UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Efforts towards the total synthesis of nosiheptide Hwang, Hee Jong Jason 2015

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
24-ubc_2015_september_hwang_heejongjason.pdf [ 2.82MB ]
Metadata
JSON: 24-1.0167208.json
JSON-LD: 24-1.0167208-ld.json
RDF/XML (Pretty): 24-1.0167208-rdf.xml
RDF/JSON: 24-1.0167208-rdf.json
Turtle: 24-1.0167208-turtle.txt
N-Triples: 24-1.0167208-rdf-ntriples.txt
Original Record: 24-1.0167208-source.json
Full Text
24-1.0167208-fulltext.txt
Citation
24-1.0167208.ris

Full Text

EFFORTS TOWARDS THE TOTAL SYNTHESIS OF NOSIHEPTIDE  by  Hee Jong (Jason) Hwang B.Sc., The University of British Columbia, 2012    A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF  MASTER OF SCIENCE  in  The Faculty of Graduate and Postdoctoral Studies (Chemistry)   THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  May 2015  © Hee Jong (Jason) Hwang, 2015 ii   Abstract  This thesis described here aims to achieve the total synthesis of nosiheptide, a representative member of the so-called E-series of thiopeptide antibiotic. Our group has established various techniques for the assembly of their complex molecular framework, and demonstrated the total syntheses of two D-series thiopeptide antibiotics. However, there is no precedent total synthesis of compound belonging to E-series. The part of the reason is that the presence of an additional 3-OH substituent not only adds significant synthetic complications, but also bars the use of the previously established technology. The present work details the assembly of the pyridine-thiazole cluster of E-series thiopeptide substance through a modified Hantzsch reaction that delivers the complete pyridine segment in a triply convergent fashion. Optimization studies related to this pyridine formation have further enhanced the conciseness and simplicity of the method. This chemistry thus developed can be applied to medicinal chemistry investigations in thiopeptide antibiotic field; an endeavor that will possibly unveil valuable new antibiotics.          iii  Preface  Hee Jong (Jason) Hwang has performed all experiments reported in this document, planned tactical synthetic routes described, and wrote this dissertation. Marco A. Ciufolini has provided technical suggestions. The work described in Chapter 3 has been published. Hwang, H-J. and Ciufolini, Marco. A. (2015) A Route to the Heterocyclic Cluster of the E-series of Thiopeptide Antibiotics. Journal of Organic Chemistry, 80: 4184-4188.              iv  Table of Contents Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iii Table of Contents ......................................................................................................................... iv List of Tables ................................................................................................................................ vi List of Figures .............................................................................................................................. vii List of Schemes ............................................................................................................................. ix List of Abbreviations .................................................................................................................. xii Acknowledgements .................................................................................................................... xvi 1.  Introduction ........................................................................................................................1 1.1      Background ...............................................................................................................1 1.2      The Thiopeptide Antibiotics .....................................................................................1 1.3      Objective of the Present Studies ...............................................................................6 2.  Previous Syntheses of Thiopeptide Antibiotics ...............................................................8 2.1      Background ...............................................................................................................8 2.2      Background on Pyridine Synthesis .........................................................................10 2.3      Synthesis of 3-hydroxypyridine Cores of the Type Found in E-series  ..................18 3.  Synthesis of the Pyridine Core of the E-series of Thiopeptides  ..................................24 3.1      Eiden-Herdeis Approach .........................................................................................25 3.2      The Hantzsch Route ................................................................................................27 3.3      Model Study to Restore Hydroxypyridine ..............................................................35 3.4      Approaches to the Assembly of the Real Pyridine .................................................38  v  3.5      Restoration of 3-hydroxypyridine ...........................................................................49 4.  Indole Segment of Nosiheptide  ......................................................................................51 5.  Conclusion  .......................................................................................................................53 References .....................................................................................................................................54 Appendix .......................................................................................................................................58                     vi  List of Tables Table 1.   Optimization Studies of the Pyridine Formation ...........................................................33 Table 2.   Attempted Optimization Sequences Leading to the OMOM Pyridine  .........................47                       vii  List of Figures Figure 1.   Representative Thiopeptide Antibiotics  ........................................................................2 Figure 2.   Examples of A, B, and C-type Thiopeptides Under Hansen’s Classification  ...............4 Figure 3.   Biosynthesis of Pyridine and Tetrahydropyridine Units of Thiopeptide Antibiotics .....5 Figure 4.   Hypothetical Desired Pyridine Core of E-series Thiopeptides ......................................7 Figure 5.   Some Thiopeptides that Have Been Succumbed by Total Synthesis  ............................9 Figure A1.   1H NMR of 105..........................................................................................................62 Figure A2.   13C NMR of 105 ........................................................................................................62 Figure A3.   1H NMR of 100..........................................................................................................65 Figure A4.   13C NMR of 100 ........................................................................................................65 Figure A5.   1H NMR of 109..........................................................................................................68 Figure A6.   13C NMR of 109 ........................................................................................................68 Figure A7.   1H NMR of 114..........................................................................................................70 Figure A8.   13C NMR of 114 ........................................................................................................70 Figure A9.   1H NMR of 116..........................................................................................................73 Figure A10.   13C NMR of 116 ......................................................................................................73 Figure A11.   1H NMR of 117........................................................................................................75 Figure A12.   13C NMR of 117 ......................................................................................................75 Figure A13.   1H NMR of 121........................................................................................................78 Figure A14.   13C NMR of 121 ......................................................................................................78 Figure A15.   1H NMR of 122........................................................................................................79 Figure A16.   1H NMR of 91..........................................................................................................82  viii  Figure A17.   13C NMR of 91 ........................................................................................................82 Figure A18.   1H NMR of 89 at Room Temperature .....................................................................85 Figure A19.   1H NMR of 89 at 65oC in d3-acetonitrile (400MHz) ...............................................85 Figure A20.   13C NMR of 89 at Room Temperature  ...................................................................86 Figure A21.   13C NMR of 89 at 65oC in d3-acetonitrile (100MHz) ..............................................86 Figure A22.   1H NMR of 142........................................................................................................89 Figure A23.   13C NMR of 142 ......................................................................................................89 Figure A24 .  1H NMR of Suspected Dimer of Crude 164 ............................................................90 Figure A25.   1H NMR of 141 at Room Temperature in d3-acetonitrile ........................................93 Figure A26.   1H NMR of 141 at 80oC in d6-DMSO (400MHz) ...................................................93 Figure A27.   13C NMR of 141 at 80oC in d6-DMSO (100MHz) ..................................................94 Figure A28.   Expanded HSQC of 141 ..........................................................................................94 Figure A29.   HPLC Trace of Pyridine 141 ...................................................................................95 Figure A30.   1H NMR of 171 at Room Temperature in d3-acetonitrile  .......................................98 Figure A31.   1H NMR of Partially Released 171 at 80oC in d6-DMSO (400MHz)   ...................98 Figure A32.   1H NMR of 172 at Room Temperature in d3-acetonitrile  .....................................101 Figure A33.   1H NMR of 172 at 65oC in d6-DMSO (400MHz) .................................................101 Figure A34.   13C NMR of 172 at 65oC in d6-DMSO (100MHz) ................................................102 Figure A35.   1H NMR of 177......................................................................................................105 Figure A36.   13C NMR of 177 ....................................................................................................105 Figure A37.   1H NMR of 178......................................................................................................108 Figure A38.   13C NMR of 178 ....................................................................................................108   ix  List of Schemes Scheme 1.   Biosynthesis of Thiazole Moieties of Thiopeptide Antibiotics ....................................6 Scheme 2.   Building Blocks of Kelly’s Synthesis of Micrococcinic Acid  ..................................11 Scheme 3.   Kelly’s Attempt to Synthesize Aryl Stannyl Derivatives and Formation of                     Homodimers ...............................................................................................................12 Scheme 4.   Kelly’s Synthesis of the Pyridine Core of Microccocinic Acid .................................13 Scheme 5.   Bach Synthesis of Pyridine Core of ent-GE2270A ....................................................14 Scheme 6.   Moody Synthesis of Pyridine Core of Promothiocin A .............................................15 Scheme 7.   Moody’s Biomimetic Synthesis of Pyridine Core of Amythiamicin D .....................16 Scheme 8.   Nicolaou’s Biomimetic Synthesis of Pyridine Core of Thiostrepton .........................17 Scheme 9.   Ciufolini’s de novo Pyridine Synthesis of Micrococcin P1 and Thiocillin I .............18 Scheme 10.   Shin’s Route to the Pyridine Core of Nosiheptide ...................................................20 Scheme 11.   Arndt’s Hetero Diels-Alder Approach to the Pyridine Core of Nosiheptide ...........21 Scheme 12.   Arndt’s Failed Attempt for the Construction of Bis-Thiazolo-Pyridine ..................21 Scheme 13.   Arndt’s Hetero Diels-Alder to Pyridine Core of Nosiheptide ..................................22 Scheme 14.   Hypothetical Approaches to the Pyridine Core of Nocathiacin and Nosiheptide ....24 Scheme 15.   Representative Eiden-Herdeis Reaction ...................................................................25 Scheme 16.   Modified Eiden-Herdeis Synthesis that May Lead to a 3-oxygenated Pyridine ......26 Scheme 17.   Failed Attempt of Eiden-Herdeis Reaction ..............................................................27 Scheme 18.   Retrosynthetic Hypothesis for the Construction of Pyridine ...................................28 Scheme 19.   Preparation of Model Substrates ..............................................................................29 Scheme 20.   Preparation of Model 3-ethoxypyridine ...................................................................29  x  Scheme 21.   Retrosynthetic Analysis of Thiazole Enone Fragment .............................................30 Scheme 22.   Preparation of Thiazolyl Enone 117.........................................................................31 Scheme 23.   Unexpected Thiazole Anion Behavior .....................................................................31 Scheme 24.   Formation of 1,5 Diketone of Model Tris-Thiazole System ....................................32 Scheme 25.   Unexpected Outcome of Condensation of Pyridine 121 ..........................................33 Scheme 26.   Hypothetical Mechanism of Elimination of the Pyridine.........................................34 Scheme 27.   Hypothetical Mechanism of Desethoxypyridine Formation in Tris-Thiazole                       System ......................................................................................................................35 Scheme 28.   Envisioned Deprotection Sequence ..........................................................................36 Scheme 29.   Unfruitful Deprotection Sequences Attempted ........................................................37 Scheme 30.   Retrosynthetic Analysis of Pyridine Core Exhibiting More Labile Protecting                       Group .......................................................................................................................38 Scheme 31.   Retrosynthetic Analysis of the ‘Real’ Pyridine ........................................................38 Scheme 32.   Preparation of Thioamide 144 ..................................................................................39 Scheme 33.   Hantzsch Thiazole Synthesis ....................................................................................39 Scheme 34.   Racemization of α-amino Thiazoles .........................................................................40 Scheme 35.   The Holzapfel-Meyers-Nicolaou (HMN) Variant of Hantzsch Thiazole                       Synthesis  .................................................................................................................40 Scheme 36.   Preparation of Ketone 89..........................................................................................41 Scheme 37.   Envisioned Synthesis of Enone From the Corresponding Carboxylic Acid ............41 Scheme 38.   Unsuccessful Functional Group Interconversion Due to Rapid Decarboxylation ...42 Scheme 39.   Synthesis of Enone 159 ............................................................................................43 Scheme 40.   Unexpected Outcomes of Elimination Pathways and Circumvention .....................44  xi  Scheme 41.   Formation of Dimeric Adduct 164 and Formation of Enone 142 ............................45 Scheme 42.   Possible Insights into the Reactivity of Aromatic Aldehyde ...................................46 Scheme 43.   Summary of the Route to Pyridine 141 ....................................................................48 Scheme 44.   Successful Peng Deprotection of Phenolic MOM-Ether ..........................................49 Scheme 45.   Fully Deblocked Hydroxypyridine for Full Characterization ..................................50 Scheme 46.   The Desired Indole Fragment of Nosiheptide ..........................................................51 Scheme 47.   Synthesis of Indole Fragment of Nosiheptide 178 ...................................................52 Scheme 48.   Anticipated Completion of Nosiheptide ...................................................................53                xii  List of Abbreviations [α]  specific rotation Ac  acetyl (acac)  acetylacetonate  aq  aqueous Bn  benzyl Boc  tert-butyloxycarbonyl b  broad Bu  butyl oC  degrees Celsius cat.  catalytic cald  calculated cm-1  wavenumbers δ  chemical shift (parts per million down from tetramethylsilane) d  doublet D-A  Diels-Alder reaction DBU  1,8-diazobicyclo[5.4.0]undec-7-ene DCE  dichloroethane DCM  dichloromethane DDQ  2,3-dichloro-5,6-dicyano-1,4-benzoquinone  xiii  DIBAL diisobutylaluminum hydride DMAP  4-N,N-dimethylaminopyridine DMF  N,N-dimethylformamide DMSO  dimethyl sulfoxide dppp  1,3-bis(diphenylphosphino)propane EBP  ethyl bromopyruvate Ef  elongation factor eq  equivalents Et  ethyl FCC  Flash Column Chromatography g  gram(s) h  hour(s) HFIP  1,1,1,3,3,3,-hexafluoro-2-propanol HRMS  high resolution mass spectrometry Hz  Hertz (s-1) i  iso (as an alkyl group) IR  Infrared J  coupling constant LDA  lithium diisopropylamide LRMS  loss resolution mass spectrometry m  multiplet  xiv  M  molar (moles per liter); mega mCPBA meta-chloroperoxybenzoic acid Me  methyl min  minute(s) MOM  methoxymethyl mp  melting point MP  micrococcin P n  normal (as an alkyl group) NBS  N-bromosuccinimide n-BuLi  n-butyllithium NMR  nuclear magnetic resonance p  para (as a benzene substituent)   Pg  unspecified protecting group Ph  phenyl PPA  polyphosphoric acid ppm  parts per million PPTS  pyridinium p-toluenesulfonate Pr  propyl py  pyridine q  quartet Rf  retention factor  xv  RNA  ribonucleic acid r.t  room temperature s  singlet sat.  saturated t  triplet t  tertiary (as an alkyl group) TBAF  tetra-n-butylammonium fluoride TBS  tert-butyldimethylsilyl t-BuLi  tert-butyllithium TES  triethylsilyl Tf  trifluoromethanesulfonyl TFAA  trifluoroacetic anhydride TFE  2,2,2-trifluoroethanol  THF  tetrahydrofuran THP  tetrahydropyran Thz  unspecified thiazole units TIPS  triisopropylsilyl TLC  Thin Layer Chromatography TMS  trimethylsilyl μ  micro   xvi  Acknowledgements  I would like to thank my supervisor, Marco A. Ciufolini, for taking me into his group and for letting me explore the world of chemistry with intellectual freedom. Years spent his group not only enabled me to re-evaluate the true meaning of passion and dedication that Marco emanates, but also taught me to appreciate many aspects of life. I feel truly fortunate to work under his supervision.  I would also like to extend my gratitude towards my fabulous lab members, past and present. Memories I have engraved with you have been a great journey: from chemistry consultation to life in general. Dr. Matthew Smith, who taught me a great deal of practical lab experience, Dr. Josh Zaifman for enormous consultation and joyful conversation, Taka for sharing his chemistry wisdom and random chit-chatting filled with laughter, Leanne for her funny jokes and support, Patrick for his confidence which I have applied in my experiments, Marco for properly introducing me great Italian food and culture, Dimitri for regular coffee with croissant and unforgettable chats, and Nikita for being super understanding and bearing with me in A322.   This journey could not have been achieved with the support of my family: My father who always listens, and gives unconditional support, my mother who always provides emotional support, and my brother for being my best friend.  Thank you my love, Bonnie Park, who has given me strength and reason to excel. I could not have gone this far without you. Thank you for her family for countless support.    Lastly, I am indebted for many of my friends who have been a great support. 1  1. Introduction 1.1   Background  The discovery of penicillin by Sir Alexander Fleming, in the 1920's, ushered in the so-called antibiotic era, and for the first time in human history provided the medical establishment with effective weapons to combat a wide variety of bacterial ailments. This not only saved millions of lives, but also stimulated intense research in the field of antibiotics. Soon after the introduction of the first antibiotics, however, it became apparent that bacterial pathogens become resistant to such drugs over time.1 It is now established that microorganisms gain resistance through genetic mutations.2 That disturbing observation should have stimulated a quest for new antibiotics; yet – paradoxically – research in the field, both in the public and the private sectors, abated greatly in the 1970's and 1980's, and very few new antibiotics have been discovered and developed since. The inevitable – if entirely predictable – consequence is that the world is now facing an "antibiotic crisis,"3 which threatens to return humankind to the "dark age" of pre-antibiotic times. The problem is especially acute in hospitals, where patients already weakened by various conditions tend to contract lethal infections caused by strains of Staphylococcus, Enterococcus, and other bacterial species that have become resistant even to vancomycin: an antibiotic that is reserved as a drug of last resort.4 Such "nosocomial infections" constitute a major public health concern.5    1.2 The Thiopeptide Antibiotics The foregoing state of affairs has rekindled interest in the search of new antibacterial resources. In that connection, the so-called thiopeptide antibiotics are actively being investigated as possible sources of new anti-infective drugs.6 Thiopeptides are highly modified, sulfur-rich,  2  cyclic peptides produced by Streptomyces and Bacillus species, and exemplified below by microccoccin P1 [MP1], 1, thiocillin I, 2, nosiheptide, 3, and nocathiacin I, 4. They are characterized by the a heterocyclic core consisting of pyridine, which may be present in partially– or completely reduced form, decorated with various thiazole and/or oxazole residues, and embedded within a macrocyclic array.7 Various dehydroamino acid subunits are also present.  The first thiopeptide to be discovered is 1, which was described in 1948. Since then, approximately 80 members of the family have been reported.   Figure 1. Representative Thiopeptide Antibiotics  All thiopeptides are potent inhibitors of protein synthesis, seemingly as a consequence of their tight binding to particular regions of the ribosome, ultimately impairing the proper functioning thereof. In that respect, they may be distinguished into 2 classes: those that target the  3  L11 domain of the 23S ribosomal RNA subunit,8 and those that bind to the elongation factor (Ef).9 The first class of thiopeptides are believed to stabilize a particular ribosomal conformation, thereby hindering the conformational changes required for the elongation step of protein synthesis. The second group of compounds bind strongly to the elongation factor known as Ef-Tu factor. This prevents the transfer of aminoacyl residues to the nascent peptide chain. In either case, protein synthesis is halted, resulting in the death of the organism. Therefore, thiopeptides are potent antiprotozoal, antifungal, and even antitumor agents.6 In addition, they possess elevated antimicrobial activity, but only against Gram-positive bacteria. Even so, their potency against methicillin-resistant Staphylococcus aureus (MRSA; a strain of great current concern) makes them attractive as starting points for the development of new antibiotics. Hensens suggested a classification of thiopeptide antibiotics into five different groups, A-E, depending on the oxidation state of the pyridine nucleus.10 Group A includes compounds that incorporate a fully reduced pyridine; e.g., thiopeptin A3a, 5. The B series encompasses substances that display a tetrahydropyridine core; e.g., thiostrepton A, 6. The compound, Sch 40832, 7, is the only member of the C family, which is characterized by the presence of an imidazopiperidine core (Figure 2). The D class thiopeptides counts the largest number of members, all of which exhibit a 2,3,6-trisubstituted pyridine, as seen in micrococcin P1, 1, and thiocillin I, 2. Finally, a 3-hydroxypyridine core defines the E series of antibiotics, exemplified by nosiheptide, 3, and nocathiacin, 4.  4   Figure 2. Examples of A, B, and C-type Thiopeptides Under Hansen’s Classification The biosynthesis of thiopeptides is believed to involve a series of post-translational modifications of a large peptide. Bycroft and Gowland first suggested in 1978 that an aza Diels-Alder reaction may be involved in the biosynthesis of the pyridine core in the thiopeptide antibiotics11. This hypothesis was confirmed only recently, thanks to the work of Walsh12 and others.  Micrococcin P1, 1, and thiocillin I, 2, are now accepted to emanate from a polypeptide  5   Figure 3. Biosynthesis of Pyridine and Tetrahydropyridine Units of Thiopeptide Antibiotics  6  such as 8, which undergoes a double dehydration leading to 9, wherein the letters in parentheses [(C), (T), (V)] represent either actual amino acid residues, or amino acid-derived thiazolines or thiazoles.13 Within an appropriate enzyme, peptide 9 undergoes amide tautomerization and folding into conformation 10, which promotes intramolecular cycloaddition leading to 11.  Two possible fates now await the newly formed 11: dehydration and reduction (path A) lead to the A and B series of natural products (structure 12), whereas dehydration and elimination of the acylamino moiety (path B) yield the fully aromatic pyridine 13 seen in the D series. Interestingly, Nicolaou and Moody have duplicated these biosynthetic routes to the pyridine core of thiopeptides in their own synthetic work (vide infra).  On the other hand, the thiazole residues are likely to arise through the cyclization of N-acyl cysteine segments 14 to thiazolines 15, which subsequently undergo aromatization to form thiazole 1614 (Scheme 1).  Scheme 1. Biosynthesis of Thiazole Moieties of Thiopeptide Antibiotics  1.3   Objective of the Present Study  The present study aimed to devise a rapid synthesis of the 3-hydroxypyridine core of the E-series of thiopeptide antibiotics, in a form suitable for the conduct of a total synthesis of representative members of the family. A plausible form of the desired pyridine would thus be 17.  7   Figure 4. Hypothetical Desired Pyridine Core of E-series Thiopeptides It should be noted that very few thiopeptides have succumbed to total synthesis, and that no syntheses of E-type compounds have yet been achieved. Even exploratory studies in the area are few. There can be little doubt that this is a consequence of the difficulties encountered in the assembly of pyridines 17. The section that follows elaborates the point by outlining key aspects of known syntheses of thiopeptides; by illustrating how the technology is devised in that connection shows how entirely inadequate it is for the construction of compound 17.            8  2. Previous Syntheses of Thiopeptide Antibiotics 2.1 Background Thiopeptide antibiotics began to attract attention from the synthetic community only in the late 1990's. Even so, only seven thiopeptides have fallen to total synthesis as of this writing: promothiocin A, 18,15 in 1998, amythiamicin D, 19,16 and thiostrepton A, 6,17 in 2005, Siomycin A , 2018 and GE2270A22, 21, in 2007, microccocin P1, 1,19  in 2009, and thiocillin I, 2,20 in 2011 (Figure 5).   9   Figure 5. Some Thiopeptides that Have Been Succumbed by Total Synthesis Early synthetic efforts in the area uncovered an important strategic principle: the key to a successful synthesis of thiopeptide compounds is the rapid assembly of their pyridine-thiazole  10  core cluster. Despite the fact that the synthesis of pyridines has been studied for a long time, the preparation of such complex pyridine-thiazole arrays remains a challenging proposition.  Furthermore, the numerous difficulties that it presents are best addressed through the development of new chemical methodology. Indeed, the main players in the field, C. J. Moody, K. C. Nicolaou, and M. A. Ciufolini, have all devised new techniques for the construction of those crucial subunits. The nature of such problems, and some noteworthy solutions developed in response to these, are highlighted in paragraphs that follow.  2.2 Background on Pyridine Synthesis  Two major strategies have been followed for the assembly of thiopeptide core clusters: the modification of a pre-existing pyridine through metal-mediated coupling reactions,21,22 and the de novo assembly of the pyridine with the full complement of thiazoles. The first approach is apparent in the earliest recorded study in the thiopeptide field: the Kelly synthesis of micrococcinic acid, 35,21 a product of acid hydrolysis of micrococcin P1. The route to 35 relied upon sequential Stille coupling23 of thiazole and pyridine units. This required fairly complex pyridyl- and thiazolylstannanes, the preparation of which proved to be lengthy and arduous. Even, the preparation of building blocks required 18 independent operations (Scheme 2).  11   Scheme 2. Building Blocks of Kelly’s Synthesis of Microccocinic Acid  Significant complications arose when all attempts to advance 29 to stannane 30, or 31 to stannane 32, were unsuccessful. The problem was that bromides 29 and 31 were largely converted into the corresponding homodimers in the presence of Pd catalysts. Evidently, species such as 30 are so electrophilic that most of the initially formed stannane reacts with more 29 to yield 33 (Scheme 3).  12   (a) 5mol % (Ph3P)2PdCl2, 2 eq 27, 55%  (b) Me3SiI, 72%  (c) POBr3, 88%, (d) 5mol % (Ph3P)2PdCl2, 2 eq 27, 65%, (e) Br2, 68%,   Scheme 3. Kelly’s Attempt to Synthesize Aryl Stannyl Derivatives and Formation of Homodiers A moderately successful stratagem to bypass such obstacles entailed the reaction of a 1:1 mixture of 29 and 31 with (Me3Sn)2 in the presence of a catalytic amount of (Ph3P)2PdCl2, whereupon the desired product 34 was isolated in 25% yield over 3 steps (Scheme 4).  From the standpoint of process chemistry, Kelly’s methodology is less than appealing.  13   (a) 4eq (Me3Sn)2, 31, 11mol % (Ph3P)2PdCl2, 49%, (b) aq. HNO2, 97%  (c) Tf2O, 58%, (d) 10mol % (Ph3P)4Pd, 25, 89%  (e) H3O+, 80% Scheme 4. Kelly’s Synthesis of the Pyridine Core of Micrococcinic Acid Similar difficulties have plagued a more recent synthesis of GE2270A by Bach and coworkers22 (Scheme 5). Key steps in this work are two Negishi and one Stille cross-coupling reactions to elaborate 2,3,6-tribromopyridine 36 into the trisubstituted pyridine core 43. Complete C-3 regioselectivity was observed during an initial halogen-lithium exchange of tribromopyridine 36, and the resultant 3-pyridyllithium was successfully converted into the corresponding Zn organometallic. The first Negishi coupling with 37 performed well to afford 38 in 81% yield. However, the second Negishi coupling of 38 with 39 occurred with moderate C-6 regioselectivity (6:5:1) and produced considerable quantities of homodimers. This led to complex mixtures of products, which were difficult to separate. Laborious purification efforts and the use of expensive transition metal catalysts clearly overshadow this approach.  14   (a) n-BuLi, THF, ZnCl2, PdCl2(PPh3)2, 81% (b) BnNH2, DIBAL-H, THF, DCM, 86% (c) PdCl2(PPh3)2, THF, 78% (d) Pd(PPh3)4, dioxane, 80oC, 61% Scheme 5. Bach Synthesis of Pyridine Core of ent-GE2270A A de novo construction of the pyridine unit can circumvent many of the foregoing difficulties. On the other hand, it requires the development of appropriate pyridine-forming methodology. A convincing demonstration is provided by the Moody synthesis of Promothiocin A 18.15 Thus, a modified Bohlmann-Rahtz reaction involving the union of ynone 45 with enamine 44 created pyridine 47 in a single step. The process is likely to involve Michael-type addition of the enamine to the ynone, followed by cyclization / aromatization of the resultant 46.  15  Compound 47 was rapidly advanced to 48, which was then elaborated to promothiocin A (Scheme 6).  (a) LiOH, 92% (b) EtO2CCl, Et3N, then NH4OH, 85% (c) Lawesson’s Reagent, 59% (d) EBP, 72% Scheme 6. Moody Synthesis of Pyridine Core of Promothiocin A  Moody and Nicolaou independently described a remarkable biomimetic avenue to thiopeptide cores through aza-Diels-Alder reaction. The centerpiece of Moody's synthesis of amythiamicin D, 19,16 is the cycloaddition of 49 to 50, followed by eliminative aromatization of intermediate dihydropyridine 51 (Scheme 7).  16   Scheme 7. Moody’s Biomimetic Synthesis of Pyridine Core of Amythiamicin D The technology is especially valuable for the construction of the partially reduced pyridines found in thiostrepton and related antibiotics. The Nicolaou synthesis of thiostrepton embodies an especially impressive application of the technique.17 Thus, treatment of thiazolidine 53 with Ag2CO3 and DBU produced transient azadiene 54, which underwent sequential hetero Diels-Alder dimerization – imine hydrolysis to afford the desired aminopiperidine 57 in a single step (Scheme 8).   17   Scheme 8. Nicolaou’s Biomimetic Synthesis of Pyridine Core of Thiostrepton Finally, the Ciufolini synthesis of Micrococcin P119 introduced a modification of the classical Hantzch pyridine synthesis that enabled the merger of fragments 58 and 59 into pyridine 60 in two steps, while the synthesis of Thiocillin I20,24 demonstrated an even more concise route to pyridine 62 through yet another modification of the Bohlmann-Rahtz reaction (Scheme 9).    18   (a) cat Li2CO3, EtOAc, 99% (b) NH4OAc, EtOH then DDQ, toluene, 98% Scheme 9. Ciufolini’s de novo Pyridine Synthesis to Micrococcin P1 and Thiocillin I   2.3 Synthesis of 3-hydroxypyridine Cores of the Type found in E-series The pyridine-forming techniques outlined above lead to 2,3,6-trisubstituted pyridines.  However, the heterocyclic core of the E-series of thiopeptides, such as nocathiacin, 4, and nosiheptide, 3 (Figure 1), is based on a tetrasubstituted, 3-hydroxypyridine. The presence of the OH substituent greatly complicates the assembly of the target pyridines and bars the use of previously developed technology. Perhaps it is for this reason that no total synthesis of any E-type thiopeptide has been achieved to date. Indeed, few chemists have attempted the synthesis of  19  such compounds, and activity in the area has been largely confined to the preparation of simpler building blocks.25, 26, 27,28 Routes to E-type, 3-hydroxypyridine thiopeptide cores have been described by Shin and by Arndt. Shin and collaborators favored a linear approach that involves the modification of a preformed pyridine through classical transformations. Their route (Scheme 10) leads to compound 70 through a lengthy, 14-step sequence. Furthermore, product 70 suffers from two major shortcomings. First, the substance is obtained as a bis-ethyl ester. These two ester functions will have to be differentiated at a later stage of the synthesis, but as demonstrated by Nicolaou,29 this is extremely difficult to do with materials of the type 70. Translation of the Shin avenue to 70 into a total synthesis will necessitate a retailoring of the sequence, in such a way that the two ester groups (or their equivalents) emerge in a suitably differentiated form. Second, the 3-pyridinol segment in 70 is O-protected as an ethyl ether. Our own experience (vide infra) strongly suggests that this form of protection is entirely inappropriate, in that attempts to cleave the ethyl ether and release the free pyridinol have uniformly resulted in destruction the molecule.    20   (a) CuCN/DMF, 85% (b) Et2SO4, K2CO3/THF, (c) H2S/py, Et3N, 80% over 2 steps (d) EBP/EtOH, 81% (e) m-CPBA/CH2Cl2, 100% (f) TMSCN, Et3N/MeCN, 83% (g) EBP, K2CO3/THF, TFAA-py/THF (h) Ac2O, 81% over 2 steps (i) Tf2O, iPrNEt2/DMAP (j) ethyl vinyl ether, Pd(OAc)2, dppp, Et3N/DMF, 64% (k) NBS, H2O/THF, (l) EtOH  Scheme 10. Shin’s Routes to the Pyridine Core of Nosiheptide  It should be mentioned that Shin and collaborators employed similar linear strategies and classical transformations also in their synthesis of micrococcin-type cores.30 The length of the resulting sequences render their approach less attractive relative to the ones delineated in Schemes 2 to 5 above.  21  Arndt and his group have focused on a convergent route that relies on 1-azadiene cycloaddition technology. Thus, the union of readily available oxime 72 with appropriate acetylenes 71 led directly to 7426,31 (Scheme 11).   Scheme 11. Arndt’s Hetero Diels-Alder Approach to the Pyridine Core of Nosiheptide Two limitations plague this elegant strategy. First, the cycloaddition step proceeds with good regioselectivity only when the acetylenic R1 substituent is the carbonyl group of a ketone or an ester. Furthermore, the R3 group can only be an ester. Thus, Arndt's original plan for the assembly of 77 through the union of 75 and 76 was found to be unworkable, seemingly due to unfavorable steric interactions involving the thiazolyl group in azadiene 76 (Scheme 12).  The methodology thus fails to provide access to fully substituted E-type pyridines.  Scheme 12. Arndt’s Failed to Attempt for the Construction of Bis-Thiazolo-Pyridine To overcome such limitations, the authors resorted to a hetero-Diels-Alder reaction of ynone 75 and oxime 79, leading to pyridine 77 (Scheme 13). Eight additional steps advanced 77  22  to 82. The final incorporation of third thiazole was done through additional 6 steps to furnish 83. The longest linear step entailed in this synthesis is a long 17 steps.  (a) Toluene, 180oC, 3eq 79 (b) Tf2O, NEt3 (c) TIPSOTf, NEt3 (d) NBS, THF/H2O  Scheme 13. Arndt’s Hetero Diels-Alder to Pyridine Core of Nosiheptide  It is apparent from the foregoing that a de novo pyridine construction, especially one involving new methodology, generally results in a shorter and more efficient avenue to  23  thiopeptide cores. It is with this guiding principle in mind that we set out to devise a concise route to the core of E-type thiopeptides.                 24  3. Synthesis of the Pyridine Core of the E-series of Thiopeptides The logic that governed our approach to the E-series of thiopeptide hydroxypyridines rested on three principles. First, nosiheptide-type cores, 85, would derive from nocathiacin-like ones, 84, by OH  SH interconversion, perhaps through a Mitsunobu reaction.32 Second, previous synthesis of MP1, 1, and thiocillin I, 2, had shown that differentiation of the carboxy groups is best achieved by introducing the "northwestern" COOH as a protected primary alcohol.    Scheme 14.  Hypothetical Approaches to the Pyridine Core of Nocathiacin and Nosiheptide  25  Therefore, the target structures 84, 85 would be obtained from 86 (Scheme 14). Third, our routes to the pyridines cores of MP1 1 and thiocillin I 2 relied on a sequence characterized by a high degree of convergency, which translated into a rather concise synthesis. It was essential to retain such a desirable feature in an avenue to E-series hydroxypyridines. Such an objective seemed attainable though a Hantzsch-like construction, which required either precursor 87 or 88. The latter would be prepared through the union of ketone 89 either with ynone 61 or enone 90. In turn, fragment 89 would emanate from the condensation of ester 91 with organolithium species 92. In this manner, pyridine 86 would arise via a triply convergent sequence that involves the merger of segments 89 and 61, or either 89 and 90.  3.1 Eiden-Herdeis Approach  The elaboration of ketone 89 and ynone 61 into 87 was envisioned to proceed by a variant of the Eiden-Herdeis pyridine synthesis.33 In its original form, this useful transformation starts with a 1,4 addition of the enolate of a ketone 93 to an ynone 94, leading to 1,5-dicarbonyl compound 95 (Scheme 15). Reaction of the latter with — typically — NH4OAc then furnishes pyridine 96.  Whereas the process cannot directly deliver 3-hydroxy (or alkoxy) pyridines, it was  Scheme 15.  Representative Eiden-Herdeis Reaction  26  surmised that dihydroxylation of adduct 95 and acetylation of the emerging diol might produce 97, treatment of which with NH4OAc in all likelihood would lead to acetoxypyridine 99 (Scheme 16).  Scheme 16. Modified Eiden-Herdeis Synthesis that May Lead to a 3-oxygenated Pyridine As it happens, fragments 100 and 61 were already available in our laboratory, having been prepared during earlier investigations in the thiopeptide field. Therefore, they were conveniently employed in an exploratory study that aimed to ascertain the feasibility of the above modification of the Eiden-Herdeis reaction. To our dismay, it quickly became apparent that such an approach suffered from numerous chemical and technical complications.  Compound 101 was sensitive to silica gel and chromatographic purification was not possible. In addition, it appeared to equilibrate with isomer 102, which in turn existed as two geometric isomers. Attempted osmylation of this mixture of isomers produced an even more complex mixture of products. Clearly, the Eiden-Herdeis approach was neither chemically nor technically viable. Therefore, it was abandoned and our focus shifted to a route to 86 from 89 and 90 (Scheme 17).  27   Scheme 17. Failed Attempt of Eiden-Herdeis Reaction  3.2 The Hantzsch Route  As seen previously in Scheme 9, Ciufolini employed a modified Hantzsch pyridine synthesis for the construction of the heterocyclic core cluster of MP1. By extension of that logic, one may surmise that the union of ketone 89 and enone 90 should lead to an ultimate pyridine of the kind found in the E-series of thiopeptides (Scheme 18).  28   Scheme 18. Retrosynthetic Hypothesis for the Construction of Pyridine 86 The feasibility of such an approach was explored with model substrates 100 and 107, both of which were readily prepared as shown in Scheme 19. Condensation of ethyl bromopyruvate with thioacetamide, reduction of the emerging thiazole 104 (DIBAL), and protection of the resultant alcohol afforded 105.  This material can be regioselectively metallated at the methyl group.35 Condensation of lithiated 106 with ester 104 thus returned the model ketone 100 in 56% yield over 3 steps.  Model enone 107 was made in just one step by addition of lithiated ethyl vinyl ether to commercial amide 106.34   29   (a) thioacetamide, EtOH, 100%  (b) DIBAL-H, THF (c) TBS-Cl, imidazole, DCM, 85% over 2 steps (d) n-BuLi, THF, 104, 60%  (e) t-BuLi, THP/THF, ethyl vinyl ether, 65%     Scheme 19. Preparation of Model Substrates The lithium enolate of ketone 100, prepared by deprotonation with LDA, smoothly underwent Michael addition to enone 107 to furnish 1,5 diketone 108 as a mixture of diastereomers and of keto-enol– as well as ring-chain tautomers. Subjection of 108 to NH4OAc in refluxing ethanol under an O2 atmosphere (balloon) afforded pyridine 109 in a moderate, but satisfactory, 38% yield (Scheme 20).  (a) LDA, THF, 107, -78oC to 0oC (b) NH4OAc, EtOH, O2, 60oC, 38% over 2 steps  Scheme 20. Preparation of Model 3-ethoxypyridine  30  This encouraging result induced us to research whether the technique could be extended to the preparation of a tris-thiazolyl pyridine more closely resembling the core of thiopeptides; i.e., whether a thiazolyl enone of general structure 110 might be serviceable in the above sequence (Scheme 21).  Scheme 21. Retrosynthetic Analysis of Thiazole Enone Fragment Past experience had shown that enones similar to 110, but lacking the ethoxy functionality, are problematic substrates in Michael chemistry. They are extremely electrophilic, and this causes them to polymerize easily under a variety of nucleophilic conditions.35 At the present juncture, it was unclear whether the alkoxy group might alleviate or exacerbate such unpleasant tendencies. Indeed, the ethoxy unit might moderate the electrophilic reactivity of the enone,36 thus retarding Michael polymerization, or it might destabilize the enolate resulting from an initial 1,4-addition,37 so as to trigger a number of possible side reactions. Regardless, a suitable form of 110 seemed to be available by reaction of amide 111 with lithiated ethyl vinyl ether. Accordingly, a sequence very similar to that seen in Scheme 19, but employing thioformamide in lieu of thioacetamide, led to thiazole 115 in 20% overall yield (Scheme 22). Deprotonation of the latter and reaction with N,N-diethyl carbamoyl chloride produced 116, which upon addition of lithiated ethyl vinyl ether advanced to 117.    31   (a) P2S5, Et2O. (b) EBP, EtOH, 2 steps over 37%. (c) DIBAL-H, THF, -78oC to r.t (d) TBS-Cl, imidazole, DCM, 90% over 2 steps. (e) n-BuLi, Et2NCOCl, 50%. (f) t-BuLi, THP/THF, ethyl vinyl ether, 77%    Scheme 22. Preparation of Thiazolyl Enone 117   An unpleasant problem was uncovered during the transformation of 115 into 116: the reaction of lithiated 115 with N,N-diethyl carbamoyl chloride returned significant quantities of ketone 118: the product of double addition of 119 to the electrophile (Scheme 23). A slow addition of carbamoyl chloride to a solution of 119 (–78 °C) resulted in exclusive formation of 118.  A rapid addition of the electrophile afforded an essentially 1:1 mixture of undesired 118 and desired 116.  Luckily, no such complication was observed in the addition of lithiated vinyl ether to amide 116, and the desired enone 117 was obtained smoothly and in good yield.  Scheme 23. Unexpected Thiazole Anion Behavior We were pleasantly surprised to discover that enone 117 was a well-behaved substrate in Michael chemistry. Indeed, it transpired that the best way to carry out the Michael step was to add 117, at room temperature, to a solution of the sodium enolate of 100, prepared by reaction of  32  the ketone with sodium hydride. The lithium enolate of 100, prepared with LDA as shown earlier in Scheme 20, produced inferior results due to incomplete conversion after longer reaction times. Diketone 120 was again obtained as a mixture of tautomers and diastereomers (Scheme 24).  (a) NaH, THF, enone 117 Scheme 24. Formation of 1,5 Diketone of Model Tris-Thiazole System To our surprise, subjection of 120 to the pyridine-forming conditions described above for model system 108 (Scheme 20) furnished desethoxy pyridine 122 as the major product (Scheme 25). The desired 121 was present in the reaction mixture (1H NMR, mass spectrometry), but the extent of its formation was less than 5%. The formation of 122 must be due to aromatization of the sensitive dihydropyridine intermediate through elimination of ethanol. It was surmised that metallic oxidants, such as Fe(III)[acac]3 or Cu(II)(OAc)2, might accelerate the aromatization of the Hantzsch dihydropyridine and suppress the elimination of ethanol. However, reactions carried out in the presence of such metallic agents produced a greater amount of undesired pyridine.    33   (a) NH4OAc, EtOH, O2, 60oC (Table 1) Scheme 25. Unexpected Outcome of Condensation of Pyridine 121 Additional investigations revealed that the formation of desethoxy pyridine 122 could be suppressed by running the reaction in a slightly acidic media. Table 1 below summarizes the results of representative optimization experiments. The best solvent was a 5:8 mixture of 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), and best result were obtained by operating at 60 oC. Slow addition of 1,5 diketone 120 (syringe pump) into the reaction vessel was beneficial. Also, NH4OAc was optimal among the various ammonia sources (NH4Cl, NH4COOCF3, (NH4)2CO3) examined during this study. The moderate yield of 121 thus obtained, 35% after chromatography, was deemed satisfactory at this stage, especially in light of the conciseness and simplicity of the method. Solvent Temperature Medium condition Yield (%) “121” + “122” EtOH 60 oC O2 9% + 15% AcOH 60 oC O2 3% + 0% TFE 60 oC O2 19% + 0% HFIP 60 oC O2 6%  + 0% TFE/HFIP(5:8) 60 oC O2 28% + 0% TFE/HFIP(5:8) 60 oC Under O2, slow addition 35% + 0%  Table 1. Optimization Studies of the Pyridine Formation   It is interesting to speculate as to the reason(s) why acidic media disfavor the formation of desethoxypyridine 122.  One possible — if naïve — explanation is that a basic agent (B: in  34  Scheme 26; acetate ion) might be involved in the elimination of EtOH from dihydropyridine isomer 126. Acidic media would neutralize such basic species and retard the rate of eliminative aromatization (Scheme 26).   Scheme 26. Hypothetical Mechanism of Elimination of the Pyridine However, this fails to account for the fact that desethoxy pyridine formation was observed only with substrate 121: no such problem subsisted with analog 109. The implication then is that the thiazole segment adjacent to the OEt group somehow promotes formation of 122. A rationale for this may be ventured as follows. Because the reaction medium is always acidic (excess NH4OAc), Hantzsch dihydropyridine 128 may exist in fast prototropic equilibrium with species 129 and 130. The latter, in turn, may equilibrate with 131, the enamine segment in which could promote slow expulsion of the ethoxy group, leading to 132. Progressively more acidic media would favor protonation of 131 to give 133, which may well become a/the major equilibrium species in the presence of acid. Loss of ethanol from 133 is unlikely: the only fate awaiting it would be slow aerobic oxidation to 134 (Scheme 27).  35   Scheme 27. Hypothetical Mechanism of Desethoxypyridine Formation in Tris-Thiazole System  3.3 Model Study to Restore Hydroxypyridine The sequence detailed above delivered pyridine 121, wherein the phenolic OH group is present as ethyl ether. A search of the literature failed to uncover precedent for the deblocking of  36  similar heterocyclic phenolic ethers. Therefore, compound 121 served to explore deprotection methods that might lead to free pyridinol 135 (Scheme 28).  Scheme 28. Envisioned Deprotection Sequence Initial efforts centered on classical methods to effect the foregoing transformation. A time-honored approach to the cleavage of phenolic ethers is the use of 48% aqueous HBr.38  Exposure of 121 to this reagent, at room temperature, resulted in clean release of the TBS groups, but the ethyl ether remained intact even after 12 h at 25 °C. Heating of the reaction mixture at temperatures above 50 °C, in an attempt to force the issue, resulted in rapid destruction of the substrate. Evidently, the molecule was intolerant of vigorous treatment with Bronsted acids. Other conventional deblocking methods involve the use of Lewis acidic reagents, e.g., BBr3,39 TMSI,40 and so on. The action of one equivalent of BBr3 upon pyridine 121 induced the release of a single TBS group. The addition of two more equivalents of BBr3 had virtually no effect: the desilylated derivative of pyridine 121 seemed to be perfectly stable under these conditions. However, the introduction of a total of more than 3 equivalent of BBr3 prompted rapid destruction of the substrate. Similar results were obtained when Me3SiI was employed in lieu of BBr3, indicating that the molecule was also intolerant of Lewis acidic reagents.    37  The reasons for this behavior are unclear. Compound 121 obviously presents numerous Lewis basic sites, which could sequester multiple equivalents of Lewis acidic species (Scheme 29). It thus seems reasonable that more than 2-3 equivalents of a deblocking agent would be required to release the ethyl group. What escapes a simplistic explanation is the rapid and complete obliteration of the substrate upon contact with more than 3 equivalents of such reagents.  Scheme 29. Unfruitful Deprotection Sequences Attempted The foregoing observations mandated the use of a more labile pyridinol protecting group the "real" system.  Recall that the latter would incorporate additional acid-sensitive functionality, which in any event barred the use of vigorous, poorly selective reagents. The choice of a suitable blocking group was also conditioned by the fact that such a segment would have to withstand the formation of organolithium species. Labile forms of protection, such as ester and silyl ethers, would be entirely inadequate in the present context.  To illustrate, attempted lithiation of the TBS enol ether of acetaldehyde to give organometallic agent 138 Pg = TBS, followed by interception thereof by addition to an aldehyde, afforded intractable mixtures of products. No attempt was made to determine the nature of these. In the end, we opted for Pg = CH2OMe (methoxymethoxy, MOM). The acetal nature of such a group rendered it potentially releasable under very mild  38  acidic conditions. Furthermore, organometallic compound 139, Pg = MOM, is known.41 Our confidence in the pyridine-forming sequence, and in our ability to successfully deblock a MOM-protected pyridinol, was sufficiently high that the next phase of the research focused directly on the synthesis of the actual nocathiacin / nosiheptide core (Scheme 30).  Scheme 30. Retrosynthetic Analysis of Pyridine Core Exhibiting More Labile Protecting Group  3.4 Approaches to the Assembly of the Real Pyridine  The observations summarized in the preceding sections, as well as past experience in the thiopeptide field, led us to envision compound 141 (Scheme 31) as a particularly convenient form of the pyridine-thiazole core cluster of nocathiacine and nosiheptide.  The synthesis of 141 would be achieved from fragments 89 and 142.    Scheme 31. Retrosynthetic Analysis of the ‘Real’ Pyridine   39  A route to 89 was charted from L-serine methyl ester, which was converted into the known42 thioamide 144 in 4 steps and in 54% yield (Scheme 32). This thioamide would function as a nucleophile in a subsequent Hantzsch thiazole synthesis: an especially practical method to for the preparation of thiazoles first described in 1887.43 The process entails a generally   (a) Boc2O, Et3N, DCM, 0oC to r.t, 85% (b) Dimethoxypropane, PPTS, THF, reflux (c) NH4OH/MeOH, 35oC, 85% over 2 steps (d) Lawesson’s reagent, THF, reflux, 66%   Scheme 32. Preparation of Thioamide 144 high-yielding condensation of a thioamide with an α-halocarbonyl compound, commonly in refluxing ethanol (Scheme 33). The reaction is also very simple to implement.  Scheme 33. Hantzsch Thiazole Synthesis  However, enantioenriched thioamides derived from α-amino acids suffer substantial or complete racemization during the reaction. Available evidence44 indicates that the problem occurs at the stage of thiazole 150, which upon protonation by the HBr released into the medium equilibrates with enamine 151, resulting in loss configuration (Scheme 34).  40   Scheme 34. Racemization of α-amino Thiazoles Such a difficulty may be circumvented by condensing amino acid derived thioamides 145 with an α- halocarbonyl compound at low temperature, and under gently basic conditions, and by dehydrating the emerging hydroxythiazoline 154 with trifluoroacetic anhydride (TFAA) in the presence of pyridine and triethylamine (Scheme 35). This procedure is commonly described as the Holzapfel-Meyers-Nicolaou (HMN) modification of the Hantzsch thiazole synthesis.17 Accordingly, reaction of 145 with ethyl bromopyruvate afforded thiazole 91 with no erosion of stereochemical purity.   Scheme 35. The Holzapfel-Meyers-Nicolaou (HMN) Variant of Hantzsch Thiazole Synthesis Reaction of 91 with lithiated thiazole 92, as seen previously in Scheme 19, then afforded ketone 89 (Scheme 36). It should be mentioned that compound 89 existed as a mixture of keto- and enol tautomers (1H NMR) in CDCl3 at room temperature. Furthermore each tautomer gave rise to a pair of BOC group rotamers that interconverted slowly on the NMR time scale at 25 °C:  41  this produced appreciable broadening of the 1H and 13C NMR signals of the molecule. Much sharper spectra were obtained at 60 – 80 °C from acetonitrile-d3 or DMSO-d6 solutions.   (a) EBP, KHCO3, Et3N, Pyridine, TFAA, 97% (b) n-BuLi, THF, 105, 74% Scheme 36. Preparation of Ketone 89 Analogous line broadening was observed for all subsequent compounds derived from 89. Therefore, NMR spectra of derivatives of 89 were recorded both at 25 °C and at appropriately higher temperatures. The preparation of enone 142 was less straightforward. A reasonable precursor of 142 was aldehyde 155, which had previously been utilized in the total synthesis of thiocillin I.20  An initial route to 142 envisaged oxidation of 155 to carboxylic acid 156, which after suitable activation (cf. 157, L = leaving group) would react with 139 (Scheme 37).   Scheme 37. Envisioned Synthesis of Enone from the Corresponding Carboxylic Acid  42  However, whereas the Pinnick oxidation of 155 afforded 156 in good yield, the tendency of the latter to undergo decarboxylation to 114 (Scheme 38) precluded its conversion into an acyl chloride or an acyl imidazole, in preparation for the organometallic step. Therefore, this  (a) NaClO2, NaH2PO4, t-BuOH, 2-methyl-2-butene, 80% Scheme 38. Unsuccessful Functional Group Interconversion Due to Rapid Decarboxylation approach was abandoned, in favor of one involving the postponement of the oxidative step after the reaction with a suitable organolithium species.  Lithiated ethyl vinyl ether underwent smooth addition to 155 to afford 158. There was no interference from the ester functionality. On the other hand, attempts to purify sensitive alcohol 158 resulted in unacceptable material loss (mass recovery less than 10%). Fortunately, immediate MnO2 oxidation of crude 158 delivered enone 159 in excellent yield. Although 159 was not useful for the assembly of the real pyridine (it incorporates an O-Et, instead of an O-MOM, group), it served to validate the general approach (Scheme 39).   43   (a) t-BuLi, THP/THF, ethyl vinyl ether, 89%, (b) MnO2, DCM, 85%    Scheme 39. Synthesis of Enone 159 Our attention thus refocused on the reaction of 155 with lithiated vinyl MOM ether.41  Unlike the sequence of Scheme 22, the new process suffered from a number of complications.  First, six equivalents of 139 were necessary to force the reaction to completion. Second, the alkoxide 160 thus formed underwent fragmentation to give 161 and 114 upon warming above -40 °C. This mandated the quenching of the reaction mixture containing 160 with TMSCl, at -78 °C, prior to work-up. The product thus obtained, initially assumed to be TMS-protected alcohol 162, was found to be very sensitive to both acid and base, but deprotection with TBAF reproducibly afforded 165.    44   Scheme 40. Unexpected Outcomes of Elimination Pathways and Circumvention However, this step also produced equal amounts of aldehyde 155, which was definitely not present in crude 162.  Scrutiny of reaction mixtures (1H NMR) revealed that the presumed 162 was actually a mixture of diastereomers of compound 164 (Scheme 41). A plausible mechanism for the formation of this material involves addition of lithiated 139 to 155, followed by rapid addition of alkoxide 160 to a second molecule of the extremely electrophilic aldehyde 155. The resultant hemiacetal anion appears to be unusually stable under the conditions of this reaction (it does not release starting 155), perhaps due to its existence as chelated complex 163. Interception of this intermediate by TMS-Cl leads to mixed acetal 164. Exposure of the latter to TBAF causes unraveling to an equimolar mixture of 165 and starting 155. Attempts to contain the formation of 165 by simultaneous addition of 155 and TMSCl to a solution of 139, in the hope of capturing alkoxide 160 before it might add to a second molecule of 155, were uniformly unsuccessful.  45   Scheme 41. Formation of Dimeric Adduct 164 and Formation of Enone 142 It should be noted that the literature records numerous cases, in which the addition of an organometallic species to a highly electrophilic, thiazole-, quinoline-, or pyrimidine-derived heteroaromatic aldehyde takes place in low / moderate yield.45 In one case, this has been attributed to the "reduced electrophilicity" of the aldehyde 46 even though all indications point to the conclusion that the aldehyde in question is quite electrophilic. It is tempting to speculate that the diminished yields often observed with the above substrates may be due to the rapid formation of stabilized chelates such as 168 (Scheme 42), which do not readily release the free aldehyde under typical reaction conditions.  46  Scheme 42. Possible Insights into the Reactivity of Aromatic Aldehyde Just as indicated earlier for 158, any method to purify 165 resulted in a massive loss of material. Therefore, this sensitive alcohol was immediately subjected to allylic oxidation with MnO2 to furnish enone 142. The oxidation step proceeded smoothly, but the product enone 142 was contaminated with starting aldehyde 155. In all likelihood, the aldehyde would interfere with the subsequent Michael step, by reacting with the enolates involved in the reaction. It was thus essential to remove it from 164. Attempted chromatographic purification of the latter resulted in major loss of material. Indeed, enone 142 proved to be quite intolerant of chromatographic supports such as silica and alumina. The search for a way to remove 155 from 164 without resorting to chromatography revealed that longer contact time between 164 and TBAF (3 hrs instead of 30 mins) affords 165 free from 155. We believe that the small amounts of water and/or hydroxide impurities present in TBAF react with aldehyde to form the corresponding hydrate, which is likely to be water-soluble, and is therefore readily removed during the subsequent aqueous work up. Aldehyde-free enone thus obtained degraded fairly rapidly at room temperature; therefore, it was promptly advanced to the subsequent Hantzsch reaction without further purification. To our surprise, the seemingly minor change in O-protecting groups between enones 117 and 142 had a significant impact on the efficiency of the pyridine-forming sequence. Thus, the  47  merger of ketone 89 and enone 142 under the conditions detailed earlier in Scheme 24 and 25 returned pyridine 141 in only 4% yield (Table 2). Such a poor result was attributable to the diminished efficiency of both the Michael reaction and the pyridine-forming step. Concerning the latter, it was determined that a major byproduct of the reaction was desoxypyridine 170. Observation made during model work suggested that more acidic reaction media should prevent the formation of 170. Ultimately, it was determined that conduct of the reaction in a mixture of TFE/AcOH (4:1), and in the presence of 1 mol eq. of PPTS,47 suppressed the formation of 170 and tripled the yield of desired 141 (10% over two steps).   Solvent Enone Additive Yield of 141  Yield of 170 5:8 TFE/HFIP Crude None 4% 10% AcOH Crude None 4% 0 HFIP Crude None 5% 0 4:1 TFE/AcOH Crude PPTS 10% 0 4:1 TFE/AcOH Purified PPTS 28% 0  Table 2.  Attempted Optimization Sequences Leading to the OMOM Pyridine  As far as the Michael step is concerned, it rapidly transpired that the use of enone of greater purity was the key to a cleaner, more efficient reaction, but as mentioned previously, the purification of 142 is a delicate proposition. We presumed that this was due to the acid-sensitive nature of that material. We thus developed a chromatographic method that entails a rapid elution  48  of 142 through silica gel that had been strongly deactivated with Et3N. To our relief, the method afforded enone of good (ca. 90% by NMR) purity, while containing material loss to an acceptable level. When this enone of greater purity was employed in the pyridine-forming reaction, under the optimized condition developed earlier, the desired 141 emerged in 28% chromatographed yield over 2 steps. Despite its moderate yield, this route to 141 is short (11 operation total, with the longest linear sequence encompassing 6 steps and with overall yield of 11% along this path; Scheme 43) and it employs inexpensive starting materials and reagents.  Scheme 43. Summary of the Route to Pyridine 141  We conclude this section with a reminder that the BOC unit in 141 gave rise to a pair of slow-interconverting (25 °C) rotamers. This caused broadening of 1H and 13C NMR signals. Spectra recorded at 60 – 80 °C from acetonitrile-d3 or DMSO-d6 solutions were much sharper and more readily interpreted.   49  3.5 Restoration of 3-hydroxypyridine A final aspect of this phase of the research centered on the selective deblocking of 141 to a free 3-hydroxypyridine. Once again, conventional techniques for MOM group cleavage, by the use of the acidic reagents listed in section 3.3, led either to destruction of the substrate or release of the other protecting groups. A literature search aiming to identify the mildest possible   Scheme 44. Successful Peng Deprotection of Phenolic MOM-Ether methods for phenolic MOM ether deprotection led to a noteworthy paper by Peng and coworkers. These researchers discovered that MOM derivatives of phenols are cleanly deblocked upon treatment with 25 mol % each of PPh3 and CBr4 at 40 oC in DCE.48 Despite the complexity, and the observed sensitivity of pyridine 141, the Peng deprotection performed extremely well and delivered 171 in 71% yield (Scheme 44). The survival of the TBS group under these conditions is remarkable. Line broadening due to the slow interconversion of BOC rotamers was apparent also in the room temperature NMR spectra of 171. An unpleasant surprise materialized upon heating a solution of 171 in DMSO-d6 in an NMR probe, in an attempt to obtain sharper spectra: partial release of the TBS group occurred (ca. 20% after 30 min), preventing proper characterization of what was now a mixture of 171 and 172 (Scheme 45). It was thus decided to  50  fully deblock 171 and to carry out a full characterization at the stage of 172, obtained in 65% yield from 141 over 2 steps.  Scheme 45. Fully Deblocked Hydroxypyridine for Full Characterization             51  4. Indole Segment of Nosiheptide One of the other key heterocyclic units of nosiheptide is indole 173 (Scheme 46). A good route to a differentially protected form of 173 has been described by Moody and collaborators.28  Scheme 46. The Desired Indole Fragment of Nosiheptide Their approach rests upon a Fischer indole synthesis, which employs the hydrazine derived from commercial aniline 174 and methyl -oxobutyrate (Scheme 47). The Moody sequence was duplicated without difficulty. Indeed, indole 177 thus obtained was of excellent purity and required no chromatography. Straightforward functional group manipulations, as detailed by Moody, were served to convert 177 into the desired 178. This fragment was thus assembled in an efficient manner with 6 steps over 30% yield.     52   (a) NaNO2, HCl, then SnCl2, -10 oC (b) methyl 2-oxobutanoate, AcOH/H2O, 70 oC (c) AcOH, PPA, 90 oC, 60% over 3 steps (d) H2, Pd/C, MeOH (e) BH3-SMe2, THF (f) TBS-Cl, imidazole, DMF, 50% over 3 steps   Scheme 47. Synthesis of Indole Fragment of Nosiheptide 178              53  5. Conclusion The synthesis of the pyridine core of nocathiacin and nosiheptide has been demonstrated. The route entails twelve distinct operations and six steps along the longest linear sequence. This approach is by far the most competitive one compared to known alternatives, in terms of length, yield and cost of materials. In order to complete the total synthesis of nosiheptide, two additional fragments are required: 179 and 180 (Scheme 48). Segment 179 is also found in MP1 and thiocillin I, and so it has already been synthesized in this laboratory. A good approach to compound 180 has been described by Moody and collaborators.27 Therefore, few, if any, difficulties are anticipated with the preparation of 179 and 180. Once these are in hand, strategic peptide couplings established earlier in connection with the synthesis of 141 and 178 will be used to assemble the target nosiheptide.   Scheme 48. Anticipated Completion of Nosiheptide  54  References 1. Livermore, D. M. Clin. Microbiol. Rev. 1995, 8, 557 2. Gilespie, S. H. Int. J. Antimicrob. Ag. 2001, 17, 171 3. Niccolai, D.; Tarsi, L.; Thomas, R. J. Chem. Comm. 1997, 2333 4. Chang, S.; Sievert, D. M.; Hageman, J. C.; Boulton, M. L.; Tenover, F. C.; Pouch Downes, F.; Shah, S.; Rudrik, J. T.; Pupp, G. R.; Brown, W. J; Cardo, D.; Fridkin, S. K. N. Engl. J. Med. 2003, 348, 1342  5. Drebes J.; Kunz, M.; Pereira, C. A.; Betzel, C.; Wrenger, C.  Curr. Med. Chem. 2014, 21, 1809 6. Haste, N, M.; Thienphrapa, W.; Tran, D. N.; Loesgan, S.; Sun, P.; Nam, S. J.; Jensen, P.R.; Fenical, W.; Sakoulas, G.; Nizet, V.; Hensler, M. E. J. Antibiot. 2012, 65, 593 7. Hughes R. A.; Moody. C. J. Angew. Chem. Int. Ed. 2007, 46, 7930 8. Xing, Y.; Draper, D. E. Biochemistry. 1996, 35, 1581 9. Anborgh, P. H.; Parmeggiani, A. EMBO J. 1991, 10, 779 10.  Hensens, O. D.; Albers-Schӧnbergs, G. Tetrahedron Lett. 1978, 19, 3649 11.  Bycroft, B. W.; Gowland, M.S. J. Chem. Soc., Chem. Commun. 1978, 256 12. (a) Acker, M. G.; Bowers, A. A.; Walsh, C.T. J. Am. Chem. Soc. 2009, 131, 17563  (b) Bowers, A.A.; Walsh, C. T.; Acker, M. G. J. Am. Chem. Soc. 2010, 132, 12182 13.  Ciufolini, M. A.; Lefranc, D. Nat. Prod. Rep. 2010, 27, 330 14.  (a) Mocek, U.; Zeng, Z.; O’Hagan, D.; Zhou, P.; Fan, L.-D. G.; Beale, J. M.; Floss, H. G. J. Am. Chem. Soc. 1993, 115, 7992 (b) Smith, T. M.; Priestely, N. D.; Knaggs, A. R.; Nguyen, T.; Floss, H. G. J. Chem. Soc., Chem. Commun. 1993, 1612  55  15.  Bagley, M. C.; Bashfod, K. E.; Hesketh, C. L.; Moody, C. J. J. Am. Chem. Soc. 2000, 122, 3301 16.  Hughes, R. A.; Thompson, S. P.; Alcaraz, L.; Moody, C. J. J. Am. Chem. Soc. 2005, 127, 15644 17.  (a) Nicolaou, K. C.; Zak, M.; Safina, B. S.; Estrada, A. A.; Lee, S. H.; Nevalainen, M. J. Am. Chem. Soc. 2005, 127, 11176 (b) Nicolaou, K. C.; Safina B. S.; Zak, M.; Lee, S. H.; Nevalainen, M.; Bella, M.; Estrada, A. A.; Funke, C.; Zѐcri, F.; Bulat. S. J. Am. Chem. Soc. 2005, 127, 11159 (c) Nicolaou, K. C. Angew. Chem. Int. Ed. 2012, 51, 12414 18. (a) Mori, T.; Higashibashi, S.; Goto, T.; Kohno, M.; Satouchi, Y.; Shinko, K.; Suzuki, K.; Suzuki, S.; Tohmiya, H.; Hashimoto, K.; Nakata, M. Tetrahedron Lett. 2007, 48, 1331 (b) Mori, T.; Higashibashi, S.; Goto, T.; Kohno, M.; Satouchi, Y.; Shinko, K.; Suzuki, K.; Suzuki, S.; Tohmiya, H.; Hashimoto, K.; Nakata, M. Chem. Asian J. 2008, 3, 1013 19.  Lefranc, D.; Ciufolini, M. A. Angew. Chem. Int. Ed. 2009, 48, 4198 20.  Aulakh, V. S.; Ciufolini, M. A. J. Am. Chem. Soc. 2011, 133, 5900 21.  Kelly, T. R.; Jagoe, C. T.; Gu, Z. Tetrahedron Lett. 1991, 32, 4263 22.  Heckmanm, G.; Bach, T. Angew. Chem. Int. Ed. 2005, 44, 1199 23.  Stille, J. K. Angew. Chem. Int. Ed. Engl. 1986, 25, 508 24.  Aulakh, V. S.; Ciufolini, M. A. J. Org. Chem. 2009, 74, 5750 25.  Umemura, K.; Noda, H.; Yoshimura, J.; Konn, A.; Yonezawa, Y.; Shin, C. Tetrahedron Lett. 1997, 38, 3539 26.  Lu, J. Y.; Riedrich, M.; Wojtas, P. K. Arndt, H. D. Synthesis. 2013, 45, 1300 27.  Belhadj, T.; Nowicki, A.; Moody, C. J. Synlett. 2006, 18, 3033  56  28.  Bentley, D. J.; Fairhurst, J.; Gallagher, P. T.; Manteuffel, A. L.; Moody, C. J.; Pinder, J.  L. Org. Biomol. Chem. 2004, 2, 701 29.  Nicolaou, K. C.; Dethe D. H.; Leung, G. Y. C.; Bin, Z.; Chen, D. Y. K. Chem. Asian. J. 2008, 3, 413 30.  Nakamura, Y.; Shin, C. G.; Umemura, K.; Yoshimura, J. Chem Lett. 1992, 1005 31.  Lu, J. Y.; Ardnt. H. D. J. Org. Chem. 2007, 72, 4205 32.  Mitsunobu, O.; Yamada, M.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 1967, 40, 935 33.  Eiden, F.; Herdeis, C, Arch. Pharm. 1977, 310, 744 34.  Shimano, M.; Meyers, A. I. Tetrahedron Lett. 1994, 35, 7727 35.  Ciufolini, M. A.; Shen, Y. C. J. Org. Chem. 1997, 62, 3804 36.  Appel, R.; Mayr, H. J. Am. Chem. Soc. 2011, 133, 8240 37.  Quick, J.; Jenkins, R. J. Org. Chem. 1978, 43, 2275 38.  Kawasaki, I.; Matsuda, K.; Kaneko, T. Bull. Chem. Soc. Jpn. 1971, 44, 1986,  39.  McOmie J. F. W.; West, D. E. Org. Synth., Coll. Vol. V. 1973, 412 40.  Jung M. E.; Lyster, M. A. J. Org Chem. 1977, 42, 3761 41.  Tamao, K.; Nakagawa, Y.; Ito, Y. Org. Synth. 1996, 73, 94 42.  Lin, C-C.; Tantisantisom, W.; McAlpine, S. R. Org Lett. 2013, 15, 3574 43.  Hantzsch, A.; Weber, J. H. Ber. 1887, 20, 3118 44.  Merritt, E. A.; Bagley, M. C. Synthesis. 2007, 3535 45. (a) Botta, M. et al Archiv Pharm. (Weinheim, Germany) 1991, 324, 203 (b) Bayer Pharmaceuticals Co., USA 2007 WO 2007064883 A2 (c) Kelly, M. G. et al PCT Int. Appl., 2007100758, 07 Sep 2007 (d) Baumann, K. et al., U.S. Pat. Appl. Publ.,  57  20080280948, 13 Nov 2008 (e) Mori, Tomonori et al, Chem. Asian J. 2008, 3, 984. (f) Vialard, Jorge Eduardo et al  PCT Int. Appl., 2008107478, 12 Sep 2008 46.  Jankowski, R.; Joseph, D.; Cave, C.; Dumas, F.; Ourevitch, M.; Mahuteau, J.; Morgant,  G.; Bosnjakovic Pavlovic, N.; D'Angelo, J. Tetrahedron Lett. 2003, 44, 4187   47.  Miyashita, M.; Yoshikoshi, A.; Grieco, P. A. J. Org. Chem. 1977, 42, 3772 48.  Peng, Y.; Ji, C.; Chen, Y.; Huang, C.; Jiang, Y. Syn Comm. 2004, 34, 4325                   58  Appendix Experimental Protocols Unless otherwise stated, 1H and 13C NMR spectra were recorded on Bruker model AVANCE II+ 300 (300 MHz for 1H and 75 MHz 13C) spectrometer using CDCl3 as the solvent at room temperature. Chemical shifts are reported in parts per million (ppm) on the δ scale and coupling constants, J, are in hertz (Hz). Multiplicities are reported as “s” (singlet), “d” (doublet), “t” (triplet), “q” (quartet), “dd” (doublet of doublets), “m” (multiplet), “b” (broad). Infrared (IR) spectra (cm-1) were recorded on a Perkin-Elmer model 1710 Fourier transform spectrophotometer on a Universal Sampling Accessories while optical rotations were measured on a Jasco P-1010 polarimeter at the sodium D line (589nm). Unless otherwise stated, low-resolution mass spectra (m/z) were obtained in the electrospray (ESI) mode on a Waters Micromass ZQ mass spectrometer. High-resolution mass spectra (m/z) were recorded in the electrospray (ESI) mode on a Micromass LCT mass spectrometer by the UBC Mass Spectrometry Laboratory. Melting points were measured on a Mel-Temp apparatus. All reagents and solvents were commercial products and used without further purification except THF (freshly distilled from Na/benzophenone under nitrogen), THP (small amount (~25-50ml) freshly distilled from Na), and CH2Cl2 (freshly distilled from CaH2 under nitrogen). Commercial n-BuLi, and t-BuLi were titrated against N-benzylbenzamide in THF at -40oC until persistence of a blue color was observed. Flash chromatography was performed on a Silicycle 230-400 mesh silica gel. Analytic and preparative TLC was carried out with Merck silica gel 60 plates with fluorescent indicator. Spots were visualized with UV light. Unless otherwise stated, reactions were performed under dry argon in flame- or oven-dried flasks equipped with TeflonTM stirbars. All  59  flasks were fitted with rubber septa for the introduction of substrates, reagents and solvents via syringe. Solvents, pure liquid reagents or reagents in solution, and solids were added in one portion unless otherwise stated.                 60  A.1 Preparation of 4-(((tert-butyldimethylsilyl)oxy)methyl)-2-methylthiazole (105)  Ethyl bromopyruvate (50mmol, 7.00ml, 9.69g, 1 eq) was added to the solution of thioacetamide (51.5mmol, 3.87g, 1.03 eq) in EtOH (30ml) in portions over 5 minutes. The mixture was stirred overnight at r.t. The reaction was then poured onto 2.5N HCl (30ml), stirred 30 mins, and washed with diethyl ether (3 x 30ml). The aqueous solution was then neutralized with excess solid NaHCO3 until the pH of the aqueous wash (pH paper) stabilized at 8, and extracted with EtOAc (3 x 70ml). Then, organic layer was washed with brine, dried over Na2SO4, and concentrated in vacuo to afford compound 104 as a yellow solid. The solution of compound 104 (50mmol, 8.56g, 1 eq) in THF (50ml) was slowly added to the flask containing DIBAL-H in hexane (1M, 120mmol, 120ml, 2.5 equiv) at -78oC over period of 5 minutes. Then, the mixture was stirred at 45 minutes at -78oC, and brought to r.t for 1 hr. The mixture was diluted with 350ml diethyl ether, and Fieser work up was performed to get rid of excess aluminum salts. Solid residue was filtered, and further washed with 2 x 100ml diethyl ether. The filtrate was then concentrated in vacuo to afford alcohol as brown oil.  The resulting alcohol (41.8mmol, 5.40g, 1 eq) was immediately dissolved in 60ml DCM and TBS-Cl (46mmol, 6.96g, 1.1 eq) and imidazole (83.6mmol, 5.69g, 2 eq) were added to the mixture and stirred overnight at r.t. The mixture was then diluted with 120ml DCM and transferred to a separatory funnel. Organic layer was washed with sat. aq. NaHCO3 (40ml), and brine (20ml). Organic layer was collected, dried over Na2SO4 and concentrated in vacuo. Flash column chromatography (10% EtOAc:90% Hex, Rf = 0.37) of the residue afforded a known compound 105  [Lefranc, D; Ciufolini, M. A. Angew. Chem. Int. Ed. 2009, 48, 4198] as a brown oil.  61  Yield: 10.3g (85% over 3 steps) 1H NMR: 7.00 (t, 1H, J=1.2Hz), 4.82 (d, 2H, J=1.1Hz), 2.09 (s, 3H), 0.95 (s, 9H), 0.13 (s, 6H) 13C NMR:  166.0, 152.0, 112.9, 62.3, 25.9, 19,1 18.4, -5.4 IR: 1717, 1097, 835cm-1 LRMS: 266.4 [M+Na+]                62  Figure A1. 1H NMR of 105 Figure A2. 13C NMR of 105  63  A.2 Preparation of 2-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)-1-(2-methylthiazol-4-yl)ethan-1-one (100)   Commercial n-BuLi solution (1.23 M in hexanes, 6.10 mL, 7.50 mmol) was added over 3 min to a cold (–78 °C) solution of 105 (1.82 g, 7.50 mmol, 2 eq) in THF (10 mL), the mixture was stirred at –78 °C for 40 min, then  a solution of compound 104 (0.590 g, 3.75 mmol, 1 equiv) in THF (4 mL) was slowly added over a period of 3 min. The mixture was slowly brought up to r.t over a period of 2 hours, then it was quenched with aq. sat. NH4Cl (5 mL). The mixture was diluted with EtOAc (50 mL), transferred to a separatory funnel, and carefully acidified with 0.5M HCl to pH 5. The organic layer was collected, dried over Na2SO4 and concentrated in vacuo. Flash column chromatography (5% EtOAc : 95% Hexane, Rf=0.15) of the residue afforded the desired ketone 100 as a yellow low melting solid, which existed (1H and 13C NMR) as a mixture of the keto- (major) and enol forms (minor); (ca. 2:1 ratio). Unreacted 2-methyl-4-(tert-butyldimethylsilyl-oxy)methyl thiazole 105 was recovered. (450 mg, 1.84 mmol, 49%, Rf = 0.17) Yield: 828mg (60%)  1H: [8.12(s, enol), 7.60 (s, keto) (1H)], [7.16 (s, enol), 6.96 (s, keto) (1H)], [6.72 (s, keto) (0.6H)],  [4.86-4.80 (m, contains enol), (2.4H)], [2.76 (s, enol), 2.74 (s, keto) (3H)], [0.96(s, keto), 0.94 (s, enol), (9H)], [0.14 (s, keto), 0.11 (s, enol) (6H)]  64  13C: 189.2, 168.1, 166.4, 166.1, 162.0, 156.8, 155.2, 154.9, 153.4, 150.7, 126.6, 117.4, 114.6, 110.1, 93.2, 62.3, 61.8, 44.1, 25.9, 19.3, 18.4, -5.3, -5.4 IR: 3126, 1694, 1630, 1090, 837cm-1 LRMS: 391.3 [M+Na+]  HRMS: cald for 391.0946 C16H24N2O2S2SiNa; found:  391.0950 [M+Na+]                 65  Figure A3. 1H NMR of 100 Figure A4. 13C NMR of 100  66  A.3 Preparation of 4-(((tert-butyldimethylsilyl)oxy)methyl)-2-(5-ethoxy-2-(2-methylthiazol-4-yl)-6-phenylpyridin-3-yl)thiazole (109)   Commercial n-BuLi in pentane (1.42 M, 0.105 ml, 0.149mmol, 1.1 eq) was carefully added over 3 min to a cold (–78 °C) solution of diisopropylamine (0.149mmol, 0.021ml, 1.1 eq) in THF (0.5ml), and stirred at -78oC for 10 minutes. Ketone 100 (50mg, 0.136mmol, 1 eq) in THF (0.1ml) was slowly added to the flask, and stirred at -78oC for 30 minutes. Then, the solution of the enone 107 (0.156mmol, 27.5mg, 1.15 eq) in THF (0.1ml) was added dropwise to the flask and slowly brought it back to 0oC for 2 hours. Then, the mixture was quenched with 0.5ml sat. aq. NH4Cl and further diluted with EtOAc (8ml) and transferred to a separatory funnel. The organic layer was washed with extra 1ml sat. aq. NH4Cl and was collected and dried over Na2SO4. The mixture was then concentrated in vacuo. The resulting diketone 108 was then dissolved in 3ml EtOH, and NH4OAc was added (1.36mmol, 106mg, 10 eq). The mixture was then brought up to 60oC and stirred for 14 hours under O2 balloon. The mixture was then concentrated in vacuo to get rid of excess solvent, diluted with 10ml EtOAc, and transferred to a separatory funnel. The organic layer was washed with portions of 1ml sat. aq. NaHCO3 until the pH of the aqueous washes (pH paper) stabilized at 7. The organic layer was collected, dried over Na2SO4, and concentrated in vacuo. The crude was subjected to flash column chromatograph (15% EtOAc:85% Hexane, Rf=0.30)  to afford the pyridine 109 as a yellow solid. Yield: 27mg (38%)  67  Melting point: 98-101oC 1H NMR: 8.08-8.05 (m, 2H), 7.89 (s, 1H), 7.46-7.37 (m, 4H), 7.19 (s, 1H), 4.92 (s, 2H), 4.20 (q, 2H, J=7.0Hz), 2.69 (s, 3H), 1.48 (t, 3H, J=7.0Hz), 0.96 (s, 9H), 0.14 (s, 6H) 13C NMR:  165.4, 164.7, 157.1, 153.2, 152.3, 147.8, 143.1, 137.1, 129.7, 128.8, 128.6, 127.8, 120.2, 119.3, 115.9, 64.6, 62.4, 25.9, 19.1, 18.4, 14.6, -5.3 IR: 1366, 1088, 834cm-1 LRMS: 524.3 [M+H+]  HRMS: cald for 523.1783 C27H34N3O2S2Si; found:  523.1791 [M+H+]              68  Figure A5. 1H NMR of 109 Figure A6. 13C NMR of 109  69  A.4 Preparation of ethyl thiazole-4-carboxylate (114)  P4S10 (30mmol, 13.35g, 0.4 eq) was added to the solution of formamide (75mmol, 3.00ml, 1 eq) in Et2O (60ml) at 0oC in small portions over 3 minutes. The reaction mixture was allowed to warm to r.t for 2 hours. The flask was diluted with 250ml of diethyl ether, and the solid residue was filtered, and filtrate was concentrated in vacuo to afford thioformamide as yellow smelly foam. The resulting thioformamide (1.96g, 32mmol, 1 eq) was immediately redissolved in 35mol EtOH, and neat ethyl bromopyruvate was slowly added to the flask (32mmol, 4.51ml, 1 eq) in portions over 5 minutes, and stirred overnight at r.t. Then, 25ml of 2.5N HCl was poured, stirred over 30 mins. Then, the mixture was washed with 2 x 30ml diethyl ether, and aqueous layer was neutralized with solid NaHCO3 until the pH of the aqueous washes (pH paper) stabilized at 8. The aqueous layer was extracted with diethyl ether (3 x 50ml), and washed with brine (15ml). The combined organic layer is dried over Na2SO4, and concentrated in vacuo. Flash column chromatography (50% EtOAc: 50% Hex, Rf = 0.50) of the residue afforded 114 as a brown solid. Yield: 4.35g (2 steps over 37%) 1H NMR: 8.85 (d, 1H, J=2.1Hz), 8.23 (d, 1H, J=2.1Hz), 4.41 (q, 2H, J=7.1Hz), 1.40 (t, 3H, J=7.1Hz) 13C NMR:  161.2, 153.5, 148.1, 127.2, 61.5, 14.3 IR: 1717, 1264cm-1 LRMS: 180.2 [M+Na+], HRMS: cald for 180.0090 C6H7NO2SNa; found:  180.0098 [M+Na+]    70  Figure A7. 1H NMR of 114 Figure A8. 13C NMR of 114  71  A.5 Preparation of 4-(((tert-butyldimethylsilyl)oxy)methyl)-N,N-diethylthiazole-2-carboxamide (116)  Compound 114 dissolved in 18ml THF (17.6mmol, 2.77g, 1 eq) was added dropwise to the flask containing DIBAL-H in hexane (1M, 44mmol, 44ml, 2.5 eq) at -78oC over a period of 3 minutes. Then, the mixture was stirred for 45 minutes at -78oC, and brought to r.t for 1 hour. The mixture was diluted with 100ml diethyl ether, and Fieser work up was performed to get rid of excess aluminum salts. Solid residue was filtered, and further washed with 2 x 50ml diethyl ether. The collected organic layer was then concentrated in vacuo to afford alcohol as brown oil. The resulting alcohol (11.5mmol, 1.33g, 1 eq) was immediately dissolved in 15ml DCM, and TBSCl (12.7mmol, 1.92g, 1.1 eq) and imidazole (23.0mmol, 1.58g, 2 eq) were added to the mixture and stirred overnight at r.t. The mixture was then diluted with 30ml DCM, and transferred to a separatory funnel. Organic layer was washed with sat. aq. NaHCO3 (15ml), and brine (10ml). Organic layer was collected, dried over Na2SO4 and concentrated in vacuo. Flash column chromagography (10% EtOAc:90% Hex, Rf = 0.40) of the residue afforded 115 as a faint yellow oil. (15.8mmol, 3.63g, 90% over 2 steps). Commercial n-BuLi solution (1.36 M in hexanes, 11.6 mL, 15.8 mmol) was added over 3 min to a cold (–78 °C) solution of 115 (15.8mmol, 3.63g, 1 eq) in THF (20ml), the mixture was stirred at –78 °C for 30 min, then neat diethylcarbamyl chloride (47.4mmol, 6.00ml, 3 eq) was added rapidly at once and slowly warmed to -40oC for 1 hour. The resulting mixture was then quenched with sat. aq. NH4Cl (15ml), and brought up to r.t immediately with gentle warm water bath. Extra 10ml sat. aq. NH4Cl was administrated concomitantly throughout the warming process. The mixture was then diluted with EtOAc (90ml)  72  and transferred to a separatory funnel. The organic layer was collected, dried over Na2SO4, and concentrated in vacuo. Flash column chromatography (5% EtOAc : 95% Hexane) of the residue afforded the desired amide 116 as a faint yellow oil. TLC monitored by (10% EtOAc: 90% Hex, Rf=0.21). Yield: 5.77g (45% over 3 steps) 1H NMR: 7.36 (t, 1H, J=1.0Hz), 4.84 (d, 2H, J=1.0Hz), 4.01 (q, 2H, J=7.0Hz), 3.54 (q, 2H, J=7.0Hz), 1.25 (t, 6H, J=7.0Hz), 0.95 (s, 9H), 0.12 (s, 6H) 13C NMR:  165.2, 160.1, 157.8, 118.8, 62.1, 42.9, 42.2, 25.9, 18.4, 14.5, 12.7, -5.3 IR: 1621, 1068, 835cm-1 LRMS: 351.3 [M+Na+]  HRMS: cald for 351.1538 C15H20N2O2NaSiS; found:  351.1539 [M+Na+]           73  Figure A9. 1H NMR of 116 Figure A10. 13C NMR of 116  74  A.6 Preparation of 1-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)-2-ethoxyprop-2-en-1-one (117)  Commercial tert-BuLi in pentane (0.97 M, 3.35 ml, 3.26mmol, 1.4 eq) was carefully added over 3 min to a cold (–78 °C) solution of ethyl vinyl ether (0.394ml, 4.15mmol, 1.8 eq) in dry tetrahydropyran (1.5 mL).  A bright yellow solution resulted. The mixture was stirred at –78 °C for 10 min, then it was warmed to –10 °C (NaCl/ice bath) and stirred at that temperature for 23 min, during which time the bright yellow color disappeared. The mixture was then cooled back to –78 °C and diluted with THF (2.5 mL). Then, the solution of amide 116 (2.31mmol, 760mg, 1 eq) in THF (2.5ml) was added to the mixture. The mixture was stirred at -78oC for 3 hours, and was slowly warmed to room temperature and quenched with H2O (3ml) and stirred for 1 minute. The mixture was diluted with EtOAc (20ml), and transferred to a separatory funnel. Aqueous layer was further extracted with EtOAc (2 x 10ml). The combined organic layer was dried over Na2SO4, and concentrated in vacuo. Flash column chromatography (1% Et3N: 10% EtOAc: 89% Hex, Rf = 0.34) of the residue afforded the desired enone 117 as a yellow oil. Yield: 580mg (77%) 1H NMR: 7.57 (t, 1H, J=1.0Hz), 6.05 (d, 1H, J=2.8Hz), 5.00 (d, 1H, J=2.8Hz), 4.95 (d, 2H, J=1.0Hz), 3.95 (q, 2H, J=7.0Hz), 1.47 (t, 3H, J=7.0Hz), 0.95 (s, 9H), 0.13 (s, 6H) 13C NMR:  179.2, 164.0, 159.6, 155.6, 121.3, 99.0, 64.1, 62.3, 25.9, 18.3, 14.2, -5.4 IR: 1746, 1695, 1135, 1095, 836cm-1 LRMS: 350.3 [M+Na+], HRMS: cald for 350.1222 C15H25NO3NaSiS; found:  350.1222 [M+Na+]  75  Figure A11. 1H NMR of 117 Figure A12. 13C NMR of 117  76  A.7 Preparation of 2,2'-(3-ethoxy-6-(2-methylthiazol-4-yl)pyridine-2,5-diyl)bis(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazole) (121)  Commercial NaH (60% oil dispersion, 7.8 mg, 0.195 mmol, 1.08 eq) was dispensed into a flask maintained under inert atmosphere (Ar balloon), washed with hexane (3 x 0.2 mL) to remove excess oil, and suspended in THF (0.2 mL). A solution of ketone 100 (66 mg, 0.180 mmol, 1 eq) in THF (0.2 mL) was slowly added (syringe) at r.t over ca. 2 min. Evolution of H2 was observed, and the color changed from faint green to yellow upon stirring at r.t for 15 minutes. A solution of enone 117 (67 mg, 0.205 mmol, 1.15 eq) in THF (0.2 mL) was added at r.t over a period of 1 min, whereupon the color of the solution turned from yellow to brown. The mixture was stirred for 90 min at r.t, then it was quenched with aq. sat. NH4Cl (0.2 mL), diluted with EtOAc (5 mL) and transferred to a separatory funnel. The aqueous layer was discarded, while the organic phase was washed with more aq. sat. NH4Cl (0.5 mL), dried over Na2SO4 and concentrated in vacuo. The resulting diketone 120 was then dissolved in 8:5 mixture of TFE:HFIP (1ml:0.65ml), and slowly syringe pumped over a period of 2.5 hours to the flask containing NH4OAc (2mmol, 156mg, 11 equiv) in 8:5 mixture of TFE:HFIP (0.8ml:0.5ml) under O2 balloon at 60oC. The mixture was stirred at 60oC (oil bath temperature) for 16 hours. The mixture was then concentrated in vacuo to get rid of excess solvent, diluted with 7ml EtOAc, and transferred to a separatory funnel. This solution was washed 2-3 times with 1 mL portions of aq. sat. NaHCO3 until the pH of the aqueous washes (pH paper) stabilized at 7, then it was dried over Na2SO4 and concentrated in vacuo. Flash column chromatography of the residue (1.5% Et3N: 18% EtOAc:  77  81.5% Hexane, Rf = 0.21) afforded pyridine 121 as a yellow solid. This compound was highly UV active (purple under short wavelength, and sky blue under long wavelength) Yield: 43mg (35% over 2 steps) Melting point: 98-101oC 1H NMR: 7.98 (s, 1H), 7.46 (s, 1H), 7.40 (s, 1H), 7.23 (s, 1H), 5.04 (s, 2H), 4.92 (s, 2H), 4.38 (q, 2H, J=7.1Hz), 2.69 (s, 3H), 1.63 (t, 3H, J=7.1Hz), 0.97 (s, 9H), 0.96 (s, 9H), 0.14 (s, 6H), 0.13 (s, 6H) 13C NMR:  165.4, 164.1, 162.4, 158.4, 157.3, 152.4, 151.8, 143.6, 139.9, 130.2, 121.0, 120.0, 116.4, 116.3, 65.5, 62.8, 62.3, 26.0, 25.9, 19.1, 18.41, 18.39, 14.6, -5.29, -5.31 IR: 1366, 1098, 836cm-1 LRMS: 697.4 [M+Na+]  HRMS: cald for 697.2168 C31H46N4O3S2Si2Na; found:  697.2178 [M+Na+]   2,2'-(6-(2-methylthiazol-4-yl)pyridine-2,5-diyl)bis(4-(((tert-butyldimethylsilyl)oxy)methyl)-thiazole) (122).  This material was not purified to homogeneity and it was characterized only by 1H NMR and low-resolution mass spectrometry. 1H: 8.32 (d, 1H, J=8.2Hz), 8.19 (d, 1H, J=8.2Hz), 7.60 (s, 1H), 7.34 (s, 1H), 7.23 (s, 1H), 4.96 (s, 2H), 4.91 (s, 2H), 2.68 (s, 3H), 0.98 (s, 9H), 0.16 (s, 6H), 0.13 (s, 6H).  LRMS: 654.7 [M+Na+]      78  Figure A13. 1H NMR of 121 Figure A14. 13C NMR of 121  79  Figure A15. 1H NMR of 122            80  A.8 Preparation of Tert-butyl (S)-4-(4-(2-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)acetyl)thiazol-2-yl)-2,2-dimethyloxazolidine-3-carboxylate (91)  A solution of known tert-butyl (S)-4-carbamothioyl-2,2-dimethyloxazolidine-3-carboxylate (144) [Lin, C-C.; Tantisantisom, W.; McAlpine, S. R. Org Lett. 2013, 15, 3574] (1.28 g, 5 mmol) in DME (30 mL) containing suspended KHCO3 (3.59 g, 35 mmol, 7 eq) was stirred at r.t for 10 min. Neat ethyl bromopyruvate (2.92 g, 15 mmol, 3 eq, 2.11 mL) was added dropwise over a period of 3 minutes, and the mixture was stirred for 16 h at r.t. The solvent was removed in vacuo, the residue was redissolved in EtOAc (65 mL) and the solution was sequentially washed with brine (20mL) and water (15mL), dried (Na2SO4), and evaporated in vacuo. The residue was placed under high vacuum to remove all remaining EtOAc, then it was redissolved in DME (30 mL).  The solution was cooled to 0°C and pyridine (3.05 mL, 3.00 g, 37.5 mmol, 7.5 eq) was added slowly over a period of 3 minutes. After 5 minutes, TFAA (4.14 g, 20 mmol, 2.78 mL, 4 eq) was added slowly. The mixture was stirred at 0°C for 3 h, then it was brought to r.t. Triethylamine (1.21 g, 10 mmol, 1.69 mL, 2 eq) was slowly added and stirring at r.t was continued for another hour, whereupon the reaction was complete (TLC). The mixture was evaporated and the residue was dissolved in EtOAc (70mL), washed with 1M HCl (15mL), sat. aq. NaHCO3 (15mL), and brine (15mL).  The organic phase was dried over Na2SO4 and concentrated in vacuo. Flash column chromatography (25% EtOAc : 75% Hex, Rf = 0.31) of the residue afforded 91 as a yellow solid. Yield: 1.72g (97%)  81  1H NMR: 8.09 (s, 1H), 5.36 (bs, minor rotamer) & 5.29-5.25 (m, major rotamer; 1H), 4.42 (q, 2H, J=7.0 Hz), 4.32-4.28 (m, major rotamer) & 4.17-4.14 (m, minor rotamer; 2H), 1.79 (bs, major rotamer) & 1.73 (bs, minor rotamer, 3H), 1.57 (bs, major rotamer) & 1.50 (bs, minor rotamer; 6H), 1.38 (t, 3H, J=7.0 Hz), 1.30 (bs, 6H).    13C NMR: 175.3, 161.3, 151.5, 147.2, 127.1, 95.2 (major rotamer) & 94.8 (minor rotamer), 81.4 (minor rotamer) & 81.0 (major rotamer), 69.3 (major rotamer) & 68.9 (minor rotamer), 61.5, 59.4, 28.2, 27.3 (minor rotamer) & 26.5 (major rotamer), 23.9 (minor rotamer) & 22.7 (major rotamer), 14.4.  Melting Point: 105-109oC Optical Rotation:  [α]D22 = -13.1o (CH2Cl2, c = 1.05) IR: 1701, 1364cm-1 LRMS: 379.1 [M+Na+]            82  Figure A16. 1H NMR of 91 Figure A17. 13C NMR of 91  83  A.9 Preparation of Tert-butyl (S)-4-(4-(2-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)acetyl)thiazol-2-yl)-2,2-dimethyloxazolidine-3-carboxylate (89)     Commercial n-BuLi solution (1.08 M in hexanes, 7.16 mL, 7.73 mmol) was added over 3 min to a cold (–78 °C) solution of 2-methyl-4-(tert-butyldimethylsilyl-oxy)methyl thiazole (1.88 g, 7.73 mmol, 2.1 eq) 105 in THF (12 mL), the mixture was stirred at –78 °C for 40 min, then  a solution of compound 91 (1.30 g, 3.63 mmol, 1 eq) in THF (4 mL) was slowly added over a period of 3 min. The mixture was slowly brought up to r.t over a period of 2 hours, then it was quenched with aq. sat. NH4Cl solution (5 mL).  The mixture was diluted with EtOAc (50 mL), transferred to a separatory funnel, and carefully acidified with 0.5M HCl to pH 5. The organic layer was collected, dried over Na2SO4 and concentrated in vacuo. Flash column chromatography (15% EtOAc : 85% Hexane, Rf= 0.33) of the residue afforded unreacted 2-methyl-4-(tert-butyldimethylsilyl-oxy)methyl thiazole (378 mg, 1.56 mmol, 38%, Rf = 0.50) and the desired ketone 89 as a thick yellow film. Proton and 13C NMR spectra of this material revealed that it existed as a mixture BOC rotamers of the keto- (minor) and enol forms (major; ca. 1:2 ratio).  Yield: 1.49g (74%) 1H NMR: [8.20(br. s, keto), 7.67 (br. s, enol) (1H)], [7.16 (br. s, keto), 6.95 (br. s, enol) (1H)], [6.69 (s, enol), (0.6H)],  [5.24 (br. s, keto), 5.21 (br. s, enol) (1H)], [4.85-4.80 (m, contains keto form of enol at 6.69), (2.5H)], [4.34-4.27 (m, enol), 4.22-4.16 (m, keto) (2H)], [1.81-1.77 (m, enol), 1.76-1.71 (m, keto) (3H)], [1.59, 1.51 (br. 2s of equal intensity, (6H)], 1.32 (br. s, enol and keto), (6H),  [0.95(s, enol), 0.93 (s, keto), (9H)], [0.13 (s, enol), 0.09 (s, keto) (6H)].  84  13C NMR (100 MHz, CD3CN, 65oC):  190.4, 175.3, 175.0, 169.4, 163.6, 157.9, 156.7, 156.3, 154.6, 153.1, 151.7, 128.1, 119.2, 116.6, 112.4, 96.1, 96.0, 94.0, 81.7, 69.8, 62.9, 62.4, 60.6, 60.5, 45.0, 28.8, 27.4, 26.5, 24.1, 19.2, -4.8  IR: 3126, 1694, 1630, 1090, 837cm-1 Optical Rotation:  [α]D22 = -6.09o (CH2Cl2, c = 1.78) LRMS: 554.3 [M+H+] HRMS: cald for 554.2179 C25H40N3O5SiS2; found:  554.2179 [M+H+]              85  Figure A18. 1H NMR of 89 at Room Temperature         Figure A19. 1H NMR of 89 at 65oC in d3-acetonitrile (400MHz)  86  Figure A20. 13C NMR of 89 at Room Temperature Figure A21. 13C NMR of 89 at 65oC in d3-acetonitrile (100MHz)  87  A.10 Preparation of ethyl 2-(2-(methoxymethoxy)acryloyl)thiazole-4-carboxylate (142)  Commercial tert-BuLi in pentane (1.24 M, 7.0 mL, 8.7 mmol, 5.8 eq) was carefully added over 3 min to a cold (–78 °C) solution of known methoxymethyl vinyl ether (385 mg, 8.7 mmol, 5.8 eq) [Tamao, K.; Nakagawa. Y.; Ito, Y. Org Syn. 1996, 73, 94] in dry tetrahydropyran (3.5 mL). A bright yellow solution resulted. The mixture was stirred at –78 °C for 10 min, then it was warmed to –10 °C (NaCl/ice bath) and stirred at that temperature for 23 min, during which time the bright yellow color disappeared. The mixture was then cooled back to –78 °C and diluted with THF (2.5 mL).  A solution of aldehyde 155 (277 mg, 1.5 mmol, 1 equiv) in THF (1.5 mL) was added dropwise, whereupon the color of the solution turned light red. The mixture was stirred at –78 °C for 20 min, then it was quenched with TMSCl (1.04 mL, 8.20 mmol, 5.5 eq) and stirred for 10 more min. Aqueous sat. NH4Cl (1.5 mL) was added, and the mixture was rapidly warmed to r.t (warm water bath).  Extra aq. sat. NH4Cl (3 mL) was added during the warming process. The mixture was then diluted with EtOAc (25 mL) and transferred to a separatory funnel and the aqueous layer was discarded. The organic phase was washed with more aq. sat. NH4Cl (3 mL), dried over Na2SO4 and concentrated in vacuo. In crude form, the sensitive product 164 (not fully characterized; a proton NMR is provided) was immediately taken up in THF (6 mL), treated with 1M TBAF in THF (1.80 mL, 1.80 mmol) and stirred at r.t for 3 h.  The mixture was quenched with aq. sat. NH4Cl (3 mL), diluted with EtOAc (25 mL) and transferred to a separatory funnel. Water (5 mL) was added to remove ammonium salts completely, then the organic phase was dried (Na2SO4) and concentrated in vacuo to afford crude  88  alcohol 165 (410 mg, 1.50 mmol). This compound was immediately dissolved in CH2Cl2 (15 mL) and treated with MnO2 (1.30 g, 15 mmol, 10 eq relative to 165). The mixture was stirred at r.t for 48 hours, then it was filtered over Celite. The filtrate was evaporated in vacuo and the residue was immediately applied to a column of silica gel (10 g) that had been deactivated by flushing with 3.5% Et3N in hexane (20 mL).  Elution with 3.5% Et3N: 50% EtOAc: 46.5% hexanes yielded enone 142 as a yellow oil (Rf = 0.63). Yield: 101mg (25% over 3 steps) 1H NMR:  8.43 (s, 1H), 6.57 (d, 1H, J=3.0Hz), 5.63 (d, 1H, J=3.0Hz), 5.17 (s, 2H), 4.43 (q, 2H, J=7.3Hz), 3.50 (s, 3H), 1.41 (t, 3H, J=7.3Hz) 13C NMR: 179.0, 165.9, 160.8, 153.1, 148.6, 132.9, 107.2, 94.6, 61.8, 56.5, 14.3 IR: 1733, 1721, 1658, 1610, 1152, 1011cm-1 LRMS:  272.2 [M+H+], 294.2 [M+Na+] HRMS: cald for 272.0593 C11H14NO5S; found:  272.0590 [M+H+]          89  Figure A22. 1H NMR of 142 Figure A23. 13C NMR of 142  90  Figure A24. 1H NMR of Suspected Dimer of Crude 164            91  A.11 Preparation of tert-butyl (S)-4-(4-(3-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)-6-(4-(ethoxycarbonyl)thiazol-2-yl)-5-(methoxymethoxy)pyridin-2-yl)thiazol-2-yl)-2,2-dimethyloxazolidine-3-carboxylate (141)   Commercial NaH (60% oil dispersion, 16 mg, 0.40 mmol, 1.3 eq) was dispensed into a flask maintained under inert atmosphere (Ar balloon), washed with hexane (3 x 0.4 mL) to remove excess oil, and suspended in THF (0.4 mL). A solution of ketone 89 (162 mg, 300 mol, 1 eq) in THF (0.5 mL) was slowly added (syringe) at r.t over ca. 2 min. Evolution of H2 was observed, and the color changed from faint green to yellow upon stirring at r.t for 15 minutes. A solution of enone 142 (126 mg, 0.46 mmol, 1.5 eq) in THF (0.4 mL) was added at r.t over a period of 1 min, whereupon the color of the solution turned from yellow to brown. The mixture was stirred for 90 min at r.t, then it was quenched with aq. sat. NH4Cl (0.5 mL), diluted with EtOAc (10 mL) and transferred to a separatory funnel. The aqueous layer was discarded, while the organic phase was washed with more aq. sat. NH4Cl (0.5 mL), dried over Na2SO4 and concentrated in vacuo. The crude diketone 169 was immediately taken up in a 4:1 mixture of TFE (1.8 mL) and AcOH (0.45 mL), and the solution was added over a period of 2.5 h (syringe pump) to a flask containing a warm (60 °C, oil bath temperature) solution of NH4OAc (288 mg, 3.75 mmol, 12.5 eq) and PPTS (75 mg, 300 mol) in 4:1 TFE (2 mL) - AcOH (0.5 mL), maintained under O2 atmosphere (balloon). The mixture was stirred at 60oC for 14 h, then it was concentrated in vacuo and the residue was taken up with EtOAc (15 mL). This solution was washed 2-3 times with 2 mL portions of aq. sat. NaHCO3 until the pH of the aqueous washes (pH paper) stabilized at 7, then it  92  was dried over Na2SO4 and concentrated in vacuo. Flash column chromatography of the residue (1% Et3N: 25% EtOAc: 74% Hexane) afforded pyridine 141 as a yellow solid. This compound was highly UV active (purple under short wavelength, and sky blue under long wavelength), and its elution was readily monitored by TLC (50% EtOAc: 50% Hexane; Rf=0.40).  The room temperature NMR spectra of 141 exhibited broad lines and revealed the presence of BOC rotamers.  Therefore, NMR spectra were recorded from DMSO-d6 solutions at 80 °C.  Yield: 22mg (28% over 2 stpes) Melting Point: 58 – 61oC 1H NMR (400 MHz, DMSO-d6, 80 °C): 8.61 (s, 1H), 8.20 (s, 1H), 7.91 (s, 1H), 7.48 (s, 1H), 5.51 (s, 2H), 5.15 (d of d, 1H,J=6.2Hz J=1.7Hz), 4.82 (s, 2H), 4.37 (q, 2H, J=7.1Hz), 4.22 (d of d, 1H, J=9.0Hz, J=6.2Hz), 3.96 (d of d, 1H, J=9.0Hz, J= 1.7Hz), 3.57(s, 3H), 1.63 (s, 3H), 1.52 (s, 3H), 1.41 (s, 9H), 1.36 (t, 3H, J=7.1Hz), 0.94 (s, 9H), 0.12 (s, 6H). 13C NMR (100 MHz, DMSO-d6, 80 °C): 172.0, 164.0, 162.3, 160.5, 156.3, 151.4, 150.9, 149.4, 147.1, 143.9, 139.2, 130.3, 130.2, 125.0, 120.3, 117.4, 95.4, 93.8, 79.7, 68.0, 61.0, 60.3, 58.3, 56.1, 27.6, 26.1, 25.4, 23.1, 17.5, 13.7, -5.7. IR: 1705, 1154, 1088cm-1 Optical Rotation:  [α]D21 = -11.3o (CH2Cl2, c = 1.65) LRMS: 826.4 [M+Na+]  HRMS: cald for 826.2410 C36H49N5O8SiS3Na; found:  826.2411 [M+Na+]       93  Figure A25. 1H NMR of 141 at Room Temperature in d3-acetonitrile Figure A26. 1H NMR of 141 at 80oC in d6-DMSO (400MHz)  94  Figure A27. 13C NMR of 141 at 80oC in d6-DMSO (100MHz) Figure A28. Expanded HSQC of 141  95  Column: Agilent ZORBAX Eclipse XDB-C18, 5μm, 4.6mm x 150mm Solvent: MeOH (5%), H2O (95%) Flow rate: 0.7mL/min Detection: UV, 254nm  Figure A29. HPLC Trace of Pyridine 141          96  A.12 Preparation of  tert-butyl (S)-4-(4-(3-(4-(((tert-butyldimethylsilyl)oxy)methyl)thiazol-2-yl)-6-(4-(ethoxycarbonyl)thiazol-2-yl)-5-hydroxypyridin-2-yl)thiazol-2-yl)-2,2-dimethyloxazolidine-3-carboxylate (171)  Solid PPh3 (2.6 mg, 10 mol, 0.25 eq) and CBr4 (3.3 mg, 10 mol, 0.25 eq) were added to a solution of pyridine 141 (32 mg, 40 mol) in 1,2-dichloroethane (0.7 mL) and the mixture was heated to 40 °C (oil bath temperature), with good stirring, for 3 h, whereupon TLC (50% EtOAc:50% Hexane) showed complete conversion of 141 into 171. The solution was then diluted with CH2Cl2 (4 mL), washed with aq. sat. NaHCO3 (1 mL), dried over Na2SO4 and concentrated in vacuo. Flash column chromatography (20% EtOAc:80% Hexane) of the residue afforded pyridine 171 as a yellow film. The compound, which was very streaky on TLC (Rf = 0.51-0.73 in 50% EtOAc:50% Hexane), is highly UV active. It can be visualized on TLC as a green spot under short wavelength, and a bright green one under long wavelength. Yield: 22mg (71%) 1H NMR(CD3CN): 11.58 (bs, 1H), 8.26 (s, 1H), 7.95 (s, 1H), 7.56 (s, 1H), 7.23 (s, 1H), [5.27-5.22 (m, minor rotamer), 5.16-5.11 (m, major rotamer) (1H)], 4.90 (s, 2H), 4.45 (q, 2H, J=7.1Hz), [4.21-4.13 (m, major rotamer), 4.11-4.03 (m, minor rotamor) (2H)], [1.79 (bs, major rotamer), 1.72 (bs, minor rotamer) (3H)], [1.58, 1.54 (bs, rotamers of equal intensity, 6H)], 1.46-1.40 (m, 9H), 0.96 (s, 9H), 0.13 (s, 6H) IR: 3126, 2932 1704, 1365, 839cm-1 Optical Rotation:  [α]D25 = -8.89o (CH2Cl2, c =0.350) LRMS: 760.3 [M+H+]   97  HRMS: cald for 760.2329 C34H46N5O7SiS3; found:  760.2324 [M+H+]. Heating a solution of 171 in DMSO-d6 to 80 oC in an NMR probe caused partial release of the TBS group. An NMR spectrum of the resulting mixture is provided.                98  Figure A30. 1H NMR of 171 at Room Temperature in d3-acetonitrile Figure A31. 1H NMR of partially released 171 at 80oC in d6-DMSO (400MHz)  99  A.13 Preparation of  tert-butyl (S)-4-(4-(6-(4-(ethoxycarbonyl)thiazol-2-yl)-5-hydroxy-3-(4-(hydroxymethyl)thiazol-2-yl)pyridin-2-yl)thiazol-2-yl)-2,2-dimethyloxazolidine-3-carboxylate (172)  The partially desilylated pyridine described above was redissolved in THF (0.1 mL) and was with TBAF (1M, 0.06 mmol, 0.06 mL), and was stirred at room temperature for 20 minutes. The mixture was then diluted with EtOAc (2mL), washed with sat. aq. NH4Cl (0.5mL), and water (2 x 0.5mL) to remove excess ammonium salts. The organic layer was collected, dried over Na2SO4, and concentrated in vacuo.  The residue was subject to flash column chromatography (40% EtOAc: 60% Hexane) and the elution of the desired product was monitored by TLC with (70% EtOAc:30% Hexane, Rf = 0.14-0.31, extremely streaky) as the eluent. Compound 172 appeared as purple spot under short wavelength, and as a yellow one under long wavelength. The product was isolated as yellow foam. Yield: 17mg (66% over 2 steps) 1H NMR(CD3CN): 11.71 (bs, 1H), 8.37 (s, 1H), 7.90 (s, 1H), 7.71 (s, 1H), 7.34 (s, 1H), 5.10 (d, 1H, J=5.9Hz), 4.64 (s, 2H), 4.39 (q, 2H, J=7.1Hz), 4.19 (d of d, 1H, J=9.0Hz, J=6.3Hz), 3.92 (d of d, 1H, J=9.0Hz, J=1.6Hz), 1.67 (s, 3H), 1.52 (s, 3H), 1.49 (bs, 3H), 1.41-1.37 (m, 9H) 13C (100 MHz, DMSO-d6, 65 °C): 172.3, 168.2, 162.0, 159.7, 157.8, 151.5, 151.2, 151.0, 145.4, 142.7, 133.8, 131.5, 130.1, 125.0, 120.2, 117.2, 93.9, 79.8, 68.2, 60.9, 59.4, 58.3, 27.7, 26.3, 23.1, 13.8 IR: 3424, 3115, 2934, 1702, 1366, 1101cm-1 Optical Rotation:  [α]D21 = -12.1o (CH2Cl2, c =0.850)  100  LRMS: 646.4 [M+H+]  HRMS: cald for 646.1464 C28H32N5O7S3; found:  646.1470 [M+H+]                 101  Figure A32. 1H NMR of 172 at room temperature in d3-acetonitrile Figure A33. 1H NMR of 172 at 65oC in d6-DMSO (400MHz)  102  Figure A34. 13C NMR of 172 at 65oC in d6-DMSO (100MHz)           103  A.14 Preparation of 7-chloro-2-(methoxycarbonyl)-3-methyl-1H-indole-4-carboxylic acid (177)  NaNO2 (7mmol, 0.484g) in H2O (3ml) was slowly added to the solution of 174 in conc. HCl (8ml) and H2O (5ml) very carefully over 10 minutes at -10oC (NaCl/ice bath), and the mixture was stirred at -5oC for 15 minutes. Then, the solution of SnCl2 (3.96g, 8.75mmol) in conc. HCl (4.5ml) was slowly added to the mixture, and mixture was further stirred at -5oC for 15 minutes.  Then, the resulting solid was filtered using EtOH (15ml)/Et2O (15ml) and dried under high vac to remove excess solvents to afford 175 as a brown solid. Then, hydrazine 174 (6.8mmol, 1.27g) was re-dissolved in the mixture of AcOH/H2O (24ml:4ml), and methyl 2-oxobutanoate (6.6mmol, 0.680ml) was added.  The mixture was brought up to 70oC and stirred for 1 hour. The reaction mixture was diluted with additional 40ml H2O and transferred to a separatory funnel, and aqueous layer was extracted with EtOAc (70ml x 3 ml). The combined organic layer was washed with 10ml H2O, dried over Na2SO4, and concentrated in vacuo to afford hydrazone 176 as a white solid. This white solid was redissolved in 25ml AcOH, and PPA (~2ml) was added, and the mixture was brought up to 90oC and stirred for 2 hours. The excess AcOH was removed in vacuo, and the residue was suspended in water, filtred, and dried in high vac to afford the known indole 177 as a brown solid. [Bentley, D. J.; Fairhurst, J.; Gallagher, P. T.; Manteuffel, A. L.; Moody, C. J.; Pinder, J.  L. Org. Biomol. Chem. 2004, 2, 701] Yield: 1.12g (60% over 3 steps)  104  1H NMR (Acetone-d6): 10.80 (bs, 1H), 7.59 (d, 1H, J=7.9Hz), 7.43 (d, 1H, J=7.9Hz), 3.93 (s, 3H), 2.72 (s, 3H) 13C NMR (Acetone-d6): 167.8, 161.7, 134.2, 134.1, 126.5, 126.2, 123.5, 123.1, 120.9, 120.7, 51.1, 11.4   IR: 3254, 1713, 1687, 1229cm-1 LRMS: 268.7 [M35+H+], 270.7 [M37+H+]                105  Figure A35. 1H NMR of 177 Figure A36. 13C NMR of 177  106  A.15 Preparation of 7-chloro-2-(methoxycarbonyl)-3-methyl-1H-indole-4-carboxylic acid (178)  The indole 177 was dissolved (267mg, 1mmol) in 10ml MeOH, and 27mg of (10% weight) Pd/C was carefully added to the mixture, and subsequently H2 was introduced (balloon). The mixture was stirred for 72 hours at r.t, by the time starting indole 177 was fully converted. (monitored by NMR). The resulting dechlorinated indole (0.8mmol, 176mg) was dissolved in 3ml THF, and cooled to 0oC. BH3-SMe2 (10M, 1mmol, 0.100ml, 1.25 eq) complex was added to the mixture dropwise over 3 minutes, and slowly brought up to r.t for 3 hours. The excess solvent was evaporated in vacuo, and the resulting residue was diluted with 20ml EtOAc, and sat. aq. NaHCO3 (8ml) was poured, and transferred to a separatory funnel. Organic layer was collected, and aqueous layer was further extracted with 10ml EtOAc. Combined organic layer was dried over Na2SO4, and concentrated in vacuo to afford the alcohol as a slightly yellow solid. The resulted alcohol was redissolved in 5ml DMF, and TBS-Cl (1mmol, 150mg, 1.25 eq) and imidazole (1.2mmol, 80mg, 1.5 eq) were added, and stirred overnight. The mixture was then diluted with 30ml EtOAc, and transferred to a separatory funnel. The organic layer was washed with H2O (5ml x 5). Combined organic layer was further washed with brine (3ml), and dried over Na2SO4, and concentrated in vacuo. The crude was subjected to flash column chromatograph (20% EtOAc:80% Hex, Rf=0.47) to afford the known indole 178 a white solid. [Bentley, D. J.; Fairhurst, J.; Gallagher, P. T.; Manteuffel, A. L.; Moody, C. J.; Pinder, J.  L. Org. Biomol. Chem. 2004, 2, 701]  107  Yield: 167mg (50% over 3 steps) 1H NMR (Acetone-d6): 7.42 (d, 1H, J=8.2Hz), 7.23 (m, 1H, J=8.2Hz, 7.1Hz), 7.11 (d, 1H, J=7.1Hz), 5.19 (s, 2H), 3.90 (s, 3H), 2.86 (s, 3H), 0.94 (s, 9H), 0.11 (s, 6H) 13C NMR (Acetone-d6): 162.4, 137.1, 135.8, 125.6, 124.7, 123.2, 120.0, 119.0, 111.8, 63.7, 50.7, 25.4, 18.0, 11.2, -5.9 IR: 3246, 1695, 1662, 1181cm-1 LRMS: 334.8 [M+H+]       108  Figure A37. 1H NMR of 178 Figure A38. 13C NMR of 178 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.24.1-0167208/manifest

Comment

Related Items