyloxycarbonyl-4-(2-hydroxyethyl)-3,5-dimethylpyrrole (140) (18.5 g, 68 mmoles) in pyridine (60 mL) was treated with anhydrous acetic anhydride (40 mL) and left stirring at room temperature overnight. The reaction mixture was poured in ice/water to precipitate the product as colorless crystals. The crystals were collected, dissolved in a minimum of ethanol and recrystallized by addition of water. The desired pyrrole was collected, washed with water and air dried (20.7 g, 97%). Ph M . P . : 74-75 °C (Lit. 73.5-74.5 ° C ) 1 0 1 207 1H-NMR (CDCI3) 5: 2.10 (s, 3H, -CH 3); 2.28 (s, 3H, -CH 3); 2.35 (s, 3H, -C(=0)CH3); 2.75 (t, 2H, -CH2CH20-); 4.10 (t, 2H, -CH2CH2O-); 5.35 (s, 2H, -CH2C6H5); 7.40 (m, 5H, -C6H5); 8.82 (br s, 1H, NH) LRMS (m/z): 315 (M+), 255 (M+ - CH3C02H), 91 (C 7 H 7 + ) EXACT MASS: calcd for C18H21NO4: 315.1471 found by high resolution mass spectroscopy: 315.1473 3.5 Synthesis of Dipyrromethanes and Dipyrromethenes 5,5'-Bis(benzyIoxycarbonyl)-3'-ethyl-3-methoxycarbonyImethyl-4,4'-dimethyI-2,2'-dipyrromethane (91) OCH A mixture of 5-acetoxymethyl-2-benzyloxycarbonyl-4-methoxycarbonylmethyl-3-methylpyrrole (95) (2.87 g, 8.00 mmoles) and 2-ethoxycarbonyl-4-ethyl-3-methylpyrrole (96) (2.05 g, 8.44 mmoles) in acetic acid (50 mL) was refluxed under nitrogen for 2 hours. The acetic acid was then removed under reduced pressure and the residual oil taken up in dichloromethane. The organic phase was washed with 5% sodium bicarbonate, water and dried over anhydrous magnesium sulfate. Filtration and evaporation of solvent under reduced 208 pressure gave a red oil which was purified by column chromatography (silica gel 60, dichloromethane eluant) to give the product as a colorless powder (3.51 g, 81%) upon evaporation of solvent. M.P.: 105- 106 'C 'H-NMR (CDC13) 8: 1.05 (t, 3H, -CH2CH3); 2.25 (s, 3H, -CH 3); 2.30 (s, 3H, -CH 3); 2.45 (q, 2H, -CH2CH3); 3.40 (s, 2H, -CH2C0 2 CH 3 ) ; 3.60 (s, 3H, -OCH3); 3.85 (s, 2H, methylene-H); 5.30 (s, 4H, 2 x -CH2C6H5); 7.30 (m, 10H, 2 x -C6H5); 8.60 (br s, 1H, NH); 9.20 (br s, 1H, NH) LRMS (m/z): 542 (M+), 451 (M + - C 7 H 7 + ) , 407 (M+ - CO2CH2C6H5), 91 (C7H7+) EXACT MASS: calcd for'C32H34N206: 542.2417 found by high resolution mass spectroscopy: 542.2418 3'-EthyI-4-methoxycarbonylmethyl-3,4',5-trimethyI-2,2'-dipyrromethene hydrobromide (93) 4-Ethyl-5-formyl-3-methylpyrrole (97) (3.01 g, 21 mmoles) and 2-carboxyl-4-methoxycarbonylmethyl-3,5-dimethylpyrrole (98) (4.22 g, 20 mmoles) were dissolved in THF (110 mL). The mixture was filtered to give a clear pale yellow solution and hydrobromic 209 acid (48%, 5 mL) was added. The color of the solution changed from yellow to dark brownish- yellow. The solvent was slowly removed under reduced pressure at 40 °C, adding methanol at regular intervals to replace the THF. The reaction mixture was cooled in an ice bath and the precipitated product filtered, washed with 10% methanol-ether, then ether and air dried (90%). M.P.: 160 °C (d) 1H-NMR (CDCI3) 8: 1.17 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.10 (s, 3H, -CH3); 2.35 (s, 3H, -CH 3 ); 2.70 (q, 2H, -CH2CH 3); 2.75 (s, 3H, -CH 3); 3.67 (s, 3H, -OCH3); 7.17 (s, 1H, -CH=); 7.57 (d, 1H, J = 4.0 Hz, pyr-H); 13.23 (br s, 2H, 2 x NH) LRMS (m/z): 286 (M+ - HBr), 271 (M+ - HBr - CH 3) EXACT MASS: calcd for C 1 7 H 2 2 N 2 O 2 (Ci7H23N202Br - HBr): 286.1681 found by high resolution mass spectroscopy: 286.1685 3,-EthyI-3-methoxycarbonylmethyl-4,5,-dimethyI-2,2*-dipyrromethene hydrobromide (104) H H 210 3-Ethyl-2-formyl-4-methylpyrrole (97) (1.66 g, 12.1 mmoles) and 3-methoxycarbonylmethyl-5-methylpyrrole (105) in methanol were stirred at 0 °C. Aqueous hydrobromic acid (48%, 2 mL) was added to the mixture and stirring continued for 30 minutes. The solution darkened and the product eventually crystallized out of solution. The crystals were collected, washed successively with cold methanol, ether, hexane and air dried (3.22 g, 75% in three crops). M.P.: 120-121 °C ' H - N M R (CDC13) 5: 1.22 (t, 3H, -CH2CH3); 2.15 (s, 3H, -CH 3); 2.76 (q, 2H, -CH2CH3); 2.80 (s, 3H, -CH 3); 3.80 (s, 3H, -OCH 3); 3.80 (s, 2H, -CH2CO2CH3); 6.40 (s, 1H, 4-H); 7.30 (s, 1H, meso-H); 7.62 (d, 1H, J = 3.0 Hz, 5-H); 13.50 (br s, 2H, 2 x NH) LRMS (m/z): 272 (M+ - HBr), 257 (M+ - HBr - CH 3), 243 (M+ - HBr - C 2 H 5 ) EXACT MASS: calcd for C16H20N2O2 (Ci6H2iN 2 0 2 Br - HBr): 272.1725 found by high resolution mass spectroscopy: 272.1528 Satisfactory microanalysis was not obtained for the compound. 4-(2-AcetoxyetHiyl)-3'-ethyl-3,4,^-trimethyl-2^2,-dipyrromethene hydrobromide (144) 211 2-Benzyloxycarbonyl-2-(2-acetoxyethyl)-3,5-dimethylpyiTole (141) (20 g, 63 mmoles) in THF (300 mL) and triethylamine (5 drops) was hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 2.0 g) until uptake of hydrogen was complete. The solution was filtered through a pad of celite and to the filtrate was added a solution of 4-ethyl-5-formyl-3-methylpyrrole (9.6 g, 70 mmoles) in THF (75 mL). The resulting solution was treated with hydrobromic acid (48%, 70 mL) and the THF was slowly replaced with methanol under reduced pressure until crystallization resulted. The orange crystals were collected, washed with cold methanol and air dried. A second crop of crystals was obtained and added to the first crop (total of 19 g, 79%). M.P.: 146 °C (d) 'H-NMR (CDCI3/TFA) 8: 1.18 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.00 (s, 3H, -CH3); 2.04 (s, 3H, -CH3); 2.35 (s, 3H, -C(=0)CH3); 2.65 (s, 3H, -CH 3); 2.75 (m, overlap, 4H, -CH2CH3 and -CH2CH2O-); 4.10 (t, 2H, J = 6.5 Hz, -CH2CH2O-); 6.80 (s, 1H, -CH=); 7.45 (d, 1H, J = 3.2 Hz, pyr-H); 12.80 (br s, 1H, NH); 13.00 (br s, 1H, NH) ANAL: calcd for Ci9H27N2Br02: C, 57.22; H, 6.88; N, 7.09; Br, 20.91 % found: C, 57.68; H, 6.85; N, 7.05 % 212 4-(2-Acetoxyethyl)-5'-bromo-5-bromomethyl-3,-ethyl.3,4'-dimethyl-2,2' dipyrromethene hydrobromide (145) 4-(2-Acetoxyethyl)-3'-ethyl-l3,4\5-trim hydrobromide (144) (1.5 g, 4 mmoles), trifluoroacetic acid (1.5 mL) and bromine (1.9 g, 3 equivalents) were stirred in 1,2-dichloroethane (12 mL) at room temperature with exclusion of moisture for 1 week. The solution was cooled and ether (20 mL) added slowly with stirring until crystallization of the product took place. The crystals were collected, washed with ether and redissolved in dichloromethane (70 mL), along with cyclohexene (6 mL). The dichloromethane was replaced by ethyl acetate under reduced pressure until reddish crystals resulted. The solution was cooled and the crystals collected, washed with ethyl acetate, then hexane and air dried (1.81 g, 85%). M.P.: 175 °C (d) 1H-NMR (CDCI3/TFA) 8: 1.25 (t, 3H, J = 7.5 Hz, -CH2CH3); 2.10 (s, 3H, -CH3); 2.18 (s, 3H, -C(=0)CH3); 2.40 (s, 3H, -CH 3); 2.75 (q, 2H, J = 7.5 Hz, -CH2CH3); 3.00 (t, 2H, J = 6.5 Hz, -CH2CH2O-); 4.50 (t, 2H, J = 6.5 Hz, -CH2CH2O-); 4.95 (s, 2H, -CH2Br); 7.20 (s, 1H, -CH=); 13.00 (br d, 2H, 2 x NH) ANAL: calcd for C18H23N2B13O2: C, 40.10; H, 4.30; N, 5.20; Br, 44.47 % found: C, 39.45; H, 4.15; N, 4.85 % 213 S'-Bromo-S'^-diethyl-S^SS-trimethyl^^'-dipyrromethene perbromide (143) 2-(Tert-butyloxycarbonyl)-4-ethyl-3,5-dimethylpyrrole (99) (35.7 g, 0.16 moles) in acetic acid (250 mL) was treated at room temperature over 5 minutes with a steady flow of a bromine solution (76.8 g, 0.48 moles in 100 mL of acetic acid) from a dropping funnel. The stirred reaction mixture warmed up and darkened to a bluish-purple color with evolution of fumes (CO2, HBr). A reflux condenser was then attached to the reaction flask and the mixture stirred for another 30 minutes with cooling. The product (blue crystals) was cooled in an ice bath and collected by filtration. The solid was washed with acetic acid (100 mL), ethyl acetate (100 mL), n-hexane (100 mL) and dried in a desiccator over KOH to give the perbromide (35.4 g, 79% in two crops). M.P.: 185 'C (d) 'H-NMR (CDCI3) 8:1.10 (t, 3H, -CH2CH3); 1.20 (t, 3H, -CH2CH3); 2.05 (s, 3H, -CH 3); 2.30 (s, 3H, -CH 3); 2.48 (q, 2H, -CH2CH3); 2.72 (s, 3H, -CH 3); 2.73 (q, 2H, -CH2CH3); 7.06 (s, 1H, meso-H); 13.10 (br s, 2H, 2 x NH) H Br3" H LRMS (m/z): 322 (M+ - HBr - Br2), 320 (M+- HBr - BT2) Satisfactory microanalysis was not obtained for the compound. 214 5'-Bromo-5-(bromomethyl)-3',4-diethyI-3,4'-dimethyl-2,2'-dipyrromethene hydrobromide (94) Dipyrromethene perbromide (143) (33.1 g, 59 mmoles), trifluoroacetic acid (30 mL) and bromine (14.14 g, 1.5 equivalents) were stirred in dichloromethane (100 mL) at room temperature with exclusion of moisture for 5 days. The solvent and excess bromine were removed under reduced pressure at 40 °C. The residue was dissolved in dichloromethane (50 mL) and treated with cyclohexene (5 mL) and concentrated in vacuo. Addition of ether to the cooled concentrate resulted in precipitation of the desired product. The dark, shiny red-orange crystals were collected by filtration, washed with ether and air dried (24.8 g, 87%). M.P.: 170 °C (d) 'H-NMR (CDC13) 8: 1.20 (2t, overlap, 3H each, 2 x -CH2CH3); 2.08 (s, 3H, -CH 3); 2.32 (s, 3H, -CH 3); 2.54 (q, 2H,-CH2CH3); 2.74 (q, 2H, -CH2CH3); 4.86 (s, 2H, -CH2Br); 7.16 (s, 1H, -CH=); 13.76, 13.81 (2 br s, 1H each, 2 x NH) LRMS (m/z): 402 (M+ - HBr), 400 (M+ - HBr), 398 (M+ - HBr) ANAL: calcd for Ci6H 2 iN 2 Br 3 : C, 39.95; H, 4.40; N, 5.82 % found: C, 39.82; H, 4.34; N, 5.60 % 215 3.6 Synthesis of a,c-Biladienes 2,12,18-Triethyl-8-methoxycarbonylmethyl-l,3,7,13,17,19-hexamethyl-a,c-biladiene dihydrobromide (89) o The dipyrromethane >1 (2.10 g, 3.87 mmoles) was dissolved in THF (100 mL) and hydrogenated at room temperature and atmospheric pressure over palladized charcoal (10%, 0.217 g) until uptake of hydrogen ceased. The catalyst was removed by filtration through celite and the filtrate evaporated to dryness to afford the diacid as a colorless solid, which was used without further purification. The diacid was stirred in trifluoroacetic acid (10 mL) under nitrogen and then treated with the formylpyrrole 92 (1.20 g, 2.0 equivalents) in methanol (30 mL) along with hydrobromic acid (31% in acetic acid, 6 mL). The mixture was stirred for 30 minutes and ether was then added to the cooled mixture (ice-water bath) until a,c-biladiene precipitation was complete. The product was filtered off, washed with cold ether and air dried (1.65 g, 61%). 216 M.P.: 210 °C (d) 1H-NMR (CDCI3) 8: 0.70 (t, 3H, -CH2CH3); 1.10 (2t, overlap, 6H, 2 x -CH2CH3); 2.24, 2.26, 2.31, 2.32 (4s, 3H each, 4 x -CH3); 2.45 (2q, overlap, 4H, 2 x -CH2CH3); 2.55 (q, 2H, -CH2CH3); 2.74, 2.75 (2s, 3H each, 2 x -CH 3); 3.30 (s, 3H, -OCH3); 3.79 (s, 2H, - CH2CO2CH3); 5.20 (s, 2H, methylene-H); 7.10, 7.15 (2s, 2H, 2 x -CH=); 13.12, 13.23, 13.37, 13.40 (4s, 1H each, 4 x NH) . LRMS (m/z): 540 (M+ - 2HBr) EXACT MASS: calcd for C34H44N4O2 (C34H46N402Br2 - 2HBr): 540.3464 found by high resolution mass spectroscopy: 540.3460 l-Bromo-3,8,13-triethyl-18-methoxycarbonylmethyl-2,7,12,17,19-pentamethyl-a,c-biladiene dihydrobromide (90) 217 3'-Eftyl-4-meftoxycarbonyl^ hydrobromide (93) (917 mg, 2.5 mmoles) and S'-bromo-S^bromomethylJ-S'Adiethyl-S^'-dimethyl^^'-dipyrromethene hydrobromide (94) (1.202 g, 2.5 mmoles) were dissolved in dichloromethane (200 mL) and stannic chloride (4 mL) added. The mixture was stirred for 1.5 hours with exclusion of moisture (CaCh tube), and then quenched with aqueous hydrobromic acid (4 mL of 48% HBr in 50 mL of water). The organic phase was poured into a mixture of methanol (100 mL) and hydrobromic acid (48%, 25 mL). The dichloromethane was removed in vacuo at 40 °C and the remaining solution left to cool to room temperature, then to 0 °C. The desired a,c-biladiene, a brown powder, was collected by filtration, washed with ethyl acetate, n-hexane and air dried (1.92 g, 99%). M.P.: 190 °C (d) 'H-NMR ( C D C 1 3 ) 8: 0.67, 1.12, 1.22 (3t, 3H each, 3 x - C H 2 C H 3 ) ; 1.98, 2.09, 2.29, 2.35 (4s, 3H each, 4 x -CH 3); 2.52 (q, 2H, - C H 2 C H 3 ) ; 2.64 (q, 2H, - C H 2 C H 3 ) ; 2.75 (s, overlap, 3H, -CH 3); 2.75 (q, overlap, 2H, - C H 2 C H 3 ) ; 3.48 (s, 2H, - C H 2 C O 2 C H 3 ) ; 3.72 (s, 3H, - O C H 3 ) ; 5.25 (s, 2H, -CH 2-); 7.12, 7.15 (2s, 1H each, 2 x -CH=); 13.35, 13.52, 13.70, 13.92 (4 br s, 1H each, 4 x NH) LRMS (m/z): 522 (M+ - H 5Br 3) EXACT MASS: calcd for-C33H38N4O2 (C33H43N402Br3 - H 5Br 3): 522.2995 found by high resolution mass spectroscopy: 522.2996 218 l-Bromo-3,8,18-triethyl-17-methoxycarbonylinethyl-2,7,12,19-tetraniethyl-5,15-biladienium dibromide (103) 3'-Eriiyl-3-methoxycarbonylmemyl-4\5-dimet^^ hydrobromide (104) (1.26 g, 3.56 mmoles) and 5,-b^omo-5-(bromomethyl)-3,,4-diethyl-3,4,-dimethyl-2,2,-dipyrromethene hydrobromide (94) (1.72 g, 3.56 mmoles) in dichloromethane (200 mL) were treated with stannic chloride (5 mL) for 60 minutes at room temperature with exclusion of moisture. The reaction mixture was quenched with hydrobromic acid (48%, 4 mL) in water (50 mL). The organic phase was separated and added to a solution of hydrobromic acid (48%, 25 mL) in methanol (100 mL). The resulting solution was concentrated on a rotary evaporator until a thick slurry of the product formed. The slurry was allowed to cool to room temperature, then in ice, and the crystals collected. These were washed with ethyl acetate, then hexane and air dried to give the a,c-biladiene as dark reddish-brown crystals (2.41 g, 90%). M.P.: 182 °C (d) 'H-NMR (CDC13) 8: 0.75, 0.92, 1.14 (3t, 3H each, 3 x -CH2CH3); 1.99, 2.30, 2.31 (3s, 3H each, 3 x -CH 3); 3.50 (q, 2H, -CH2CH3); 2.65 (s, 3H, -CH 3); 2.75 (2q, overlap, 2H 219 each, 2 x -CE2CH3); 3.59 (s, 3H, -OCH3); 3.95 (s, 2H, -CH2CO2CH3); 5.18 (s, 2H, - C H 2 -); 7.14, 7.15 (2s, 1H each, 2 x -CH=); 7.32 (s, 1H, P-H); 13.57, 13.65, 13.73, 13.79 (4 br s, 1H each, 4 x NH) LRMS (m/z): 508 (M+ - H 5Br 3) EXACT MASS: calcd for C 3 2 H 3 6 N 4 O 2 (C32H4iN402Br3 - H 5Br 3): 508.2828 found by high resolution mass spectroscopy: 508.2838 l-Bromo-8,18-bis(2-acetoxyethyl)-3,13-diethyI-2,7,12,17,19-pentamethyl-a,c-biladiene dihydrobrornide (153) 4-(2-Acetoxyethyl)-3'-ethyl-3,4\5-trimethyl-2,2,-mpyrromethene hydrobromide (144) (0.9 g, 2.4 mmoles) and 4-(2-acetoxyethyl)-5,-bromo-5-bromomethyl-3'-etliyl-3,4l-(limethyl-2,2'-dipyrromethene hydrobromide (145) (1.3 g, 2.4 mmoles) in dichloromethane (200 mL) were treated with SnCU (4 mL). The mixture was stirred for 90 minutes at room temperature with exclusion of moisture. The reaction mixture was then quenched with hydrobromic acid (4 mL of a 48% solution in 50 mL of water). The organic phase was separated and poured in a 220 mixture of methanol (100 mL) and hydrobromic acid (48%, 25 mL). The dichloromethane was removed under reduced pressure at 40 °C until a thick slurry of product resulted.The slurry was cooled, the crystals collected, washed with ethyl acetate, then hexane and air dried (1.60 g, 79%). M.P.: 195 °C (d) 1H-NMR (CDC13) 5: 1.20 (2t, overlap, 6H, 2 x -CH2CH3); 2.15 (s, 3H, -CH 3); 2.16 (s, 3H, -CH 3); 2.37 (s, 6H, 2 x -C(=0)CH3); 2.60-3.00 (m, overlap, 4H, 2 x -CH2CH3); 2.65 (s, 3H, -CH 3); 2.68 (s, 3H, -CH 3); 2.70 (s, 3H, -CH 3); 3.80 (2t, overlap, 4H, 2 x -CE2CH2O-); 4.40 (2t, overlap, 4H, 2 x -CH2CH2O-); 4.50 (s, 2H, =C-CH2-C=); 7.20 (s, 1H, =CH-); 7.26 (s, 1H, =CH-); 11.70-12.60 (4s, 1H each, 4 x NH) LRMS (m/z): 594 (M+ - H 5Br 3) EXACT MASS: calcd for C 3 6 H4 2 N 4 04 (C36H47N404Br3 - H 5Br 3): 594.3206 found by high resolution mass spectroscopy: 594.3208 Satisfactory microanalysis was not obtained for the compound. 221 3.7 Synthesis of Porphyrins 7,3,17-Triethyl-3-methoxycarbonylmethyl-2,8,12,18-tetramethylporphyrin (88) via the dipyrromethane route o The a,c-biladiene 89 (520 mg, 0.740 mmoles) was added to a solution of copper (U) chloride (2.6 g) in DMF (40 mL) previously heated to 145 °C and stirred for 4 minutes. The mixture was then poured into water and, after cooling, extracted with dichloromethane (3 x 100 mL). The combined organic phases were washed with water (3 x 100 mL), dried (anhydrous MgSC«4), filtered and the filtrate evaporated to give a residue which was treated with sulfuric acid (1 mL) in trifluoroacetic acid (19 mL). The solution was stirred at room temperature for 1 hour under nitrogen and was poured into water and extracted into dichloromethane. The organic phase was washed with aqueous saturated sodium bicarbonate, water, dried (MgS04), filtered and the filtrate evaporated to dryness. The resulting solid after demetalation was treated with 5% sulfuric acid in methanol (50 mL) and stirred at room temperature overnight It was then poured into aqueous sodium acetate (100 mL) and extracted into dichloromethane. The extract was washed with aqueous sodium bicarbonate, water, dried (MgS04), filtered and the filtrate evaporated to dryness. The final residue was chromatographed (silica gel 60, dichloromethane eluant) to give the desired porphyrin (90 mg, 23%). 222 M.P.: 287-288 'C 'H-NMR (CDCI3) 8: -3.77 (br s, 2H, 2 x NH); 1.86 (3t, overlap, 9H, 3 x -CH2CH3); 3.60 (s, 3H, -OCH3); 3.65 (s, 9H, 3 x -CH 3); 3.75 (s, 3H, -CH3); 4.10 (2q, 6H, 3 x -CH2CH3); 5.04 (s, 2H, -CH2CO2CH3); 10.06 (s, 2H, 2 x meso-H); 10.09, 10.13 (2s, 2H, 2 x meso-H) LRMS (m/z): 522 (M+), 463 (M+ - CO2CH3) EXACT MASS: calcd for C33H 3 8N402: 522.2995 found by high resolution mass spectroscopy: 522.2997 VISIBLE SPECTRUM (CH2C12): W(nm): 400 498 534 568 620 peak ratio: 19.2 2.05 1.60 1.07 1.00 8,13,18-Triethyl-3-methoxycarbonyImethyl-2,7,12,17-tetramethylporphyrin (88) via the dipyrromethene route 223 1 -Bromo-3,8,13-triethyl-18-methoxycarbonylmethyl-2,7,12,17,19-pentamethyl-a,c-biladiene dihydrobroniide (90) (1.60 g, 2.08 mmoles) dissolved in DMSO (200 mL) was treated with pyridine (10 mL) and allowed to stand at room temperature for 5 days in the dark. The porphyrin crystals were collected by filtration, washed with methanol and air dried (0.89g, 82% in two crops). M.P.: 289-290 'C *H-NMR ( C D C 1 3 ) 8: -3.73 (br s, 2H, 2 x NH); 1.86 (3t, 3H each, 3 x -CH2CH3); 3.60 (s, 3H, - C H 3 ) ; 3.65 (s, 9H, 3 x - C H 3 ) ; 3.76 (s, 3H, - O C H 3 ) ; 4.03-4.14 (3q, overlap, 2H each, 3 x - C H 2 C H 3 ) ; 5.05 (s, 2H, - C H 2 C 0 2 C H 3 ) ; 10.07 (s, 2H, 2 x meso-H); 11.01, 11.04 (2s, 1H each, 2 x meso-H) LRMS (m/z): 522 (M+), 507 (M+ - CH 3), 463 (M+ - C O 2 C H 3 ) ANAL: calcd for C33H38N4O2: C, 75.83; H, 7.33; N, 10.72 % found: C, 75.67; H, 7.39; N, 10.68 % VISIBLE SPECTRUM ( C H 2 C I 2 ) : AmaxOim): 400 498 534 568 620 peak ratio: 18.5 2.00 1.57 1.08 1.00 8,13,18-Triethyl-3-(2.hydroxyethyl)-2,7,12,17-tetramethyIporphyrin (146) A saturated solution of zinc acetate in methanol (2 mL) was added to a solution of ester porphyrin 88 (200 mg, 0.38 mmoles) in dichloromethane (60 mL). The mixture was stirred at room temperature and the progress of the reaction followed by U.V-vis spectroscopy until the four band (Q) spectrum became a two band spectrum. Dichloromethane (100 mL) was added, the organic phase washed twice with water and the solvent removed to afford the zinc complex (0.20 g, 90%) which was used without further purification. Into a 250-mL, two-necked, round-bottomed flask equipped with a magnetic stirring bar, an argon gas inlet and a 100-mL addition funnel was placed lithium aluminium hydride (49.0 mg, 1.28 mmoles) and THF (30 mL). The slurry was cooled to 0 °C and a solution of the zinc complex (150 mg, 0.26 mmoles) in THF (20 mL) was added dropwise. The reaction mixture was allowed to warm up to room temperature and stirred for 2 hours (reaction complete as shown by tic). Cold water was added to quench the reaction (H 2 evolved). The mixture was extracted with dichloromethane and the organic phase shaken with trifluoroacetic acid (3 mL) to demetalate the zinc complex. The organic phase was then washed twice with 5% sodium bicarbonate, twice with water and dried over anhydrous magnesium sulfate. Filtration and evaporation of the filtrate to dryness in vacuo afforded the desired porphyrin (122 mg, 95%). A sample was recrystallized from chloroform-ether to give fine purple crystals. M.P.: 275 °C (d) 'H-NMR (CDCI3) 6: -3.76 (br s, 2H, 2 x NH); 1.87 (3t, overlap, 9H, 3 x -CH2CH3); 3.61, 3.64, 3.65, 3.66 (4s, 3H each, 4 x -CH3); 4.10 (3q, overlap, 6H, 3 x -CH2CH3); 4.35 (t, 2H, -CH2CH2OH); 4.48 (t, 2H, -CH2CH2OH); 10.08 (s, 1H, meso-H); 10.09 (s, 2H, 2 x meso-H); 11.01 (s, 1H, meso-H) LRMS (m/z): 494 (M+), 477 (M+ - OH), 463 (M+ - CH 2OH) EXACT MASS: calcd for C32H38N4O: 494.3046 found by high resolution mass spectroscopy: 494.3051 VISIBLE SPECTRUM (CH2C12): Xmax(nm): 400 498 532 566 620 peak ratio: 25.2 2.78 2.24 1.65 1.00 8,13,18-Triethyl-2,7,12,17-tetramethyl-3-(2-tosyloxyethyI)porphyrin ITS 226 p-Tolucnesulfonyl chloride (0.1285 g, 0.674 mmoles) was added to a solution of 8,13,18-triemyl-3-(2-hyfroxyethyl)-2,7,12,17-tett^ (146) (50 mg, 0.1012 mmoles) in pyridine (15 mL) and the mixture was left stirring at room temperature overnight. Tic showed the reaction was incomplete. More p-toluenesulfonyl chloride (50 mg) was added to the mixture and the reaction monitored by dc every 2 hours. The reaction was stopped when the tic showed formation of a new reaction product in addition to the tosylate porphyrin after 8 hours. The reaction mixture was quenched with ice and 1.0 N HC1, and then extracted into dichloromethane. The organic phase was washed with 5% aqueous sodium bicarbonate (3 x 50 mL), water (2 x 100 mL) and dried over anhydrous magnesium sulfate. Filtration, evaporation of the filtrate and chromatography of the crude product (silica gel 60, dichloromethane eluant) gave the tosylate as the major product (40.7 mg, 62%). A by-product (8 mg) was collected as the faster running fraction. M.P.: 265 °C (d) 1H-NMR (CDC13) 6: -3.82 (br s, 2H, 2 x NH); 1.90 (s, overlap, 3H, -C6H5-CH3); 1.90 (3t, overlap, 3H each, 3 x -CH2CH3); 3.54, 3.61, 3.63, 3.67 (4s, 3H each, 4 x -CH3); 4.10 (3q, overlap, 2H each, 3 x -CH2CH3); 4.42 (t, 2H, -CH2CH2O-); 4.87 (t, 2H, -CH2CH2O-); 6.01 (d, 2H, Ar-H); 7.13 (d, 2H, Ar-H); 9.88, 10.02, 10.10, 10.11 (4s, 1H each, 4 x meso-H) LRMS (m/z): 648 (M+), 633 (M+ - CH 3) EXACT MASS: calcd for C 3 9 H 4 4 N 4 O 3 S : 648.3134 found by high resolution mass spectroscopy: 648.3129 227 VISIBLE SPECTRUM (CH 2 C1 2 ): XmaxCnm): 398 498 534 568 620 peak ratio: 19.32 2.09 1.66 1.30 1.00 3-(2-ChIoroethyl)-8,13,18-triethyl-2,7,12,17-tetramethylporphyrin (148) A mixture of 8,13,18-triethyl-3-(2-hydroxyethyl)-2J,12,17-tetramethylporphyrin (146) (200 mg, 0.405 mmoles), dichloromethane (100 mL), DMF (20 mL) and potassium carbonate (6 g) was stirred at room temperature. Thionyl chloride (6 mL) was added and stirring continued for 3 hours (completion of reaction shown by tic). The reaction rnixture was quenched with slow addition of water until effervescence ceased. The organic phase was washed four times with sodium bicarbonate, twice with water and dried over anhydrous magnesium sulfate. Filtration and removal of solvent under reduced pressure gave a purplish-brown solid which was purified by chromatography (silica gel 70-230 mesh, dichloromethane eluant). Recrystallization from chloroform-methanol gave the product as red crystals (180.4 mg, 86%). M.P.: 250 °C (d) 228 'H-NMR (CDCI3) 8: -3.75 (br s, 2H, 2 x NH); 1.88 (2t, overlap, 9H, 3 x -CH2CH3); 3.61 (s, 3H, -CH 3); 3.66 (s, 9H> 3 x -CH 3); 4.04-4.15 (3q, overlap, 6H, 3 x -CH2CH3); 4.32 (t, 2H, -CH2CH2CI); 4.52 (t, 2H, -CH2CH2CI); 10.01, 10.08, 10.09, 10.11 (4s, 1H each, 4 x meso-H) LRMS (m/z): 512 (M+), 497 (M+ - CH 3), 478 (M+ - Cl + H), 463 (M+ - CH2CI) . EXACT MASS: calcd for C32H3 7N4C1: 512.2707 found by high resolution mass spectroscopy: 512.2715 VISIBLE SPECTRUM (CH2C12): XmaxCnm): 398 498 534 566 620 peak ratio: 24.1 2.35 1.82 1.35 1.00 8,13,18-TriethyI-2,7,12,17-tetramethyl-3-vinylporphyrin (81) 229 3-(2-Chloroemyl)-8,13,18-trieftyl-2,7,12,17-tetr^ (148) (150 mg, 0.293 mmoles) was dissolved in pyridine (100 mL) and the resulting mixture was refluxed under nitrogen for 30 minutes. To this mixture was added water (5 mL) and refluxing was continued for 5 minutes. An aqueous solution of sodium hydroxide (10%, 10 mL) was then added to the boiling mixture and reflux maintained for an additional 2 hours (completion of reaction shown by dc). Aqueous acetic acid (50%, 40 mL) was added and the mixture extracted with dichloromethane (200 mL). The organic layer was washed with 5% aqueous sodium bicarbonate (3 x 200 mL), followed by water (2 x 200 mL), dried over anhydrous magnesium sulfate, filtered and evaporated to dryness. The residue was chromatographed (silica gel 60, dichloromethane eluant) to give a pink residue after solvent evaporation. Recrystallization from chloroform-methanol gave the monovinylporphyrin as purple prisms (121.3 mg, 87%). M.P.: 220 °C (d) !H-NMR ( C D C 1 3 ) 8: -3.68 (br s, 2H, 2 x NH); 1.90 (3t, overlap, 3H each, 3 x - C H 2 C H 3 ) ; 3.63, 3.66, 3.68, 3.74 (4s, 3H each, 4 x -CH 3); 4.10 (3q, overlap, 2H each, 3 x - C H 2 C H 3 ) ; 6.15 (dd, 1H, J = 1.5, 11.5 Hz, -CH=CH 2); 6.36 (dd, 1H, J = 1.5, 17.5 Hz, -CH=CH2); 8.32 (dd, 1H, J = 11.5, 17.5 Hz, -CH=CH2); 10.07 (s, 2H, 2 x meso-H); 10.15, 10.23 (2s, 1H each, 2 x meso-H) LRMS (m/z): 476 (M+), 461 (M+ - CH 3), 446 (M+ - 2 x CH 3) EXACT MASS: calcd for C 32H 3 6N4: 476.2940 found by high resolution mass spectroscopy: 476.2938 ANAL: calcd for C 3 2H 36N4: C, 80.63; H, 7.61; N, 11.76 % found: C, 80.54; H, 7.63; N, 11.74 % 230 VISIBLE SPECTRUM (CH2C12): XmaxCnm): 400 502 538 570 624 peak ratio: 18.6 2.04 1.93 1.40 1.00 7,12,17-TriethyI-3-methoxycarbonylmethyl-8,13,18-trimethylporphyrin (102) The biladiene 103 (2.00 g, 2.66 mmoles) was dissolved in DMSO (300 mL) and pyridine (10 mL) was added to the solution. The reaction mixture was kept for 5 days at room temperature in the dark to cyclize to the corresponding porphyrin. Water was added to the mixture and the purple needles were collected on a Biichner funnel. The porphyrin was chromatographed (silica gel 60, dichloromethane eluant) to give a purple solid upon solvent evaporation. The porphyrin was recrystallized from chloroform-methanol, collected and air dried to afford 1.12 g (83%) of the pure product as purple needles. M.P.: 228-230 °C 231 'H-NMR (CDCI3) 8: -3.88 (br s, 2H, 2 x NH); 1.85 (3t, overlap, 9H, J = 7.5 Hz, 3 x -CH2CH3); 3.55, 3.58, 3.62 (3s, 9H, 3 x -CH 3); 3.85 (s, 3H, -OCH3), (3q, overlap, 6H, J = 7.5 Hz, 3 x -CH2CH3); 5.16 (s, 2H, -CH2CO2CH3); 9.24 (s, 1H, Ar-H); 10.00 - 10.12 (4s, 4H, 4 x meso-H) LRMS (m/z): 508 (M+), 493 (M+ CH 3), 478 (M+ - 2x CH3), 449 (M+ - CO2CH3) ANAL: calcd for C 3 2 H 3 6 N 4 O 2 : C, 75.56; H, 7.13; N, 11.01; O, 6,30 % found: C, 75.40; H, 7.25; N, 10.86 % VISIBLE SPECTRUM (CH2C12): W(nm): 398 498 534 566 620 peak ratio: 29.9 3.00 2.37 1.80 1.00 7,12,17-Triethyl-3-(2-hydroxyethyl)-8,13,18-trimethylporphyrin (149) A saturated solution of zinc acetate (3 mL) was added to the ester porphyrin 102 (250 mg, 0.492 mmoles) dissolved in dichloromethane (50 mL). The reaction was stirred at room temperature and monitored by U.V-vis spectroscopy. Complete conversion to the 232 metaUoporphyrin (4 band spectrum becoming 2 band spectrum) was observed after 2 hours. The dichloromethane was then washed twice with water, then dried over anhydrous magnesium sulfate. Filtration and evaporation of the solvent gave a pinkish-red sotid which was dried in vacuo overnight before the reduction step. A 250-mL, two-necked flask equipped with a pressure-equalizing addition funnel, magnetic stirring bar and nitrogen inlet was flushed with dry nitrogen and charged with THF (40 mL). Lithium aluminium hydride (0.1500 g) was then added to the THF and the slurry cooled in an ice-water bath. A solution of the metaUoporphyrin (obtained from above) in THF (30 mL) was added dropwise with stirring. An extra 10 mL of THF was used to rinse the addition funnel. The reaction mixture was stirred for 2 hours at room temperature (completion of reaction shown by tic). The mixture was cooled to 0 °C and quenched by addition of ice. Water was added and the mixture was extracted into dichloromethane. The zinc (II) porphyrin complex was demetalated by addition of trifluoroacetic acid (5 mL). The demetalated porphyrin in dichloromethane was then washed twice with 5% sodium bicarbonate and once with water. The organic phase was dried over anhydrous magnesium sulfate, filtered and the filtrate evaporated in vacuo to provide the desired porphyrin as purple crystals (205 mg, 95%). M.P.: 273-274 °C 1H-NMR (CDC13) 5: -3.75 (br s, 2H, 2 x NH); 1.88 (2t, overlap, 9H, 3 x -CH2CH3); 3.62, 3.65, 3.69 (3s, 9H, 3 x -CH 3); 4.10 (3q, 6H, 3 x -CH2CH3); 4.45 (t, 2H, -CH2CH2OH); 4.62 (t, 2H, -CH2CH2OH); 9.19 (s, 1H, Ar-H); 10.08-10.16 (4s, 4H, 4 x meso-H) LRMS (m/z): 480 (M+), 465 (M+ - CH3), 449 (M+ - CH 2OH) EXACT MASS: calcd for C31H36N4O: 480.2889 found by high resolution mass spectroscopy: 480.2892 233 VISIBLE SPECTRUM (CH2C12): Amaxfom) : 400 498 532 568 620 peak ratio: 28.17 2.29 2.05 1.78 1.00 3-(2-ChIoroethyl)-7,12,17-triethyl-8,13,18.trimethylporphyrin (150) Cl A mixture of 7,12J7-triemyl-3-(2-hyoVoxyethyl)-843,18-trimethylpo (149) (200 mg, 0.418 mmoles), dichloromethane (100 mL), DMF (20 mL) and potassium carbonate (6 g) was stirred at room temperature. Thionyl chloride (6 mL) was added and stirring continued for 2 hours (completion of reaction shown by tic). The reaction mixture was quenched with slow addition of water (effervescence). The organic phase was washed four times with sodium bicarbonate, twice with water and dried over anhydrous magnesium sulfate. Filtration and removal of solvent at reduced pressure gave a purplish-brown solid which was purified by chromatography (silica gel 60, dichloromethane eluant) to afford the chloroporphyrin (198 mg, 95%). M.P.: 230-232 °C 234 'H-NMR (CDCI3) 5: -3.80 (br s, 2H, 2 x NH); 1.08 (3t, overlap, 9H, 3 x -CH2CH3); 3.61, 3.65, 3.69 (3s, 9H, 3 x -CH3); 4.02-4.17 (3q, overlap, 6H, 3 x -CH2CH3); 4.51 (t, 2H, -CEL2CH2CI); 4.67 (t, 2H, -CH2CH2CI); 9.20 (s, 1H, Ar-H); 10.10-10.14 (4s, 4H, 4 x meso-H) LRMS (m/z): 498 (M+), 483 (M+ - CH3), 462 (M+ - HC1) • EXACT MASS: calcd for C31H35N4CI: 498.2550 found by high resolution mass spectroscopy: 498.2552 VISIBLE SPECTRUM (CH2C12): Xmax(nm): 400 502 542 570 626 peak ratio: 30.1 2.52 2.42 1.73 1.00 7,12,17-Triethyl-8,13,18-trimethyl-3-vinylporphyrin (101) 235 A solution of 3-(2-cWoroemyl)-7,1247-triethyl-8J3,18-trim (150) (195 mg, 0.390 mmoles) was reflux ed under nitrogen for a few minutes and water (10 mL) added cautiously. An aqueous solution of sodium hydroxide (10%, 10 mL) was added after 5 minutes and the mixture refluxed for 3 hours (completion of reaction shown by tic). Aqueous acetic acid (50%, 40 mL) was added and the mixture extracted with dichloromethane (200 mL). The organic layer was washed with 5% aqueous sodium bicarbonate (3 x 200 mL), followed by water (2 x 200 mL), dried over anhydrous magnesium sulfate and evaporated to dryness. The residue was chromatographed (silica gel 60, dichloromethane eluant) to give a pink residue after solvent evaporation. This residue was recrystallized from dichloromethane-methanol to give the desired vinylporphyrin (158 mg, 88%). M.P.: 221-222 °C 1H-NMR (CDC13) 8: -3.66 (s, 2H, 2 x NH); 1.90 (2t, overlap, 9H, 3 x - C H 2 C H 3 ) ; 3.60, 3.64, 3.66 (3s, 9H, 3 x - C H 3 ) ; 4.15 (2q, overlap, 6H, 3 x - C H 2 C H 3 ) ; 6.40 (d, 1H, J = 12.0 Hz, -CH=CH2); 6.63 (d, 1H, J = 17.0 Hz, - C H = C H 2 ) ; 8.47 (dd, 1H, J = 12.0, 17.0 Hz, -CH=CH2); 9.44 (s, 1H, Ar-H); 10.06, 10.07, 10.10, 10.26 (4s, 4H, 4 x meso-H) LRMS (m/z): 462 (M+), 447 (M+ - CH 3), 432 (M+ - 2 x CH 3) EXACT MASS: calcd for C31H34N4: 462.2783 found by high resolution mass spectroscopy: 462.2784 VISIBLE SPECTRUM (CH2C12): Xmax (nm): peak ratio: 402 504 544 570 630 22.35 2.27 2.51 1.69 1.00 236 8,18-diethyl-3,13-Bis(methoxycarbonylmethyl)-2,7,12,17-tetramethylporphyrin (108) via Fischer's route. OCH3 3'-Emyl-4-memoxycarbonylnremyl-3,3\5-trm hydrobromide (93) (1.75 g, 4.7 mmoles) was suspended in anhydrous formic acid (35 mL) in a 100-mL round-bottomed flask. Bromine (1.52 g, 2.0 equivalents) was added slowly to the stirred suspension and the mixture re fluxed for 2.5 hours. The condenser was then removed and the reaction mixture boiled to dryness within one hour. The residue was dissolved in methanol (80 mL) containing concentrated sulfuric acid (5 mL), treated with trimemylorthoformate (15 mL) and allowed to stir at room temperature overnight. The mixture was poured into dichloromethane (150 mL) and water (100 mL). The organic phase was neutralized with triethylamine, washed with water and evaporated to dryness. The resulting residue was purified by column chromatography (silica gel 60,98:2 dichloromethane-methanol eluant) to give the desired porphyrin as a red solid (326 mg, 24%) after evaporation of solvents. An analytically pure sample was recrystallized from dichloromethane-ether as deep red needles. 237 M.P.: 302-303 °C iH-NMR (CDCI3) 8: -3.78 (br s, 2H, 2 x NH); 1.90 (t, 6H, 2 x -CH2CH3); 3.64 (s, 6H, 2 x -CH3); 3.68 (s, 6H, 2 x -CH3); 3.80 (s, 6H, 2 x -CO2CH3); 5.06 (s, 4H, 2 x -CH2C02CH3); 10.14 (s, 2H, 2 x meso-H); 10.16 (s, 2H, 2 x meso-H) LRMS (m/z): 566 (M+), 551 (M+ - CH3), 507 (M+ - CO2CH3) EXACT MASS: calcd for C34H38N4O4: 566.2893 found by high resolution mass spectroscopy: 566.2895 VISIBLE SPECTRUM (CH2C12): AmaxOim): 398 498 534 562 618 peak ratio: 18.3 1.72 1.49 1.21 1.00 3,13-Bis(2-acetoxyethyl)-8,18-diethyl-2,7,12,17-tetramethyIporphyrin (154) OAc OAc 238 The biladiene 153 (1.55 g, 1.85 mmoles) was dissolved in DMSO (200 mL) and then treated with pyridine. After 5 days in the dark at room temperature in an open erlenmeyer flask, the reaction mixture was filtered and the purple crystals collected were washed with 50% methanol-hexane and air dried (890 mg, 81%). Tic of the product showed the presence of one major component, which is the bis(acetoxyemyl)methylporphyrin, and two minor components. A pure sample of the bis(acetoxyeuyl)diethylporphyrin for characterization was obtained by column chromatography (silica gel 60, 95:5 dichloromethane-methanol eluant) of a small amount of the product. M.P.: 320 °C (d) l H NMR ( C D C 1 3 ) 8: -3.80 (br s, 2H, 2 x NH), 1.90 (t, 6H, J = 8.0 Hz, 2 x - C H 2 C H 3 ) ; 2.10 (s, 6H, 2 x -C(=0)CH3); 3.65 (s, 6H, 2 x -CH 3); 3.70 (s, 6H, 2 x -CH 3); 4.15 (q, 4H, J = 8.0 Hz, 2 x -ch2ch3); 4.38 (t, 4H, J = 8.0 Hz, 2 x -ch2ch2o-); 4.90 (t, 4H, J = 8.0 Hz, 2 x -ch2ch2o-); 10.10 (s, 2H, 2 x meso-H); 10.15 (s, 2H, 2 x meso-H) L R M S (m/z): 594 (M+), 552 (M+ - COCH 3 - H), 536 (M+ - OCOCH 3 + H), 521 (M+ -c h 2 o c o c h 3 ) ANAL: calcd for C26H42N4O4: C, 72.70; H, 7.12; N, 9.42; O, 10.76 % found: C, 72.52; H, 7.69; N, 9.08% VISIBLE SPECTRUM (CH2C12): 534 568 620 2.10 1.52 1.00 A-max (nm): 398 498 peak ratio: 26.81 2.66 239 8,18»Diethyl-3,13-Bis(2-hydroxyethyl)-2,7,12,17-tetramethyIporphyrin (151) OH OH Method 1 The di-ester porphyrin 108 (212 mg, 0.375 mmoles) in refluxing dry chloroform (60 mL) was treated with a saturated solution of zinc acetate (3 mL). The metalation was complete in one hour (as shown by U.V-vis spectroscopy). The chloroform was washed twice with water, dried over anhydrous magnesium sulfate, filtered and the filtrate evaporated to dryness to afford the zinc complex as a pinkish-red solid (222.6 mg, 94%). A 250-mL two-necked flask equipped with a pressure-equalizing addition funnel, magnetic stirring bar and a nitrogen inlet was flushed with dry nitrogen and charged with dry THF (20 mL). Lithium aluminium hydride (199.7 mg, 5.2 mmoles) was added to the stirred THF and the resulting slurry cooled to 0 °C. A solution of the zinc complex in THF (30 mL) was added dropwise with stirring. THF (10 mL) was used to rinse the addition funnel and the solution warmed to room temperature.and stirred until completion of reaction (as shown by dc, 2 hours). The reaction mixture was cooled to 0 °C and quenched with ice, followed by water (effervescence). The mixture was extracted with dichloromethane and the organic phase treated 240 with trifluoroacetic acid (5 mL) to produce the demetalated porphyrin. The organic phase was then washed twice with 5% sodium bicarbonate, twice with water, dried over anhydrous magnesium sulfate, filtered and the filtrate evaporated in vacuo to afford the desired diol as purple crystals (112 mg, 64%). Method 2 The bis(acetoxyethyl)porphyrin 154 (600 mg, 1.01 mmoles) was dissolved in methanol (150 mL) containing 5% (w/v) concentrated sulfuric acid and stirred in the dark for 16 hours (dc showed completion of reaction at that time). The mixture was poured slowly into a slurry of ice and water (150 g) and the aqueous phase extracted with dichloromethane. The organic phase was washed with 5% aqueous sodium bicarbonate and dried over anhydrous magnesium sulfate. The solution was filtered and the filtrate evaporated to dryness to give a purplish-red solid which was recrystallized from chloroform-ether to give the bis(hychoxyemyl)porphyrin (505 mg, 98%). M.P.: 240 °C (d) 'H-NMR (CDC13) 5: -3.40 (br s, 2H, 2 x NH); 1.75 (t, 6H, 2 x - C H 2 C H 3 ) ; 3.66 (s, 6H, 2 x -CH 3); 3.71 (s, 6H, 2 x -CH 3); 4.16 (q, 4H, 2 x -CH2CH3); 4.56 (t, 4H, 2 x -CH2CH2OH); 5.04 (t, 4H, 2 x -CH2CH20H); 10.65 (s, 2H, 2 x meso-H); 10.70 (s, 2H, 2 x meso-H) LRMS (m/z): 510 (M+), 495 (M+ - CH 3), 479 (M+ - CH2OH), 448 (M+ - 2 x CH 2OH) EXACT MASS: calcd for C 32H 3 8N 402: 510.2995 found by high resolution mass spectroscopy: 510.2999 241 VISIBLE SPECTRUM (CH3OH): W ( n m ) : 394 496 530 564 618 peakratio: 18.41 2.14 1.65 1.28 1.00 3,13-Bis(2-chloroethyl)-8,18-diethyl-2,7,12,17-tetramethyIporphyrin (152) A stirred suspension of the bis(hydroxyethyl)rx)rphyrin 151 (500 mg, 0.98 mmoles) and potassium carbonate (20 g) in dichloromethane (350 mL) and DMF (30 mL) was treated with thionyl chloride (20 mL) at room temperature for 3 hours. Water was carefully added to the mixture (effervescence)'which was then extracted with dichloromethane. The organic phase was washed twice with saturated aqueous sodium bicarbonate, twice with water and dried over anhydrous magnesium sulfate. The solution was filtered and evaporated to dryness. The crude product was purified by column chromatography (silica gel 60, dichloromethane eluant). The product obtained was crystallized from chloroform-methanol to afford the product as reddish-purple crystals (465 mg, 85%). M.P.: 324-325 'C 1H-NMR (CDCI3) 8: -3.75 (br s, 2H, 2 x NH); 1.88 (t, ,6H, J = 7.5 Hz, 2 x -CH2CH3); 3.65 (s, 6H, 2 x -CH 3); 3.68 (s, 6H, 2 x -CH 3); 4.13 (q, 4H, J = 7.5 Hz, 2 x -CH2CH3); 4.32 (t, 4H, J = 8.0 Hz, 2 x -CH2CH2CI); 4.50 (t, 4H, J = 8.0 Hz, 2 x -CH2CH2CI); 10.03 (s, 2H, 2 x meso-H); 10.13 (s, 2H, 2 x meso-H) EXACT MASS: calcd for C32H36N4C12: 546.2317 found by high resolution mass spectroscopy: 546.2307 242 ANAL: calcd for C32H36N4CI2: C, 70.19; H, 6.63: N, 10.23 % found: C, 70.25; H, 6.58; N, 10.20 % VISIBLE SPECTRUM (CH2C12): Xmax(nm): 398 498 534 570 620 peak ratio: 17.53 1.69 1.43 1.16 1.00 8,18-Diethyl-2,7,12,17-tetramethyl-3,13-divinylporphyrin (106) To a refluxing solution of the bis(chloroethyl)porphyrin 152 (439 mg, 0.80 mmoles) in pyridine (100 mL) was slowly added water (15 mL). After 10 minutes, a solution of sodium hydroxide (30 mL, 10% w/v) was added and the refluxing continued for 2 hours. Aqueous acetic acid (10% v/v) was added to neutralize the excess sodium hydroxide, followed by water. The reaction mixture was extracted with dichloromethane and the organic layer washed twice with saturated sodium bicarbonate solution, twice with water and dried over anhydrous magnesium sulfate. The filtered organic phase was taken to dryness in vacuo and the residue purified by column chromatography (silica gel 60, dichloromethane eluant). The fractions 243 containing the product were combined, concentrated and the product crystallized by addition of petroleum ether (b.p. 30-60 °C) (320 mg, 84%). M.P.: 225 *C (d) 'H-NMR (CDC13) 8: -3.66 (br s, 2H, 2 x NH); 1.90 (t, 6H, 2 x -CH2CH3); 3.68 (s, 6H, 2 x -CH3); 3.72 (s, 6H, 2 x -CH3); 4.15 (q, 4H, 2 x -CH2CH3); 6.15 (dd, 2H, J = 2,0, 12.0 Hz, -CH=CH2); 6.34 (dd, 2H, J = 2.0, 18.0 Hz, -CH=CH2); 8.32 (dd, 12.0, 18.0 Hz, 2H, 2 x -CH=CH2); 10.15 (s, 2H, 2 x meso-H); 10.23 (s, 2H, 2 x meso-H) 13C-NMR (10% TFA-CDCI3) 8 : 11.71, 12.61 (4C, 4 x -CH3); 16.18 (2C, 2 x -CH2CH 3 ) ; 20.07 (2C, 2 x -CH 2 CH 3 ); 99.09, 99.48 (4C, 4 x meso carbons); 127.59 (2C, 2 x -CH=CH2); 128.23 (2C, 2 x -CH=CH2); 138.34, 138.60, 140.52, 141.54, 142.82, 145.00 (16C, a- and P-pyrrolic carbons) LRMS (m/z): 474 (M+), 459 (M+ - CH3), 444 (M+ - 2 x CH3) ANAL: calcd for C 3 2 H 3 4 N 4 : C, 80.98; H, 7.22; N, 11.80 % found: C, 80.41; H, 7.38; N, 11.65 % VISIBLE SPECTRUM (CH2C12): Xmax(nm): 402 506 546 572 626 peak ratio: 19.2 2.05 2.68 1.80 1.00 244 3.8 Synthesis of Chlorin Derivatives 8,13,18-Triethyl-23-hydro-21-methoxycarbonyl-2,7,12,17-tetramethyl-22-phenylsulfonyI-monobenzo[b]porphyrin 8,13,18-Triemyl-2,7,12,17-tetrametty^ (81) (50 mg, 0.105 mmoles) and methyl (E)-P-phenylsulfonylacrylate (1.19 g, 5.25 mmoles) were suspended in degassed toluene (20 mL) and heated at 110 °C in a sealed tube for 3 days. The reaction mixture was evaporated to dryness and the residue was applied to a silica gel column and eluted with 1% methanol in dichloromethane. The unreacted vinylporphyrin, excess dienophile and cycloadducts were collected in this order of elution. The fractions containing the cycloadducts were combined, dried and subjected to chromatography on a chromatotron (1 mm silica gel plate, dichloromethane eluant) to give 14.0 mg (19%) of DAEST1 (Rf = 0.38) as the first product, followed by 15.48 rag (21%) of DAEST2 (Rf = 0.35) as the other product (total yield = 40%). DAEST1: H PhOjS H C H J O J C M.P.: 140-141 °C 245 1 H - N M R ( C D C I 3 ) 5: -2.65 (br s, 2H, 2 x NH); 1.76 (3t, overlap, 3H each, 3 x - C H 2 C H 3 ) ; 2.05 (s, 3H, angular-CH3); 2.54 (s, 3H, - C O 2 C H 3 ) ; 3.05 (ddd, 1H, J = 6.0, 8.0, 14.0 Hz, 23-H), 3.32 (ddd, 1H, J = 2.0, 12.0, 14.0 Hz, 23-H), 3.42, 3.45, 3.55 (3s, 3H each, 3 x - C H 3 ) ; 3.85 (q, 2H, -CII2CH3); 3.98 (2q, overlap, 2H each, 2 x - C H 2 C H 3 ) ; 4.00 (m, overlap, 1H, 22-H); 4.81 (d, 1H, J = 7.0 Hz, 2!-H); 7.41 (dd, 1H, J = 2.0, 8.0 Hz, 24-H); 7.66-8.18 (m, 5H, Ar-H); 8.97, 9.31, 9.72, 9.73 (4s, 1H each, 4 x meso-H) 13C-NMR (10% T F A - C D C I 3 ) 8: 11.01 (3C, 3 x - C H 3 ) ; 15.89 (3C, 3 x - C H 2 C H 3 ) ; 19.50 (3C, 3 x - C H 2 C H 3 ) ; 23.54 (1C, C-23) ; 27.10 (1C, ang. -CH 3), 48.85 (1C, C-2l); 52.57 (2C, C-2 and C-22); 60.22 (1C, -OCH 3); 88.61, 89.65, 103.34, 104.28 (4C, 4 x meso-carbons); 126.67, 128.61, 129.98, 135.29, 158.09 (7C, phenyl carbons and C-24); 134.29, 135.76, 136.44, 137.58, 139.07, 140.23, 140.80, 142.54, 143.54, 146.36, 147.27, 147.48, 147.95 (15C, 8a- and 7f3- pyrrolic carbons); 173.00 (1C, -CC>2CH 3) LRMS (m/z): 702 (M+), 561 (M+ - S O 2 C 6 H 5 ) EXACT MASS: calcd for C42H46N4O4S: 702.3220 found by high resolution mass spectroscopy: 702.3245 VISIBLE SPECTRUM (CH2CI2): W(nm): 402 502 534 600 658 peak ratio: 27.78 2.04 2.52 1.00 6.54 246 PAEST2; M.P.: 145-146 °C 'H-NMR (CDCI3) 8: -2.62 (br s, 2H, 2 x NH); 1.78 (3t, 3H each, 3 x -CH2CH3); 1.98 (s, 3H, angular-CH3); 3.34 (m, 2H, 2 x 23-H); 3.42, 3.45, 3.55 (3s, 3H each, 3 x -CH 3); 3.66 (d, 1H, J = 12 Hz, 2J-H); 3.88 (2q, overlap, 2H each, 2 x -CH2CH3); 4.00 (q, 2H, - CH2CH3); 4.74 (m, 1H, J = 6,10,12 Hz, 22-H); 7.07 (t, 1H, J = 6.0 Hz, 24-H); 7.42-8.00 (m, 5H, Ar-H); 8.72, 9.18, 9.69, 9.74 (4s, 1H each, 4 x meso-H) 1 3 C - N M R (10% TFA-CDC13) 8: 11.07 (3C, 3 x -CH 3); 15.70 (3C, 3 x -CH2£H3); 19.49 (3C, 3 x - £ H 2 C H 3 ) ; 23.28 (1C, ang. -CH 3) ; 24.51 (1C, C-23), 47.45 (1C, C-21); 52.21 (1C, C-2); 53.18 (1C, C-22); 59.08 (1C, - 0 £ H 3 ) ; 87.88, 90.73, 103.23, 104.81 (4C, 4 x meso-carbons); 129.11, 129.80, 135.21, 155.09 (7C, phenyl carbons and C-24); 134.26, 136.35, 137.37, 137.62, 138.03, 139.41, 139.62, 140.59, 141.91, 143.91, 145.95, 146.51, 147.43,148.35 (15C, 8a- and 7B- pyrrolic carbons); 172,80 (1C, -C02CH3) LRMS (m/z): 702 (M+), 561 (M+ - SO2C6H5) 247 EXACT MASS: calcd for C42H46N4O4S: 702.3220 found by high resolution mass spectroscopy: 702.3238 VISIBLE SPECTRUM (CH2CI2): Xmax (nm): peak ratio: 27.20 400 498 2.21 2.15 534 600 1.00 6.30 656 21-Cyano-8,13,18-triethyl-23-hydro-2,7,12,17-tetramethyl-22-phenylsulfonyl-monobenzo[b]porphyrin from (E)-dienophile (160-C) PhOjS 8,1348-Triemyl-2J,12J7-tetramethyl-3-vmylporphyrin (81) (50 mg, 0.105 mmoles) and (E)-P-phenylsulfonylacrylonitrile (1 g, 5.25 mmoles) were suspended in degassed toluene (20 mL) and heated at 110 °C in a sealed tube for 3 days. The reaction mixture was evaporated to dryness and the residue was applied to a silica gel column and eluted with dichloromethane. A small amount of unreacted vinylporphyrin was recovered, followed by the excess dienophile. The solvent polarity was increased to 2% methanol in dichloromethane. Fractions absorbing at 656 nm were combined and evaporated to dryness. Further purification was carried out by chromatography on a chromatotron (1 mm silica gel plate, dichloromethane H H NC 248 eluant) to give the pure cycloadduct which was recrystallized in cMoroform-methanol to give bluish-purple crystals (42.1 mg, 60%). M.P.: 226-228 °C 1H-NMR (CDC13) 8: -2.65 (br s, 2H, 2 x NH); 1.80 (3t, overlap, 3H each, 3 x -CH2CH3); 2.08 (s, 3H, angular-CH3); 3.40 (ddd, 1H, J = 4.5, 10.0, 18.0 Hz, 23-H); 3.44, 3.48, 3.56 (3s, 3H each, 3 x -CH3); 3.60 (ddd, 1H, J = 3.0, 6.0, 18.0 Hz, 23-H); 3.87 (d, 1H, J = 11 Hz, 21-H); 3.90-4.02 (3q, overlap, 2H each, 3 x -CH2CH3); 4.38 (ddd, 1H, J = 3.0, 10.0, 11.0 Hz, 22-H); 7.15 (dd, 1H, J = 4.5, 6.0 Hz, 24-H); 7.60-8.10 (m, 5H, Ar-H); 9.20, 9.52, 9.75, 9.77 (4s, 1H each, 4 x meso-H) 13C-NMR (10% TFA-CDCI3) 8: 11.33 (3C, 3 x -CH 3); 15.68 (3C, 3 x -CH2CH3); 19.81 (3C, 3 x -C.H2CH3); 24.78 (2C, ang. - C H 3 , C-23) ; 35.01 (1C, C-2l); 51.14 (1C, C-2); 57.97 (1C, C-22); 88.48, 90.96, 103.16,104.81 (4C, 4 x meso-carbons); 117.22 (1C, -ON); 129.01, 130.25, 135.63, 154.56 (7C, phenyl carbons and C-24); 134.18, 134.94, 136.86, 137.76, 138.12, 138.39, 139.81, 140.00, 140.63, 141.26, 144.28, 146.79, 147.38, 147.77, 148.67 (15C, 8a- and 7B- pyrrolic carbons) L R M S (m/z): 669 (M+), 528 (M+ - SO2C6H5), 527 (M+ - SO2C6H5 - H), 511 (M+ -SO2C6H5 - 2H - CH 3) EXACT MASS: calcd for C41H43N5O2S: 669.3138 found by high resolution mass spectroscopy: 669.3128 249 VISIBLE SPECTRUM (CH2C12): Xmax (nm): peak ratio: 400 27.02 2.15 500 2.13 532 596 1.00 656 6.31 21-Cyano.8,13,18-triethyl-23-hydro-2,7,12,17.tetramethyI-22.phenylsulfonyI-monobenzo[b]porphyrin from (Z)-dienophile (161-D) 8,1348-Triemyl-2J,12J7-tetramethyl-3-vmylporphyrin (81) (50 mg, 0.105 mmoles) and (Z)-|3-phenylsulfonyl acrylonitrile (1 g, 5.25 mmoles) were suspended in degassed toluene (20 mL) and heated at 110 °C in a sealed tube for 3 days. The reaction mixture was evaporated to dryness and the residue was applied to a silica gel column and eluted with 1% methanol in dichloromethane. A small amount of unreacted vmylpomhyrin was recovered, followed by the excess dienophile and the cycloadduct Further purification was carried out by chromatography on a chromatotron (1 mm silica gel plate, dichloromethane eluant) to give the pure cycloadduct which was recrystalhzed in dichloromethane-methanol to give bluish-purple crystals (30.9 mg, 40%). PhOjS H - . . J NC M.P.: 158-159 'C 250 1H-NMR (CDCI3) 8: -2.70 (br s, 2H, 2 x NH); 1.75 (3t, overlap, 3H each, 3 x -CH2CH3); 2.10 (s, 3H, angular-CH3); 2.85 (m, 1H, 23-H); 3.25 (m, 1H, 23-H); 3.50 (m, overlap, 1H, 22-H); 3.39, 3.40, 3.52 (3s, 3H each, 3 x -CH 3); 3.80-4.00 (3q, overlap, 2H each, 3 x - C H 2 C H 3 ) ; 4.18 (d, 1H, J = 10 Hz, 2*-H); 7.02 (dd, 1H, 24-H); 7.64-8.12 (m, 5H, Ar-H); 9.10, 9.31, 9.70, 9.73 (4s, 1H each, 4 x meso-H) 1 3 C - N M R (10% TFA-CDC13) 8: 11.30 (3C, 3 x -CH 3); 15.89 (3C, 3 x - C H ^ H ^ ; 19.44 (3C, 3 x -CH 2 CH 3 ) ; 24.35 (1C, C-23) . 25.52 (1C, ang. -CH 3), 34.64 (1C, C-2*); 51.80 (1C, C-2); 56.98 (1C, C-22); 87.92, 90.40, 103.40, 104.67 (4C, 4 x meso-carbons); 116.40 (1C, -ON); 128.78, 129.82, 135.25, 155.36 (7C, phenyl carbons and C-24); 134.08, 136.34, 136.70, 137.52, 137.83, 139.33, 140.41, 140.73, 141.00, 142.86, 143.72, 146.03, 147.16, 148.07 (15C, 8a- and 7p- pyrrolic carbons) LRMS (m/z) : 669 (M+), 527 (M+ - SO2C6H5 - H), 511 (M+ - SO2C6H5 - 2H - CH 3) EXACT MASS: calcd for GiiH^NsChS: 669.3138 found by high resolution mass spectroscopy: 669.3142 VISIBLE SPECTRUM (CH2C12): XmaxOim) : 400 498 534 600 656 peak ratio: 26.89 2.11 2.10 1.00 6.28 251 2 1 -Cyano-8 ,13 ,18-tr iethyl -23 ,2 '» ,3- tr ihydro-2 ,7 ,12 ,17-tetramethyl-22 . phenylsulfonyl-monobenzo[b]porphyrin (163) PhCXjS Cycloadduct 160-C (30 mg, 0.045 mmoles) was dissolved in THF (10 mL). To this solution was added 10% palladium-carbon (5 mg) and the mixture was hydrogenated at room temperature and atmospheric pressure. Completion of reaction was established by U.V-vis. spectroscopy (disappearance of the band at 522 nm). The mixture was filtered through celite and the celite washed with THF. The combined organic phases were evaporated to dryness and the residue was submitted to chromatography on a chromatotron (1 mm silica gel plate, 1% methanol in dichloromethane as eluant), followed by recrystallization from chloroform-methanol to give the product as bluish-purple crystals (25.7 mg, 85%). M.P.: 185 "C (d) iH-NMR (CDC13) 8: -2.60 (br s, 2H, 2 x NH); 1.75 (3t, overlap, 3H each, 3 x -CH2CH3); 2.27 (m, 1H, 23-H); 2.52 (s, 3H, angular-CH3); 2.65 (m, 1H, 23-H); 2.75 (d, overlap, 1H, 2*-H); 2.75 (m, overlap, 1H, 24-H); 3.30 (m, 1H, 24-H); 3.41, 3.42, 3.53 (3s, 3H each, 3 x -CH 3); 3.72 (m, 1H, 22-H); 3.83-4.05 (3q, overlap, 2H each, 3 x -CH2CH3); 4.65 (t, 3H, angular-H); 7.37-7.80 (m, 5H, Ar-H); 8.73, 9.15, 9.75, 9.76 (4s, 1H each, 4 x meso-H) 252 LRMS (m/z): 671 (M+), 529 (M+ - S O 2 C 6 H 5 - H), 511 (M+ - S O 2 C 6 H 5 - 4 H - CH 3) EXACT MASS: calcd for C41H45N5O2S: 671.3294 found by high resolution mass spectroscopy: 671.3293 VISIBLE SPECTRUM (CH2CI2): W(nm): 392 492 588 642 peak ratio: 24.92 2.18 1.00 6.05 21-Cyano-8,13,18-triethyl-23,24,3-trihydro-2,7,12,17-tetramethyl-monobenzo[b]porphyrin (168) The hydrogenated cycloadduct 163 (20 mg, 0.0298 mmoles) was dissolved in dichloromethane (10 mL). To this solution was added 10 drops of DBU and the mixture was stirred at room temperature in the dark. Upon completion of reaction (as shown by tic), the solution was diluted with dichloromethane and washed with sodium bicarbonate, followed by water and dried over anhydrous magnesium sulfate. The solvent was removed under reduced 253 pressure and the residue chromatographed on a chromatotron (1 mm silica gel plate, 1% methanol in dichloromethane as eluant). RecrystaUization from chloroform-hexane gave the product as greenish-blue crystals (15.0 mg, 95%). M.P.: 163-165 *C 'H-NMR ( C D C 1 3 ) 8: -2.57 (br s, 2H, 2 x NH); 1.78 (3t, overlap, 3 H each, 3 x - C H 2 C H 3 ) ; 2.40 (s, 3 H , angular-CH3); 2.50 (m, 2H, - C H 2 at C-23); 2.71 (m, 1H, 24-H); 2.98 (m, 1H, 2 4-H); 3.85-4.00 (3q, overlap, 2H each, 3 x -CH2CH3); 4.95 (t, 1H, J = 5.0 Hz, angular-H); 6.80 (t, 1H, J = 5.0 Hz, =CH-); 8.88, 9.45 (2s, 1H each, 2 x meso-H); 9.75 (s, 2 H , 2 x meso-H) LRMS (m/z): 529 (M+), 514 (M+ - CH 3) EXACT MASS: calcd for C35H39N5: 529.3205 found by high resolution mass spectroscopy: 529.3207 VISIBLE SPECTRUM (CH 2C1 2): Xmax(nm): 392 494 620 644 peak ratio: 23.80 2.67 1.00 6.93 254 3.9 Synthesis of Benzoporphyrin Derivatives 7,12,17-Triethyl-21,22-bis(methoxycarbonyl)-8,13,18-trimethyl-monobenzo[b]porphyrin (171) 7,12J7-Triemyl-843,18-trimemyl-3-vinylporphyrin (101) (50 mg, 0.108 mmoles) and dimethyl acetylenedicarboxylate (767 mg, 5.40 mmoles) were suspended in degassed toluene (20 mL) and the mixture heated at 110 °C in a sealed tube for 24 hours. After removal of the toluene in vacuo, the residue was chromatographed (silica gel 60, dichloromethane eluant). After elution of the excess dienophile, the solvent polarity was increased to 2% methanol in dichloromethane to afford the benzoporphyrin as pure purple crystals (52 mg, 80%) after solvent removal. A sample was recrystallized from chloroform-methanol. M.P.: 327-328 'C 'H-NMR (CDCI3) 8: -4.2 (br s, 2H, 2 x NH); 1.84 (t, overlap, 6H, 2 x -CH2CH3); 1.95 (t, 3H, -CH2CH3); 3.51, 3.55, 3.70 (3s, 9H, 3 x -CH 3); 3.95-4.80 (2q, overlap, 4H, 2 x -CH2CH3); 4.20 (s, overlap, 3H, -CO2CH3); 4.20 (q, overlap, 2H, -CH2CH3); 4.64 (s, 3H, -CO2CH3); 8.71 (d, 1H, J = 8.0 Hz, 23-H); 9.32 (d, 1H, J = 8.0 Hz, 24-H); 9.91-10.40 (4s, 4H, 4 x meso-H) 255 LRMS (m/z): 602 (M+), 587 (M+ - CH3), 572 (M+ - 2 x CH3), 544 (M+ - C02CH3 + H) EXACT MASS: calcd for C37H38N4O4: 602.2893 found by high resolution mass spectroscopy: 602.2895 VISIBLE SPECTRUM (CH2CI2): Amaxtnm): 410 512 548 576 630 peak ratio: 20.1 1.27 2.64 1.27 1.00 21,22-Bis(ethoxycarbonyI)-7,12,17-triethyl-8,13,18-trimethyl. monobenzo[b]porphyrin (172) 7,12J7-Triethyl-843,18-trimemyl-3-vinylporphyrin (101) (50 mg, 0.108 mmoles) and diethyl acetylenedicarboxylate (919 mg, 5.40 mmoles) were suspended in degassed toluene (20 mL) and the niixture heated at 110 °C in a sealed tube for 24 hours. After removal of the toluene in vacuo, the residue was chromatographed (silica gel 60, dichloromethane eluant). After elution of the excess dienophile, the solvent polarity was increased to 2% methanol in dichloromethane to elute the major reaction products. The crude product (after 256 evaporation to dryness) was further purified by chromatography on a chromatotron (silica gel plate, 2% methanol in dichloromethane as eluant). After collecting some starting material, the following fractions afforded, after solvent removal, the desired benzoporphyrin which was recrystallized from chloroform-methanol as purple crystals (51 mg, 75%). M.P.: 230-232 °C 'H-NMR (CDC13) 8: -4.01 (br s, 2H, 2 x NH); 1.62 (t, 3H, J = 8.0 Hz, -CO2CH2CH3); 1.71 (t, 3H, J = 8.0 Hz, -CO2CH2CH3); 1.80-1.92 (3t, overlap, 9H, J = 7.5 Hz, 3 x -CH2CH3); 3.54, 3.64, 3.75 (3s, 9H, 3 x -CH 3); 3.97-4.20 (3q, overlap, 6H, J = 7.5 Hz, 3 x -CH_2CH3); 4.65 (q, 2H, J = 8.0 Hz, -C0 2 CH2CH 3 ) ; 5.18 (q, 2H, J = 8.0 Hz, -C02CH2CH3); 8.73 (d, 1H, J = 8.0 Hz, 23-H); 9.4 (d, 1H, J = 8.0 Hz, 24-H); 9.93-10.47 (4s, 4H, 4 x meso-H) LRMS (m/z): 630 (M+), 615 (M+ - CH 3), 602 (M+ - C 2 H 5 + H) EXACT MASS: calcd for Q Q R ^ C M : 630.3206 found by high resolution mass spectroscopy: 630.3201 VISIBLE SPECTRUM (CH2C12): W(nm): 410 512 548 576 628 peak ratio: 20.9 1.30 3.19 1.45 1.00 257 21,22-Bis(tert-butoxycarbonyl).7,12,17-triethyl-23,24-(iihydro-8,13,18-trimethyl-monobenzo[b]porphyrin (174) 7,12,17-Triemyl-8,13,18-trimemyl-3-v^ (50 mg, 0.108 mmoles) and di-tert-butyl acetylenedicarboxylate (1.20 g, 5.40 mmoles) were suspended in degassed toluene (20 mL) and the mixture heated at 110 °C in a sealed tube for 24 hours. After removal of the toluene in vacuo, the residue was chromatographed (silica gel 60, dichloromethane eluant). After elution of the excess dienophile, the solvent polarity was increased to 2% methanol in dichloromethane to elute the crude product of the reaction. This was then further purified by chromatography on a chromatotron (silica gel plate, 1% methanol in dichloromethane). The first fraction gave the unreacted starting material (5 mg). The second fraction consisted of the minor product (rhodo spectrum indicating the formation of a benzoporphyrin). The third fraction yielded the porphyrin, upon evaporation of solvent, as purple crystals (33.4 mg, 45%). M.P.: 120-121'C iH-NMR (CDCI3) 5: -3.65 (br s, 2H, 2 x NH); 1.73 (s, 9H, -C(CH 3) 3); 1.78 (s, 9H, -C(CH 3) 3); 1.85 (3t, overlap, 9H, 3 x -CH2CH3); 3.33 (t, 2H, J = 10 Hz, 23-H); 3.98-4.12 258 (3q, overlap, 6H, 3 x -CH2CH3); 4.25 (t, 2H, J = 10 Hz, 24-H); 9.97-10.50 (4s, 4H, 4 x meso-H) LRMS (m/z): 688 (M+), 632 (M+ - H2C=C(CH3)2), 576 (M+ - 2 x H2C=C(CH3)2) EXACT MASS: calcd for C43H52N4O4: 688.3988 found by high resolution mass spectroscopy: 688.3996 VISIBLE SPECTRUM (CH2C12): XmaxOim): 408 510 550 574 636 peak ratio: 24.65 2.30 3.57 2.31 1.00 21,22-Bis(tert-butoxycarbonyl)-7,12,17-triethyl-8,13,18-trimethyl-monobenzo[b]porphyrin (173) The minor product isolated from the reaction of di-tert-butyl acetylenedicarboxylate with the monovmylporphyrin in the previous experiment gave 11.14 mg (15% yield) of purple crystals. This compound was characterized as the desired benzoporphyrin. 259 M.P.: 170°C (d) 1H-NMR (CDCI3) 8: -3.70 (br s, 2H, 2 x NH); 1.82 (s, 9H, -C(CH 3) 3); 1.84-1.97 (3t, overlap, 9H, 3 x -CH2CH3); 2.00 (s, 9H, -C(CH3)3); 3.57, 3.72, 3.74 (3s, 9H, 3 x -CH 3); 3.98-4.28 (3q, overlap, 6H, 3 x -CH2CH3); 8.57 (d, 1H, J = 8.0 Hz, 23-H); 9.43 (d, 1H, J = 8.0 Hz, 24-H); 10.06 - 10.92 (4s, 4H, 4 x meso-H) LRMS (m/z): 686 (M+ not observed), 556 (M+ - C 2 H 5 - C02C(CH3)3), 541 (M+ - C 2 H 5 -C02C(CH3)3 - CH 3), 484 (M+ - 2 x C02C(CH3)3) FAB (m/z): 687 (M+ + 1) VISIBLE SPECTRUM (CH2C12): Amax(nm): 408 508 546 574 628 peak ratio: 21.93 1.53 3.06 1.57 1.00 Aromatization of adducts 160-C and 161-D 21-Cyano-8,13,18-triethyl-7,12,17-trimethyI-monobenzo[b]porphyrin (162) 260 Adduct 160-C or 161-D (15 mg, 0.0224 mmoles) and benzene (10 mL) containing 10 drops of DBU were refluxed for 30 minutes. D D Q (2.0 equivalents) was then added to the mixture and refluxing continued for 2 hours. The reaction mixture was evaporated to dryness and the residue subjected to flash column chromatography (silica gel 60, dichloromethane eluant) to give a fast running fraction exhibiting a rhodo spectrum. Evaporation of the solvent gave 2.29 mg of the benzoporphyrin (20%). M.P.: 300°C (d) ' H - N M R (CDC13) 5: -3.85 (br s, 2H, 2 x NH); 1.87, 1.91, 2.00 (3t, 3H each, 3 x -CH2CH3); 3.58, 3.74, 3.76 (3s, 3H each, 3 x -CH 3); 8.13 (t, 1H, Ar-H at C-23); 8.40 (d, 1H, Ar-H at C-22); 9.60 (d, 1H, Ar-H at C-24); 10.14, 10.15, 10.30, 10.39 (4s, 1H each, 4 x meso-H) LRMS (m/z): 511 (M+), 496 (M+ - CH3) EXACT MASS: calcd for C34H33N5: 511.2739 found by high resolution mass spectroscopy: 511.2742 VISIBLE SPECTRUM (CH2CI2): Xmax(nm): 408 508 542 570 626 peakratio: 22.10 1.59 2.94 1.52 1.00 261 3.10 Synthesis of Bacteriochlorin Derivatives 8,18-Diethyl-23,123.dihydro-2,7,12,17-tetramethyl-21-22,12l-122.bis(N-phenyImaleimide)-dibenzo[b,I]porphyrin (179) The A,C-divmylporphyrin 106 (50 mg, 0.105 mmoles) and N-phenylmaleimide (909 mg, 5.25 mmoles) were suspended in toluene (20 mL). The system was degassed by three freeze-thaw cycles and then heated at 110 °C for 3 days. The reaction mixture was concentrated in vacuo and the residue was chromatographed (silica gel 60,2% methanol in dichloromethane as eluant) on a column to get rid of the excess dienophile. Further purification using a chromatotron (1 mm silica gel plate, same solvent as above) gave the bacteriochlorin derivative (45%) and a minor product, the mono-adduct (8%). M.P.: 185-186°C !H-NMR (CDC13) 5: -2.28 (s, 2H, 2 x NH); 1.70 (t, 6H, 2 x -CH2CH3); 2.00, 2.05 (2s, 6H, angular-CH3); 3.35 (s, 6H, 2 x ring-CH3); 3.40 (m, 4H, - C H 2 - at C-23 and C-123); 3.80 (m, 2H, H at C-22 and C-122); 3.85 (m, 4H, 2 x -CH2CH3); 4.54 (d, 2H, H at C-2* and 262 C-121); 6.60-7.00 (m, 10H, Ar-H); 7.22 (t, 2H, 2 x CH at C-24 and C-124); 8.90, 9.05 (2s, 4H, 4 x meso-H) LRMS (m/z): 820 (M+), 805 (M+ - CH 3), 647 (M+ - C10H7O2N), 474 (M+ - 2 x C10H7O2N) EXACT MASS: calcd for C52H46N6O4: 820.3724 found by high resolution mass spectroscopy: 820.3729 VISIBLE SPECTRUM (CH 2C1 2): Xmax(nm): 388 410 490 526 666 702 738 peak ratio: 16.47 19.36 2.62 1.85 1.00 1.50 11.77 21,121-Dicyano-8,18-diethyI-23,123-dihydro-2,7,12,17-tetramethyl-22,122-bis(phenylsulfonyl)-dibenzo[b,l]porphyrin (181) The A,C-divmylporphyrin 106 (50 mg, 0.105 mmoles) and (E)-|5-phenylsulfonyl-acrylonitrile (1.01 g, 5.25 mmoles) were suspended in toluene (20 mL). The system was degassed by three freeze-thaw cycles and then heated at 110 °C for 3 days. The reaction mixture was concentrated in vacuo and the residue was chromatographed (silica gel 60, 2% 263 methanol in dichloromethane as eluant). The fractions absorbing at 734 nm were combined and concentrated in vacuo. Further purification on a chromatotron (1 mm silica gel plate, same solvent as above) gave the bacteriochlorin as the major product (45%). 5% of the A,C-divinylporphyrin was recovered, together with a small fraction absorbing at 656 nm (mono-adduct) prior to bacteriochlorin elution. A sample was recrystallized from dichloromethane-methanol, affording shiny bluish-green crystals. M.P. : 225-226 °C 1H-NMR (CDCI3) 5: -2.52, -2.54 (2s, 2H, 2 x NH); 1.70 (t, 6H, 3 x -CH2CH3); 1.99, 2.01 (2s, 6H, 2 x angular-CH3); 3.36 (s, 6H, 2 x ring-CH3); 3.25-3.60 (m, 4H, - C H 2 at C-23 and C-123); 3.75 (d, 2H, H at C-21 and C-121); 3.78-3.86 (q, 4H, 2 x -CH2CH3); 4.31 (m, 2H, H at C-22 and C-122); 7.0 (m, 2H, H at C-24 and C-124); 7.50-8.03 (m, 10H, Ar-H); 9.05, 9.33 (2s, 4H, 4 x meso-H) LRMS (m/z): 860 (M+), 719 (M+ - SO2C6H5), 702 (M+ - SO2C6H5 - 2H - CH 3) FAB (m/z): 861 (M++1) EXACT MASS: calcd for C50H48N6O4S2: 860.3178 found by high resolution mass spectroscopy: 860.3181 VISIBLE SPECTRUM (CH 2C1 2): XmaxCnm): 384 412 488 520 668 798 734 peak ratio: 16.15 25.63 2.80 1.00 1.22 1.27 12.09 264 8,18-Diethyl-21,22,12l422-tetrakis(ethoxycarbonyl)-23,123-dihydro-2,7,12,17-tetramethyI-dibenzo[b,l]porphyrin (182) The A,C-divinylporphyrin 106 (50 mg, 0.105 mmoles) and diethyl acetylenedicarboxylate (750 mg, 5.25 mmoles) were suspended in toluene (20 mL). The system was degassed by three freeze-thaw cycles and then heated at 110 °C for 3 days. The reaction mixture was concentrated in vacuo and the mixture was submitted to flash chromatography (silica gel 60,2% methanol in dichloromethane as eluant). The excess diethyl acetylenedicarboxylate was eluted first and the subsequent fractions combined and concentrated in vacuo. The resulting residue was subjected to chromatography on a chromatotron (1 mm silica gel plate, same solvent as above). The mono-adduct eluted first in negligible amount (showing a band at 656 nm), followed by the bis-adduct (bacteriochlorin) which was obtained in 52% yield. A more polar fraction eluted behind the bis-adduct as a by-product of the reaction. M.P. : 175-177 'C iH-NMR (CDC13) 8: -2.51 (s, 2H, 2 x NH); 1.08 (t, 6H, -CO2CH2CH3 at C-2' and C-12'); 1.40 (t, 6H, -CO2CH2CH3 at C-2 2 and C-122); 2.00, 2.02 (2s, 6H, angular-CH3 at C-2' and 265 C-121); 3.40 (s, 6H, 2 x ring-CH3); 3.62 (m, 2H, H at C-23 and C-123); 3.85 (m, 4H, 2 x -CH2CH3); 3.95 (m, 2H, H at C-23 and C-123); 4.30-4.40 (m, 4H, 2 x -CO2CH2CH3 at C-22 and C-122); 4.42-4.62 (m, 4H, 2 x -CO2CH2CH3 at C-21 and C-121); 7.23-7.28 (m, 2H, H at C-24 and C-124); 8.95 (s, 2H, H at C-5 and C-15); 9.18 (s, 2H, H at C-10 and C-20) L R M S (m/z): 814 (M+), 798 (M+ - C H 3 - H), 782 (M+ - 2 x CH3 - 2H), 741 (M+ -CO2CH2CH3), 726 (M+ - CH3 - CO2CH2CH3) EXACT MASS: calcd for C48H54N4O8: 814.3927 found by high resolution mass spectroscopy: 814.3957 VISIBLE SPECTRUM (CH2C12): Xmax(nm): 384 406 484 538 666 698 738 peak ratio: 16.42 19.35 2.62 1.87 1.00 1.43 11.88 8,18-Diethyl-21,22,12l422-tetrakis(ethoxycarbonyI)-2l,12l-dihydro-2,7,12,17-tetramethyl-dibenzo[b,l]porphyrin (183) The bis-adduct 182 was dissolved in dichloromethane and a few drops of DBU were added. The reaction mixture was stirred in the dark and monitored by visible spectroscopy 266 (completed in 3 hours). The mixture was poured into IM HC1 and extracted with dichloromethane. The organic layer was washed twice with brine, once with water and dried over anhydrous magnesium sulfate. Filtration and evaporation of the filtrate gave a crude product which was purified by chromatography on a chromatotron (1 mm silica gel plate, 2% methanol in dichloromethane as eluant). The product was obtained in 90% yield. M.P. : 279-280 'C 1H-NMR (CDC13) 8: -1.87 (br s, 2H, 2 x NH); 0.33, 0.38 (t, 6H, -CO2CH2CH3 at C-2* and C-121); 1.46 (t, 6H, -CO2CH2CH3 at C-22 and C-122); 1.74, 1.78 (s, 6H, angular-CH3 at C-21 and C-121); 1.75 (t, 6H, 2 x -CH2CH3); 3.30-3.60 (m, 4H, -CO2CH2CH3 at C-21 and C-121); 3.35 (s, 6H, 2 x ring-CH3); 3.75-3.90 (m, 4H, 2 x -CH2CH3); 4.35-4.50 (m, 4H, -CO2CH2CH3 at C-22 and C-122); 4.90 (s, 2H, H at C-21 and C-121); 7.28, 7.78 (2d, 4H, J = 8 Hz, H at C-23, C-24, C-123 and C-124); 8.76 (s, 2H, H at C-5 and C-15); 9.13 (s, 2H,HatC-10and C-20) LRMS (m/z): 814 (M+), 726 (M+ - CH3 - C02CH 2 CH 3 ), 638 (M+ - 2 x C H 3 - 2 x -CO2CH2CH3) EXACT MASS: calcd for C48H54N4O8: 814.3927 found by high resolution mass spectroscopy: 814.3957 VISIBLE SPECTRUM (CH2CI2): XmaxCnm): 448 468 588 622 702 742 786 peak ratio: 4.78 6.89 1.66 2.36 1.11 1.00 4.40 267 REFERENCES 1. Leeper, FJ . Nat. Prod. Rep. 2, 19 (1985) 2. Krauder, B. Chimia. 4J., 277 (1987) 3. 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