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The solution photochemistry of some Diels-Alder adducts Ngan, Yuen Mui 1975

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THE SOLUTION PHOTOCHEMISTRY OF SOME DIELS-ALDER ADDUCTS BY YUEN MUI NGAN B. Eng., T....I. T. Tokyo, JAPAN, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of Chemistry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA Ap r i l , 1975 In present ing th is thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for s c h o l a r l y purposes may be granted by the Head of my Department or by h is representa t ives . It is understood that copying or pub l i ca t ion of th is thes is fo r f i n a n c i a l gain sha l l not be allowed without my wri t ten permiss ion. Department of '£}JL The Univers i ty of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 - 11 -ABSTRACT In order to extend the study of the photochemistry of substituted butadiene-a,3-unsaturated ketone Diels-Alder adducts, 3,4-dimethyl-3-cyclohexenyl phenyl ketone (47) and cis and trans-2,5-dimethyl-3-cyclo-hexenyl methyl ketone (48 and 56) were synthesized and photolyzed. The photoproducts from 3,4-dimethyl-3-cyclohexenyl phenyl ketone were obtained in very low yields with d i f f i c u c l t y , and appeared to have arisen via a hydrogen abstraction process. The photoreaction of cis-2,5-dimethyl-3-cyclohexenyl methyl ketone resulted in the formation of 8-oxa-2,5,7-3 7 trimethyltricyclo[ ' joctane (80A), 7-oxa-2,5,8-trimethyltricyclo-3 8 [ ' ]octane (81B) and 7-hydroxy-4,7-dimethylbicyclo[4.2.0]oct-2-ene (82C) , a l l in low yield. The f i r s t two products were produced by an intramolecular Paterno-Buchi reaction, while the third was formed from a Norrish Type II reaction. Photolysis of trans-2,5-dimethyl-3-cyclo-hexenyl methyl ketone afforded the major product 7-oxa-3,6,8-trimethyl-bicyclo[3.2.1]oct-2-ene (83B) in low yield, for which a mechanism involving a diradical intermediate and an unusual intramolecular 1,4-hydrogen transfer was postulated. In addition the photochemistry of the trans-1,3,5-hexatriene-p-benzoquinone Diels-Alder adduct (t-59) was investigated. Selective n-ir* excitation of this material led to a single product, tetracyclo[6.4.0. 2 11 4 9 0. ' 0 ' ]dodec-6-ene-3,10-dione (127) in high yie l d , the result of a novel regiospecific intramoleculat 2+2 photoaddition. A two step mechanism involving a diradical intermediate was proposed, and the observed photoreaction was discussed in terms of conformational and radical s t a b i l i t y effects. - i i i -TABLE OF CONTENTS Page INTRODUCTION 1 (1) General 1 (2) Background of Present Research 6 (3) Goals of the Present Research 15 RESULTS AND DISCUSSION 19 (1) The Diels-Alder Adduct of 2,3-Dlmethyl-l,3-butadiene with Phenyl Vinyl Ketone 19 A. Synthesis of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone 19 B. Photolysis of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone 20 C. Synthesis of 3,4-Dimethyl-3-cyclohexenyl-2,2,5,5-Phenyl Ketone 23 D. Photolysis of 3,4-Dimethyl-3-cyclohexenyl-2,2,5, 5-d^ Phenyl Ketone 25 (2) Methyl Vinyl Ketone-trans, trans-2,4-Hexadiene Diels-Alder Adducts 26 A. Synthesis 26 B. Photolysis of cis-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone 27 C. Photolysis of trans-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone 30 D. Discussion of the Mechanism of the Photolysis of Adducts cis and trans-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone 31 - iv -Page (3) p-Benzoquinone-trans-1,3,5-Hexatriene Diels-Alder Adduct 40 A. Synthesis of 5-Vinyl-4a,5,8,8a-tetrahydro-1,4-naphthoquinone 40 B. Photolysis 45 I. Photolysis in Benzene 45 II. Photolysis in tert-Butyl Alcohol 47 C. Discussion 48 D. Conclusions 57 EXPERIMENTAL 58 BIBLIOGRAPHY 72 - v -LIST OF SCHEMES AND FIGURES Page Scheme 1 5 2 7 3 8 4 9 5 9 6 10 7 14 8 16 9 18 10 20 11 22 12 24 13 25 14 32 15 33 16 34 17 35 18 35 19 36 20 38 , 21 39 22 41 23 44 24 49 25 50 26 51 27 53 28 54 29 56 Fig. 1 2 2 3 3 4 - v i -ACKNOWLEDGEMENTS I would like to express my sincere gratitude to Dr. J. R. Scheffer for his direction and encouragement throughout this work. I am also grateful to my friends and coworkers Dr. K. S. Bhandari, Dr. R. E. Gayler B. Jennings, J. Louwerens, H. Zakouras, A. A. Dzakpasu for their helpful discussions and cordial s p i r i t . Finally, I am further indebted to the many staff members of this department for their contributions and to the University for a Teaching Assistantship, - 1 -INTRODUCTION t (1) General The past two decades have been extremely f r u i t f u l in the f i e l d of photochemistry. During the past five years alone about 12500 papers covering organic and inorganic photochemistry were published.''" Photochemistry has developed so rapidly that i t has now become an important synthetic tool, both i n the synthesis of highly strained 2 and polycyclic ring systems as well as in the total synthesis of a 3 number of natural products., The regions of the electromagnetic spectrum that are used in organic photochemistry are generally the near ultraviolet (2000 A to 4000 A) and the v i s i b l e region (4000 A to 8000 A ) . * The absorption of light by molecules raises them from their ground electronic states to less stable states of higher electronic energy (excited states) that may result in chemical changes in the molecules. Each substance i s selective in i t s absorption of radiation, depending on the presence of functional groups (chromophores). There are two accessible electronic transitions (n-ir* and ir-u*) in the conventional absorption spectra of o o ketones (2000 A to 7000 A). The n - T r * transition usually occurs between 2700 A" to 3500 A with low intensity depending on the exact structure - 2 -while the TT-TT* transition generally occurs at wavelengths between o o 2000 A to 2500 A with high intensity. The n-ir* excitation is often the most prominent in the organic photochemistry of carbonyl compounds and as noted above usually requires the least energy."* The n-ir* excitation promotes an electron from the non-bonding orbital (n) on oxygen to the anti-bonding orbital (ir*) (Figure 1).^ The presence of an electron in the anti-bonding MO (ir*) reduces the double bond character of the carbon-oxygen bond, and the singly occupied n-orbital conveys radical-like reactivity to the oxygen atom. It is often noted that the n-ir* excited state of carbonyl compounds resembles an alkoxy radical in chemical reactivity.^ The n-ir* excited state can often be selectively populated by choosing the proper f i l t e r . I II III Figure 1. Two and three dimensional representation of the carbonyl group in i t s ground state ^1) and n-ir* excited state (II and III), o represents sp hybrid electrons coaxial with sigma C-0 bond; y*s, p electrons; •, ir-system electrons. - 3 -When a molecule absorbs a photon of v i s i b l e or ultraviolet light and an electron is promoted to an antibonding orbital with spin preserved, an excited singlet state results, while the spin unpaired state i s a 8 -9 t r i p l e t excited state (Figure 2). The average lifetime is 10 to -5 _5 _3 10 sec. for singlet excited states and 10 to 10 sec. for t r i p l e t states. Thus the t r i p l e t i s generally much longer lived than the singlet and in addition usually has lower energy than the singlet state. The t r i p l e t state is obtained mainly by intersystem crossing from the corresponding singlet excited state since direct ground state (singlet)-t r i p l e t excitation is forbidden (Figure 3). hv intersystem crossing II III Figure 2 Simple MO Description of Singlets and Triplets I = orbitally and spin paired ground state; II = orbitally unpaired, spin paired singlet state ; III =orbitally and spin unpaired t r i p l e t state. - 4 -TT —TT* S (10 5-10~3) phosphorescence Figure 3 Jablonski Diagram S . f S = singlet state; T , T 2 - triplet state; S = ground state J - —-One of the best studied photochemical reactions which often 10 originates from n-7r* excited states is the Norrish Type II reaction. This type of reaction is characterized by a hydrogen transfer to the excited carbonyl oxygen. In many cases studied the ketones or aldehydes undergo intramoleculer hydrogen atom transfer through a six-membered transition state (y-hydrogen transfer) leading to a 1,4-biradical intermediate which then suffers either intramolecular ring closure to give a cyclobutanol (path b, Scheme 1) or cleavage to afford a carbonyl compound and an olefin (path a) . 1 * Both singlet and triplet n-ir* excited states can undergo hydrogen abstraction. With aromatic ketones, for which intersystem crossing yields are generally unity, photoelimination and cyclobutanol formation occur only from triplet states. For aliphatic ketones cyclobutanol formation proceeds mainly via the triplet state while both singlet and triplet n- w * states undergo Type II elimination 10 - 5 -Sometimes for aryl-alkyl ketones either electron-donating substituents in the aryl group or high solvent polarity can cause an inversion in the order of t r i p l e t states, so that the T T - T T * state rather than the n-ir* t r i p l e t becomes the lowest state. However in this case the ketone is found to have substantially reduced reactivity toward intramolecular 12 hydrogen abstraction. It is suggested in this case that the ir-ir* state has most of i t s excitation localized in the aromatic ring and in addition has an electron-rich rather than a radical-like excited carbonyl oxygen, both of these factors tending to disfavour theprocess of hydrogen atom abstraction."^ R hv R 0. R' R' it b R' II R' OH a + R' II 0 Scheme 1 Norrish Type II Process - 6 -(2) Background of Present Research In 1971 an interest in the possible synthesis of unusual polycyclic ring systems led our research group to investigate the photochemistry 13 of the butadiene-benzoquinone Diels-Alder adduct 1(a). The photolysis of this compound had previously been reported to lead only to tar 14 and ill-defined products. Selective n—IT* excitation of this compound in benzene gave a 10% yield of two novel t r i c y c l i c ene-diones 3(a) and 4(a) in a ratio of 7:1. The reaction was suggested to proceed via intramolecular g-hydrogen abstraction by the excited oxygen atom through a five-membered transition state giving resonance-stabilized 13 diradical _2 which may in turn close to give 3(a) and 4(a). A study of the photochemistry of the 2,3-dimethylbutadiene-benzoquinone Diels-Alder adduct 1(b) provided further evidence for this mechanism.^ Photolysis of adduct 1(b) gave r i s e to analogous products 3(b) and 4(b) and an additional t r i c y c l i c enone alcohol 5_. These photoproducts were found to undergo novel thermal and photochemical interconversions, and the ratios of these three products were remarkably solvent dependent. In tert-butyl alcohol 80% of ene-dione 4(b) was isolated and only trace amounts of 3(b) and 5_ were formed. In contrast, photolysis of 1(b) in benzene afforded only a 35% yield of ene-dione 3(b) and a 22% yield of keto-alcohol 5_. It was tentatively suggested that tert-butyl alcohol stabilized an intermediate such as 6_ by hydrogen bonding and solvation of the positive charge at Cg leading to preferential formation of photoproduct 4(b), while in benzene, in the absence of hydrogen bonding, an intermediate diradical such as 7_ (possibly inductively stabilized at the C, position by the methyl - 7 -group) leads to preferential formation of photoproducts 3(b) and 5. 6 7 - 8 -The intermediacy of enol 5_ was supported by the study of the irradiation of 1(b) in tert-butyl alcohol-O-d. Photolysis of 1(b) in tert-butyl alcohol-O-d led to ene-dione 9_ with one deuterium atom substituted exclusively in the exo-4 position. This experiment ruled out a mechanism involving i n i t i a l hydrogen abstraction by C„. Further evidence for the mechanism was provided by the irradiation of tetradeuterated adduct 10 in benzene and tert-butyl alcohol. Photolysis of 10_ in benzene afforded tetradeuterated photoproduct 13, whereas irradiation of 10 in tert-butyl alcohol gave photoproduct 15_ with 3.0 D substituted (Scheme 4). The process of 3-hydrogen abstraction through a five-membered transition state is very rare. Prior to the investigation mentioned above only one example of photochemical 3-hydrogen abstraction had been reported. The photochemical reation studied by Padwa and Eisenhardt^ involves trans-N-tert-butyl-2-phenyl-3-benzoylaziridine 16 as shown in Scheme 5. Interest in this type of reaction led us to extend our study to other Diels-Alder adducts. Scheme 4 H - 1 0 -In order to study substltuent effects on the g-hydrogen abstraction, the trans,trans-2,4-hexadiene-p-benzoquinone Diels-Alder adduct 27 was also investigated in our laboratory.^ This molecule introduces methyl groups into the $ and g' positions, so that both g and y-hydrogen abstractions are possible. However only one photoproduct 30^  was isolated in 26% yield. This is the product resulting from exclusive Y-hydrogen abstraction by oxygen. Evidence for the intermediacy of enol 29_ was provided by the f i n -ding of one deuterium atom incorporated at the position when the photolysis of compound 27 was carried out in tert-butanol-O-d. - 11 -Preferential y-hydrogen abstraction in the case of 27 was explained on the basis of conformational control. Of the three conformations shown in eq 2, conformation 33_ with two methyl groups in a pseudo-equatorial position and conformation 34_ (quasi-chair) are favoured. These two favourable conformations bring methyl hydrogens closer to oxygen than the 3-hydrogens. The severe methyl-methyl bowsprit-flagpole-like interaction i n conformation 32^ makes 3-hydrogen abstraction unlikely from this conformer. These ideas were supported by the inertness of compound 35_ when irradiated under the same conditions. eq 2 eq 3 In previously studied reations in our laboratory, intramolecular a l l y l i c hydrogen abstraction was observed only for a,3 -unsaturated ketones, and the carbonyl groups were restricted by a ring, e. g. compounds 1 and 27. When the analogous saturated compound 3_6 was irradiated under the same conditions, no intramolecular hydrogen - 12 -18 abstraction was observed. Apparently the generation of the diradical intermediate requires the presence of the ene-dione double bond for reasons of stabilization. hv no reaction eq 4 19 In 1971 Jeger and coworkers reported a possible example of a 3-hydrogen abstraction process in the y,g-saturated ketone 38. In this reaction the excited ketone group might have abstracted the a l l y l i c Cg hydrogen to form the observed photoproduct cyclopropanol 39. 38 39 eq 5 In view of the novel and extremely interesting chemistry observed for the variously substituted butadiene-benzoquinone Diels-Alder adducts described above, i t next became of interest to our group to extend this work to the study of the photochemistry of monocyclic Diels-Alder adducts such as the 2,3-dimethylbutadiene-methyl vinyl ketone adduct 40. Diels-Alder adduct 40 can be considered to be analogous - 13 -to compound 1(b). However compound 4_0 differs from 1 (b) in that (a) i t possesses greater conformational freedom compared with 1(b), i^e., the acetyl group can rotate around the C^-C^ bond, and (b) i t lacks a double bond conjugated to the carbonyl group. Inspection of the molecular model of compound 40_ reveals that the acetyl group can occupy an equatorial position (conformer 41) or an axial position (conformer 42). Based on the relative a l l y l i c hydrogen to oxygen distances in conformations 41 a n& 42_, the expectation was that conformation 41^  would favour 3-hydrogen abstraction from and that conformation 42 would f a c i l i t a t e y-hydrogen abstraction from C,.. However in contrast to our expectations, Dr. K.S. Bhandari in our laboratory found that photolysis of compound 40 in hexane afforded exclusively the cyclic ether 43; neither of the anticipated hydrogen abstraction processes was 20 observed (eq 6). In order to further elucidate the reaction mechanism and the structure of the photoproduct, Dr. Bhandari studied the photochemestry 20 of the tetradeutero analogue 44. Irradiation of compound 44_ gave a single product 46_ with no loss i n overall deuterium content. The 21 characteristics of the nmr spectrum were consistent with structure 46. A possible mechanism for this reaction i s shown in Scheme 7. This involves the addition of the excited carbonyl oxygen to of the double bond ( f i r s t step of intramolecular oxetane formation) to give diradical species 45_ which in turn undergoes an intramolecular hydrogen (or deuterium) shift yielding the observed photoproducts 4_3 and 46. Since the present thesis describes further experiments which bear on this problem, a detailed discussion of this mechanism is deferred to the Results and Discussion section. - 14 -- 15 -(3) Goals of the Present Research The goals of my research which led to this thesis were threefold; each i s outlined in detail below. I n i t i a l l y we chose to study the photochemistry of phenyl ketone 47. Recalling that ketone 4fj fai l e d to take part in hydrogen abstraction, i t was reasoned that such a process would l i k e l y be more favorable for j4_7_ due to increased stabilization of the diradical which would be produced ( cf. photochemical inertness of adduct 36_ ). 40 47 A second goal of my research was to investigate the photochemistry of a monocyclic Diels-Alder adduct analogous to bicy c l i c adduct 27. For this purpose we chose the a l l cis-2,4-hexadiene-methyl v i n y l ketone Diels-Alder adduct 48^ . As in the case of compound 40) ( p . 14 ), the cis adduct 48_ i s also characterized by a free rotating acetyl group and an a, g-saturated carbonyl group. Inspection of the molecular model of 48^  shows that in either of the conformations corresponding to conformations 41 and 42 discussed earlier, the n-orbital of oxygen can be brought very close to the methyl hydrogens by the free rotation of the acetyl group (see conformers 49_ and 50). Therefore, y or 6 methyl hydrogen abstraction i s a distinct p o s s i b i l i t y . y-Methyl hydrogen abstraction in the case of adduct 48^  would lead to diradical 5_1 and hence possibly to cyclobutanol 52_. In addition in the half-chair - 16 -conformer 4£ , the 3-allylic hydrogen is situated moderately close to the oxygen n-orbital, although not nearly as close as the Y or <S methyl hydrogens in conformers 49_ and 5_0 respectively. Scheme 8 Suggested Route of Hydrogen Abstractions for 48 17 -It was also of Interest to us to study the photochemistry of the trans-epimer 56 of Diels-Alder adduct 48_. Again, depending on the conformation which ketone 5_6_ adopts, various hydrogen abstraction processes appear feasible upon irradiation. These include Y - m e t h y l and / or 3 - a l l y l i c hydrogen abstraction from conformer 5_7 and Y - a l l y l i c hydrogen 0^ CH„ 56 abstraction from conformer 58. H 57 The third goal of my research was to study the photochemistry of the trans-1,3,5-hexatriene-p-benzoquinone Diels-Alder adduct t-59. This compound promised to be of interest for two main reasons: (a) Abstraction of a d i a l l y l i c 3-hydrogen atom from C^ would lead to the very interesting diradical species 60. This intermediate could i n turn lead to several new and unusual ring systems via diradical recombination as outlined in Scheme 9. (b) In certain conformations of Diels-Alder adduct t-59, the side chain vinyl group can approach the ene-dione double bond quite closely. For this reason i t seemed possible that an intramolecular 2+2 photocycloaddition might occur since in general ot, 3-unsaturated ketones undergo this sort of reaction very readily (both inter and intramolecularly) upon irradiation. Intramolecular cyclobutane formation in the case of t-59 could lead t-59 to two cycloadducts (C 2"C 9 and C^IO b o n d i n 8 or C 2-C 1 0 and C 3"C g - 18 -bonding) both of which possess highly interesting and novel ring systems. Scheme 9 - 19 -RESULTS AND DISCUSSION (1) The Diels-Alder Adduct of 2,3-Dimethyl-l,3-butadiene with Phenyl Vinyl Ketone (47) A. Synthesis of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone (47) As mentioned in the Introduction section, hydrogen abstraction was not a major process in the photolysis of the aliphatic ketone 40. We therefore chose to study the corresponding phenyl ketone 47 for comparison. Compound 47 was synthesized through the Diels-Alder reaction between 2,3-dimethyl-l,3-butadiene 6^5 40 47 23 and phenyl vinyl ketone 66^ . The latter material was prepared from 24 3-dimethylaminopropiophenone hydrochloride 67 according to the 25 method of G.C. Mannich and G. Heilter. The procedure of synthesis is outlined in Scheme 10. After purification ( d i s t i l l a t i o n under reduced pressure), compound 47_ was obtained as a colourless liquid, bp 130 °C/ 0.75 mm Hg ( L i t . 2 6 bp 163-165 °C/ 6 mm Hg), in 80% yield. The nmr spectrum showed two multiplets at T 1.95-2.18 and 2.35-2.70 due to the a, 6 and y protons of the phenyl group, a six proton singlet at T 8.40 (vinyl methyl groups), a one proton multiplet centered at x 6.50 (methine hydrogen adjacent to carbonyl), and a multiplet at T 7.60-8.30 due to the six methylene protons. A strong carbonyl stretch - 20 -at 6.Op ln the i r spectrum and a weak uv absorption at 320 nm due to the n-ir* transition (e=100) indicated the presence of the carbonyl group. Scheme 10 CH3C0C6H5 + CH20 + (CH3)2NH«HC1 + HC1 ethanol, reflux C6H5C0CH2CH2-NH<^3 CJ? 67 1 steam d i s t i l l a t i o n C6H5-C-CH=CH2 + (CH3)2NH'HC1 66 CH2=C(CH3)C(CH3)=CH2 + 65 sealed tube 100°C 47 B. Photolysis of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone (47) Compound 4J_ was irradiated in d i s t i l l e d benzene at a concentration of 3-4 mg/ml through a Pyrex f i l t e r (A>290 nm). The reaction was followed by glpc ( 5'-DEGS, 20%, 150°C, 150 ml/min ). A new peak (E) appeared in the early stages of the photolysis. This peak gradually diminished as the photolysis progressed, and at the same time another new peak (D) appeared and increased steadily. At the f i n a l stage of the photolysis when the peak of the starting material had vanished, four photoproducts, A, B, C and D, in order of increasing glpc retention 27 time were formed in the ratio 1 : 1 : 1 : 10. A l l the photoproducts were separated as colourless o i l s by glpc using the same column and - 21 -conditions as above. The yield of the major product D could never be increased beyond 5%; for each of the three minor products (A, B and C), the yields were less than 1%. The spectra of the major product D showed the following characteristics: A strong i r absorption at 6.0 y (1670 cm "*") indicated the presence of a con-jugated carbonyl group. The mass spectrum showed a parent peak at m/e 214 identical to the molecular weight of the starting material. The nmr spectrum (CDCl^) displayed two multiplets due to the aromatic protons (at T 1.95-2.18 and 2.35-2.70), and a methine multiplet centered at T 6.60. Other signals were a new two proton doublet (T 5.20, J=6 Hz), l i k e l y due to an exo-methylene which appeared at higher f i e l d than the six proton methyl singlet (T 8.40) in the nmr spectrum of the starting material, and a multiplet due to the remaining seven protons at T 7.30-8.60. The uv spectrum (ethanol) showed an n-ir* absorption maximum at 320 nm (e=29), the same position as that of the starting material (A. =320 nm, e=100). This information was indicative of a structure of either 69_ or 70_ (P. 23). Since the spectral data could not distinguish between structures 69_ and 70^, and also because both 69_ and 70, had cis and trans isomers, further evidence to conform the structure for photoproduct D was necessary. Because of the low yields of the three minor products (less than 1%) , none of these products could be isolated in amounts sufficient for their characterization. However, interruption of the photolysis when photoproduct E was at i t s maximum concentration allowed i t s isolation in small amounts by preparative glpc. Compound E showed the following spectral data: i r (film) 6.0 (s, C=0) y; mass spectrum (70 eV) m/e parent 214; uv (ethanol) group doublet (T 8.85, J=7 Hz) max - 22 -max 320 nm (e=20); nmr spectrum (CDCl^) T 1.95-2.10 (m, 2, phenyl a-protons), 2.30-2.80 (m, 3, phenyl 3 and y-protons), 4.65 (m, 1, viny l ) , 6.05 (m, 1, methine), 8.38 (t, 3, J=4 Hz, C 3 methyl), and 9.03 (m, 3, methyl). This data indicated that the >CHC0Ph moiety was s t i l l intact and suggested either structure 7JL or 72_ for E. Interestingly, photolysis of a CDCl-j solution of E in an nmr tube (Pyrex f i l t e r ) led to photo-product D as shown by glpc retention time comparisons. There is literature analogy for a reaction such as _71 > 69, although i t is certainly not a well documented process. R.L. C a r g i l l and his coworkers reported an example of a photochemically induced intramolecular hydrogen 29 transfer in the 3, Y - u n s a t u r a t e d ketone 7_3. Irradiation of 7_3 i n hexane (X>260 nm) produced four photoproducts 74(a), 74(b), 75(a) and 75(b) i n which 74(a) predominated ( 70% ). It was proposed that the formation of photoproducts 7_4_ and 7_5 could be accounted for by an orbital symmetry allowed [1, 3] suprafacial sigmatropic reaction. The selectivity observed (70% of the reaction being migration of the hydrogen which is farthest from the carbonyl group and on the face of the ir system away from the carbonyl) might be interpreted as being due to an interaction between the carbonyl group and the ol e f i n i c group in the reacting species. Scheme 11 hv - T - T ^ T - A + B + C + D A>290 nm | maj or - 23 -Scheme 11 In order to obtain more evidence on the structures of the photo-products from compound 47 and to elucidate the mechanism of their formation, the corresponding tetradeuterio compound J76_ was synthesized through the Diels-Alder reaction between 2,3-dimethylbutadiene-l,l,4,4-d^ and phenyl vinyl ketone. The former was in turn prepared by the - 24 -general procedure of Cope. 30-32 The synthesis is outlined in Scheme 12. Starting with compound 7_8_ with deuterium content 75%, compound _76, a colourless l i q u i d , purified by d i s t i l l a t i o n under reduced pressure (142 °C/0.45 mm Hg), was obtained in 15% yield. It showed i r (film) 6.0 (s, C=0) u; mass spectrum (70 eV) m/e parent 215, 216, 217 and 218; nmr (CDC13) T 1.95-2.10 (m, 2, phenyl a-protons), 2.38-2.70 (m, 3, phenyl g and y-protons), 6.32-6.75 (m, 1, methine), 8.40 (s, 6, methyl), 7.70-8.23 (m,-"^, unsubstituted methylene protons). The deuterium content of compound 76. was ca. 70% based on the nmr spectrum 33 and 68.7% based on the mass spectrum. Scheme 12 + 79 DO, dioxane 170°C A D D 78 77 76 - 25 -D. Photolysis of 3,4-Dimethyl-3-cyclohexenyl-2,2,5,5-d7| Phenyl Ketone (76) Compound _76 was irradiated i n benzene under the same conditions as those previously described. The reaction was followed by glpc ( 5'-DEGS, 20%, 150 °C, 150 ml/min ) which showed a trace essentially identical to that obtained in the irradiation of the non-deuterated material. Unfortunately the yields in the deuterated system were even lower than previously obtained, and only deuterated photoproduct D could be isolated in very small amounts. It showed a strong carbonyl stretch at 6.0 y in the i r , and the nmr spectrum (CDCl^) showed T 1.90-2.10 (m, 2, phenyl a-protons), 2.40-2.50 (m, 3, phenyl 6 and Y ~ P r o t o n s ) 5.23 (d, -1.3 H, J=9 Hz, viny l ) , 6.45-6.80 (m, 1, methine), 7.50-8.70 (m, -6.3 H), 8.90 (m, 3, methyl). T f the pathway suggested in eq 8 occurred, two vinyl protons should be present in the major photoproduct D from photolysis of the tetradeuterated compound 47_. However, scrambling had occurred, since there was approximately one v i n y l proton integrated in the nmr of the photoproduct D from 47. The nmr spectrum of photoproduct D i s not simple to explain, and the reactions br i e f l y described i n this section, while interesting from a mechanistic and theoretical point of view, proceeded in such low yields as to make their further investigation not worthwhile. Thus this project was abandoned. It should be noted however, that i n a sense, the original idea of substituting phenyl for-methyl ( cf_. hl_ vs 40_ ) in the hope of observing i n i t i a l hydrogen abstraction rather than intramolecular oxetane formation proved to be valid since both the D and E type photoproducts almost certainly arise in this way. hv P Scheme 13 J 76 ^ r — A + B + C + D X> 260 nm -E-- 26 -(2) Methyl Vinyl Ketone-trans, trans-2,4-Hexadiene Diels-Alder Adducts 48 and 56 A. Synthesis of 48_ and 56_ The Diels-Alder adduct 2,5-dimethyl-3-cyclohexenyl methyl ketone was prepared by heating the mixture of methyl vinyl ketone and trans, trans-2,4-hexadiene in a sealed glass ampoule at 100-105 °C for 20 hrs. The Diels-Alder adducts obtained in 80% yield were separated into two pure compounds in a ratio of 48_ : 5_6_ = 2 : 1 by glpc ( 20'-Carbowax, 20%, 135 °C, 150 ml/min ). As a general rule, kenetically controlled Diels-Alder reactions proceed via erido addition while exo addition stereochemistry i s favoured 34 under conditions of thermodynamic control. We thus assumed that the major adduct formed at 100 °C possessed the endo stereochemistry 48. This was confirmed by the finding that sealed tube thermolysis of either 48_ or 56_ at 180 °C gave an identical equilibrium mixture of 48 : 56_ = 1 : 2 . This can be accounted for on the basis of a dissociation-recombination mechanism or on the basis of an enolization-ketonization mechanism with ketone 56_ being less crowded and hence preferred. Compound 48_, the a l l cis-isomer, was isolated as colourless crystals with melting point 23-25 °C. Ir (film) 5.80 (s, C=0), 6.25 (w, vinyl) p; nmr (CDCl^ T 9.00 (d, 3, J=7 Hz, CH3 at C 2 or C 5), eq 10 - 27 -9.10 (d, 3, J=6 Hz, CH3 at C 2 or C5>, 7.83 (s, 3, CH3>, 7.10-8.80 (m, 5, methine and methylene), 4.50 (m, 2, vinyl); uv (hexane) max 285 nm (e=22, n-ir*); mass spectrum (70 eV) m/e parent 152. Compound 5_6, the trans-epimer , i s a colourless liquid at room temperature. It showed i r (film) 5.80 (s, C=0), 6.25 (w, vinyl) u; nmr (CDC13) T 9.00 (d, 3, J=6 Hz, CH3 at C 2 or C 5), 9.10 (d, 3, J=6 Hz, CH3 at C 2 or C 5), 7.82 (s, 3, CH 3), 7.40-8.40 (m, 3, methine and methylene), 4.50 (m, 2, v i n y l ) ; mass spectrum (70 eV) m/e parent 152. B. Photolysis of cisr-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone (48) As mentioned earlier, i t was of interest to examine the photo-chemical behavior of the monocyclic a l l cis Diels-Alder adduct 4iJ in analogy to the b i c y c l i c compound 27_. It was also of interest to determine the effect, i f any, which the more fle x i b l e ring system of 48_ has on possible hydrogen abstraction processes. As previously ' o pointed out i n the Introduction Section, the CH3 2 ^ molecular model study revealed the p o s s i b i l i t i e s of Y and 6-methyl hydrogen abstraction. In order to find the optimum conditions for reaction, t r i a l photolyses were carried out i n methanol, benzene, pentane and hexane solution. Because of side reactions in methanol and due to the long periods of irradiation required i n benzene (Pyrex f i l t e r necessary), the photolyses were carried out in pentane or hexane; identical results were obtained in each solvent. Thus photolyses of compound 4^5 were carried out at a concentration - 28 -of 1-2 mg/ml using a Corex f i l t e r (X>260 nm) and a medium-pressure mercury immersion lamp. The reaction was followed by glpc ( 20'-Carbowax, 20%, 135 °C, 150 ml/min ). Under conditions of selective n-ir* excitation, compound 4_8_ gave three v o l a t i l e photoproducts A, B, and C which were isolated in about 10% total yield by glpc using the same column as described above. The relative ratio of the photoproducts based on glpc was A : B : C = 6 . 7 : 1 : 3.3. The i r spectrum of the major product A (a colourless liquid) showed that the carbonyl group had been lost. In addition, the weak vinyl band at 6.25 y was no longer observed. The nmr spectrum of photoproduct A showed a one proton doublet at T 5.80 as the only low f i e l d absorption. This could be due to the methine proton (C^) adjacent to the oxygen 35 in an oxetane such as 8OA (Lit. x 5.58). 36 A three proton singlet at T 8.53 ( L i t . CH3-C-0, T 8.60) and two doublets at T 9.00 (d, 3, J=ll Hz) and 9.10 (d, 3, J=10 Hz) could be due to the methyl groups at and C_ or C, respectively. The remaining six protons gave ri s e to a complex multiplet at x 7.50-8.40. Thus structure 80A appeared to be in accord with the spectral data. The mass spectrum (70 eV) gave a parent peak of m/e 152, again consistent with structure 80A. However this data did not rule out another oxetane 8IB resulting from oxygen addition to in the starting material. 81B - 29 -Photoproduct B, which was also a colourless liquid, could not be rigorously purified because of i t s low yield and i t s glpc overlap with photoproduct A. From the spectral data (nmr spectrum of a mixture of A : B = 2 : 3), i t appeared that this material was also oxetane-like. Again, no distinction could be made between 8OA and 81B on the basis of this information. Colourless liquid C was an alcohol. Its i r spectrum displayed a broad OH stretch at 2.90 U, while no carbonyl absorption was observed. The nmr spectrum (CDCl^) showed a two proton vinyl multiplet at T 4.40, a singlet and a doublet at T 8.80 (s, 3) and 9.00 (d, 3, J=7 Hz) respectively due to methyl groups (methyl at C^ and methyl at C^ of structure 82C), and a complex multiplet of seven protons at T 6.80-8.40. This data is well accommodated by structure 82C, a cyclobutanol produced by Y-hydrogen abstraction ( see 8 Introduction Section ). A parent peak of m/e 2 152 in the mass spectrum further confirmed 3 structure 82C. Equation 11 outlines the overall course of reaction of Diels-Alder 82C adduct 48. H 80A 8 IB 82C eq 11 48 - 30 -C. Photolysis of trans-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone (56) The photochemistry of the trans-epimer 56 was also investigated on the assumption that i t might undergo i n i t i a l y and/or g - a l l y l i c hydrogen abstraction (see Introduction Section). Photolyses of com-pound 5_6 were also carried out in dilute hexane or n-pentane solutions through which nitrogen was bubbled to remove dissolved oxygen. Under the conditions of selective n-n* excitation (X>260 nm), the starting material was rapidly consumed, and three new peaks A, B and C due to the photoproducts were found on glpc ( 20'-Carbowax, 20%, 135 °C, 150 ml/min ). The amount of peak A was too l i t t l e to permit i t s isolation; peaks B and C (ratio 2 : 1) were isolated by preparative glpc using the column described above. Both were v o l a t i l e , colourless liquids. Photoproduct B, the major product, was believed to be the cyclic ether 83B on the basis of i t s spectral data and by analogy to compound 37 43. The spectral features from which structure 83B were deduced are the following: no i r carbonyl absorption, weak vinyl absorption at 6.08 u; the nmr spectrum (CDCl^) T 4.45 ( m, 1, vinyl proton), 6.05 (m, 2, and methines), 7.70-8.30 (m, 4, C 5 methylene and C^, and Cg methines), 8.35 (s, 3, v i n y l methyl), 8.80 (d, 3, J=6 Hz, C 3 or C Q methyl) and 9.00 (d, 3, J=6 Hz, C_ or C 0 methyl); mass spectrum (70 eV) m/e parent 152. Structure 83B 83B was assumed to have the exo-C^-methyl configuration shown again in ana-38 logy to compound 43_. The minor photoproduct C displayed a strong i r carbonyl stretch at 5.80 y and a parent peak at m/e 152 in the mass spectrum. The nmr spectrum (CDC13> showed x 4.65 (m, 1, viny l ) , 7.90 (s, 3, methyl), 8.40 (m, 3, methyl), 9.10 (m, 3, methyl) and 7.70-9.00 (m, 6). The nmr spectrum of photoproduct C was very similar to that of the starting material 5_6 except that (a) one signal due to a methyl group was shifted downfield from T 9.00 to T 8.40, and (b) only one vinyl proton was apparent in the spectrum of photoproduct C (T 4.65). Therefore photoproduct C is l i k e l y a double bond isomer of compound _56_, probably either structure 84C or 85C. Further investigation or to confirm the structure of photoproduct C was restricted by the small amounts of 84c 85C material available. V 61 84C or + A 85C 83B y eq 12 i 56 D. Discussion of the Mechanism of the Photolysis of Adducts 48 and 56 The y - nydrogen abstraction process which was favoured in the i r r a -diation of compound 27_ (see Introduction Section) seemed not to be the major process for the photolysis of i t s monocyclic analogue, compound 48. The photoreaction leading to oxetanes 80A and 81B can be thought of 39 as an intramolecular Paterno-Buchi photocycloaddition. The Paterno-Buchi reaction involves addition of an excited carbonyl oxygen to an - 32 -electron-rich or an electron-deficient ol e f i n to give an oxetane. As pointed out in the Introduction Section, a nonbonding electron of the carbonyl oxygen is promoted to the antibonding T  orbital in the n-ir* excited state. This imparts radical-like reactivity to the oxygen atom. The reaction pathway for the photocycloaddition reaction of carbonyl compounds to electron-rich olefins can be described as the addition of the excited carbonyl oxygen atom to an electron-rich unsatu-rated system yielding a diradical intermediate such as 86_ (Scheme 14). Subsquent closure of the diradical intermediate leads to the product 39 oxetane 87. RCOR- hv RCOR ( n . i r * ) 1 . RCOR RCOR ( n . T r * ) 1 ( n , i r * ) 3 (n T T * ) 1 ° R 3 RCOR^n,ir ; + Scheme 14 Paterno-Buchi Reaction * : D » U ' K R— —R*. I 86 R' -R' -R' R R' 87 In the case of 4J5, since the two methyl groups at and C,. are electron-donating substituents, the formation of the photoproduct oxetanes 80A and 81B can be viewed as proceeding via the same pathway described above, i . e. addition of the carbonyl oxygen to the relatively electron-rich double bond at or C^ leading to the diradical intermediates 88 or 89 respectively which then close yielding oxetanes 80A and 8IB (Scheme 15). Many literature examples are known which indicate that oxetane - 33 -Scheme 15 89 81B formation occurs via diradical intermediates. One such was reported by 40 Turro in 1970. In the photocycloaddition of acetone to cis and trans-1-methoxy-l-butene (90 and 91), four isomeric oxetanes were obtained in good yield (Scheme 16). The stereochemistry of the cis or trans-methoxybutene (90 or 91) was completely lost. This result was explained by a two step mechanism involving diradical intermediates 92^ and j)3_ i n which the loss of configuration occurs by the bond rotations shown. Another classic example involves the photocycloaddition of vinyl ethers to acetone, propionaldehyde and cyclohexanone.^ A ca; 1:3 mixture of 2- and 3-alkoxyoxetanes 101 and 100 was obtained (Scheme 17). The preferential formation of the major product 100 was explained on the basis of the expected s t a b i l i t y of the possible intermediate diradicals, the alkoxy substituent presumably leading to the more stable diradical 98_ which subsquently closes to give the major product. Additions to - 34 Scheme 16 C H 3 ° H (CH3)2CO + H > e = C < C 2 H 5 91 hv A? R ( n , T r * ) 1 or 3 93 /R=CH30 or C 2H 5\ ^R'=CH30 or C ^ J C„H 2"r 0CH3 H ~0CH„ H H 94 C 2H 5 H 95 H" ( C f t 3 ) 2 C O + CH 30 > C = C <C 2H 5 90 hv R R' ( n . i r * ) 1 or 3 92 -H i 0CH3 C 2H 5 96 H- -C 2 H 5 0CH3 H 97 ketene diethylacetal seem to be more regiospeclfic, and only 3,3-dialkoxy oxetanes 103 could be isolated. It was presumed that the comparatively higher s t a b i l i t y of the alkoxy diradical 102 was again the important regiospeclfic factor. Returning to the discussion of the photochemistry of Diels-Alder adduct 48, the minor photoproduct 82C obtained in this study is simply a product arising from the well known Norrish Type II reaction ( see Scheme 1, path b)."^ As in the case of adduct 27_, the excited carbonyl oxygen abstracts a y-bydrogen atom from the methyl group leading to the - 35 -Scheme 17 OEt 103 102 biradical intermediate 104 (Scheme 18) which undergoes subsquent bond closure yielding the expected cyclobutanol 82C. Scheme 18 The photolysis of the exo trans, trans-2,4-hexadiene-methyl vinyl ketone Diels-Alder adduct 5_6_ gave two photoproducts. The major product 83B is analogous to product 43_ obtained in the photolysis of 40 (see Introduction Section). The formation of the product 83B can be accounted for by the same mechanism postulated for the analogous product 43^  observed previously, i . e. addition of the excited carbonyl oxygen to of the double bond leads to a diradical intermediate 104 which then undergoes an intramolecular hydrogen shift to give the cyclic ether 83B (Scheme 19). Scheme 19 104 56 As mentioned in the Introduction Section, the photochemistry of the analogous tetradeuterated compound 4-4 was studied. This study showed that there was no total deuterium content change in the photo-product 46 when compared with the starting material. This suggests an intramolecular reaction and also confirms the structure of the cyclic ether 462^. The f i r s t step of the postulated mechanism (Scheme 19) for the reaction 5_6 to 83B is the addition of the excited carbonyl oxygen - 37 -to the double bond leading to a diradical intermediate 104. This step is of course identical to the f i r s t step of the Paterno-Buchi reaction. At this point we can see that both the cis and trans-epimers 48 and 56 undergo addition of the excited carbonyl oxygen to the double bond producing the epimeric diradical intermediates 88_ and 104. The diradical intermediate 88^  produced from the cis-isomer 48, possesses a methyl group at Cg (see Scheme 15) which is situated underneath the seven-membered ring (side opposite the bridge) while i n the diradical 104, the methyl group at Cg is above the seven-membered ring. Thus diradical 104 possesses the stereochemistry at C , necessary for the intramolecular hydrogen transfer leading to 83B. Diradical 88_ however i s stereoche-mically incapable of this process, and as a result bonding occurs between the radical centres giving oxetane 8QA. Intramolecular hydrogen atom shifts of the type postulated (cf. 104—»-83B) are apparently unknown in b i c y c l i c systems. However, they are f a i r l y well established in acyclic systems. For example, an intra-molecular hydrogen shift was observed in an acyclic oxatetramethylene 42 diradical . The photoreaction of biacetyl with 2-methyl-2-butene resulted in the formation of the acyclic ether 108 (50%) and the corresponding oxetane 109 (25%). The formation of the acyclic ether 108 was postulated to proceed through the addition of the excited carbonyl oxygen to the electron-rich olefin giving a diradical inter-mediate 107 which undergoes an intramolecular hydrogen transfer v ia a six-membered transition state (Scheme 20). Liao and Mayo have reported a similar intramolecular hydrogen trans-fer in an acyclic thiotetramethylene diradical . Irradiation of - 38 -Scheme 20 COCH H-transfer 108 0 COCH 3 109 adamantanethione and a _methylstyrene In benzene gave the thietane 114a and 2-adamantyl-2'-phenyl a l l y l sulphide 113a. It was suggested that the cyclization of the hypothetical diradical intermediate 112a led to product 114a, while the intramolecular hydrogen transfer gave product 113a. This was unequivocally demonstrated by irradiation of adamantanethione and a-trideuteriomethylstyrene 111b which afforded trideuteriated products 113b and 114b (Scheme 21). After this study had been completed, another analogous intra-molecular hydrogen transfer was reported i n an acyclic oxatetramethylene d i r a d i c a l . ^ In addition, a hydride shift analogous to the 104—-83B hydrogen atom shift has recently been reported.^ In conclusion, the study of the photochemistry of compounds 48 and _56 has confirmed the mechanism previously postulated for 20 compound 40 (see Introduction Section). and has shown, for ali p h t i c alkyl ketones analogous to 1(b) and 27, that photochemical intramole-cular hydrogen abstraction i s not a major process. - 40 -(3) p-Benzoquinone-trans-1,3,5-Hexatriene Diels-Alder Adduct t-59 A. Synthesis of 5-Vinyl-4a,5,8,8a-tetrahydro-l,4-naphthoquinone (t-59) The starting material 1,3,5-hexatriene 115 was synthesized from l,5-hexadien-3-ol 116 which was in turn prepared from a l l y l bromide.^ The procedure is outlined in Scheme 22. The colourless, v o l a t i l e 1,3,5-hexatriene was obtained as a mixture of cis and trans isomers in the ratio of cis : trans = 3 : 7 (50% yield ) . Without further purification, this mixture was subjected to Diels-Alder reaction with p-benzoquinone. p-Benzoquinone and a three molar excess of 1,3,5-hexatriene were stirred and heated at 60 °C for two hours. After removal of the excess 1,3,5-hexatriene and r e c r y s t a l l i -zation of the resulting crude solid from petroleum ether (bp 67-68 °C), a single product (yellow crystals, mp 58-60°C) was obtained in 40-60% yie l d . The gross structure of adduct _59_ was indicated by the following spectroscopic data: mass spectrum (70 eV) m/e parent 188; uv (ethanol) max 355 nm (e=75, n-ir*); i r (CHC13) 5.95 (s, C=0) and 6.20 (w, vinyl) y; and nmr spectrum (CDCl^) x 3.30 (d, 2, J=2 Hz, vinyl protons at and C 3 ) , 4.10-4.55 (m, 3, vinyl protons at C 6,C 7 and C g), 4.85-5.10 (m, 2, vinyl protons at C^Q)> 6.50-7.05 (m, 3, methine protons at C ^ A , C„ and C q) and 7.10-8.00 (m, 2, methylene protons at C f l). + eq 13 59 41 -CH2=CHCH2Br + Mg CH2=CHCH2CH(OH)CH=CH2  116 C 6H9 S(CH 3) 2CH 2C 6H 5Br"-ether CH2=CHCH2MgBr CH2=CHCHO H20, H 2S0 4 PBr , (CH_).NCH0C,H(. 3 C 6H 9Br 3 2 2 6 5 aqueous NaOH A CH2=CH-CH=CH-CH=CH2 115 C 6H 5CH 2N(CH 3) 2 Scheme 22 Both the cis and trans-1,3,5-hexatriene isomers exist as an equilibrium mixture of cisoid and transoid conformers (eq 14 and eq 15), trans cisoid half-cisoid eq 14 transoid 117 118 119 cis 1 2 ^ 4 cisoid half-cisoid eq 15 transoid 120 121 122 Although the transoid conformers 119 and 122 are the most stable, only the cisoid conformers (117 and 120) and the half-cisoid conformers steric influences (overlap of the spheres of influence of the vinyl group on C-4 and the hydrogen atom on C-l), the cis-isomers (120 and 121) are less reactive than the trans-isomers (117 and 118) respectively. An example of the lowered reactivity of cis-substituted dienes in additions to dienophiles involves cis and trans-1,3-pentadiene. The reactivity of the trans-1,3-pentadiene 123 towards tetracyanoethylene differed f r om the corresponding cis—isomer 124 by a factor of almost 5 47 10 (eq 16 and eq 17) . Similarly, in the case of 1,3,5-hexatriene we may expect that the trans-isomer in the cisoid and half-cisoid conformations (117 and 118) w i l l take part most readily in the diene addition. (118 and 121) can undergo diene addition. 34 As a result of purely yield + 91% eq 16 123 TCNE CN CN CN 0% eq 17 124 Theoretically two epimers, trans-59 (t-59) and cis-59 (c-59), may be formed in the reaction of benzoquinone with the cisoid or half-cisoid conformations of trans-1,3,5-hexatriene (117 and 118). These - 43 -isomers, shown in eq 18 and eq 19, result from endo and exo addition respectively. However the Diels-Alder reaction between p-benzoquinone eq 18 eq 19 and 1,3,5-hexatriene afforded only a single product. Woodward and Hoffmann have ascribed the preference for endo rather than exo addition in the Diels-Alder reaction to secondary forces arising from interaction of frontier orbitals, the highest occupied orbital 48 (HOMO) and the lowest vacant molecular orbital (LVMO). The frontier orbitals often contribute most to the overall energy change as a transformation occurs. As shown in Scheme 23 for the Diels-Alder addition of butadiene to i t s e l f , the endo approach i s distinguished from the exo approach by the energy-lowering interaction of the proximate g and g'orbitals. Similarly, in the case of the Diels-Alder reaction between p-benzoquinone and 1,3,5-hexatriene the energy-lowering secondary interaction leads to the preferential endo-addition. Thus the stereochemistry of 5_9_ was tentatively assigned as t-59. - 44 -Scheme 23 endo-addltion exo-addition A similar stereoselectivity was found in the case of the 1,3,5-49 hexatriene-maleic anhydride Diels-Alder adduct 125. Structure t-125 with the vinyl group trans to the two bridgehead hydrogens was exclusively obtained, although again two isomers could have been formed. eq 20 t-125 - 45 -B. Photolysis of t-59 1. Photolysis iii Benzene Compound t-59 was irradiated in benzene (200 mg of t-59 in 200 ml benzene) through a Corning 7380 f i l t e r (X>340 nm). The reaction was followed by t i c (alumina or s i l i c a gel, eluting solvent: ethyl acetate). Both t i c and glpc (5'-QF-l, 5%, 150 °C, 150 ml/min, the peak due to starting material i s not eluted) indicated that only one photoproduct was produced after two hours of irradiation. The photoproduct was purified f i r s t by column chromatography using ethyl acetate as eluent (200 mg of starting material/20g of alumina), and then by recrystallization from the solvent system petroleum ether : ethyl ether = 9 : 1 . The product was obtained as colourless crystals, mp 91.5-92.5 °C, in |0% y i e l d . This photoproduct displayed the following spectral features: i r (CCl^) 5.82 (s, C=0) y; uv (ethanol) max 283 nm (e=30, n-ir*); mass spectrum (70 eV) m/e parent 188; and nmr spectrum (CDCl^) T 4.25 (m, 2, vinyl) and 6.40-9.00 (m, 10). By comparing the spectral data of the photoproduct with that of the starting material, the following conclusions could be drawn: (a) the shift of the carbonyl stretch to 5.82 y i n the i r of the photoproduct from 5.95 y in the starting material plus the blue shift in the uv of the photoproduct (from 355 nm to 283 nm) indicated the loss of the ene-dione C ^ - ^ ) double bond in the starting material; (b) the mass spectrum of the photoproduct showed that the photoproduct has the same molecular weight as that of the starting material; (c) the nmr spectrum of the photoproduct showed no vinyl hydrogen signals due to the side chain vinyl group or the C 0-C 0 double bond. On the - 46 -other hand the signals attributable to the vinyl hydrogens at C, and o of the starting material were intact in the photoproduct spectrum. This information suggested that a 2+2 cycloaddition reaction had occurred between the side chain vinyl group and the ene-dione double bond. Either structure 126 or structure 127 could be suggested for the photoproduct (eq 21). hv X>340 nm or eq 21 t-59 126 127 For further information, the europium shift reagent 5 0 E u ( f o d ) 3 5 1 (in CCl^ solution) was added to the CCl^ solution,of the photoproduct. 52 A significant four proton downfield shift was observed. These four protons could be the four protons adjacent to the two carbonyl groups in either structure 126 or 127, since the Eu(fod) 3 i s expected 5 0 to coordinate most closely with the carbonyl groups, and those hydrogen atoms closest to the coordination s i t e are shifted furthest downfield. 5 0 It was expected that structures 126 and 127 could be distinguished by i r spectroscopy since in structure 126 the two carbonyl groups are in different size rings (five-membered and six-membered rings) while in structure 127, both carbonyl groups are in six-membered rings. 53 Thus one would predict that 126 would have carbonyl stretches at 5.83 u (1715 cm"1) and 5.72 y (1745 cm"1), whereas 12J7 should have a - 47 -single carbonyl stretch at 5.83 y (1715 cm 1 ) . The i r of the photopro-duct in CCl^ showed a single carbonyl stretch at 5.82 y while the i r in KBr gave rise to a carbonyl absorption at 5.82 y with a shoulder at 5.76 y. Thus on the basis of the spectral data alone i t was impossible to decide between structures 126 and 127. Therefore the photoproduct was submitted for an X-ray crystal structure determination. The X-ray analysis was performed by Professor J.Trotter and S.J. Rettig of this department, to whom the author would like to express her thanks. The result of the X-ray analysis confirmed structure 127 for the photoproduct. II. Photolysis in tert-Butyl Alcohol Irradiation of compound t-59 in tert-butanol using a Corning 7380 f i l t e r (X>340 nm) produced the same photoproduct 127 as in the case of the photolysis in benzene. The yield was 60% for the crude product (isolated by column chromatography, A^O^, eluting solvent: ethyl acetate). H eq 22 t-59 127 - 48 -C. Discussion Although an intramolecular hydrogen abstraction process seemed most l i k e l y , neither the photoproduct from intramolecular d i a l l y l i c g-hydrogen abstraction nor the photoproduct from mono-allylic g-hydrogen abstraction was detected in the irradiation of compound t-59. The only photoproduct, 127, i s obviously produced by intramolecular 2 2+2 cycloaddition of the ene-dione double bond to the side chain vinyl group. Examination of the molecular model of t-59 reveals a possible reason for the failure to observe g-hydrogen abstraction. Thus only in the boat-conformers A and B (Scheme 24) is the d i a l l y l i c o g-hydrogen accessible to the carbonyl oxygen (2.7 A in the conformer A ° 54 and 3.2 A in conformer B ). In conformers A and B, the side-chain vinyl group i s situated at a relatively unfavourable pseudo-axial position. Furthermore,the carbon-diallylic hydrogen bond is nearly orthogonal to the adjacent carbon-carbon double bond p-orbitals, thus reducing incipient resonance stabilization in the abstraction process. Both of these factors disfavour abstraction. With regard to the failure to observe mono-allylic g-hydrogen atom abstraction, about a l l that can be said is that i t is entirely conceivable that this process has , a higher activation energy than the observed process of intramolecular 2+2 cycloaddition. Turning now to the regiospecific intramolecular 2+2 cycloaddition undergone by compound t-59 upon photolysis, two main pathway may be 22 considered. These are (a) a stepwise process involving the intermediacy of diradicals, and (b) a concerted 2+2 cycloaddition. Each of these processes may or may not be proceeded by intramolecular exciplex - 49 -(excited state charge-transfer complex) formation and/or geometric •. „. • 22 xsomerization. Scheme 24 E Pathway (b), concerted photoaddition, appears unlikely for a variety of reasons. One major argument excluding concerted inter-molecular 2+2 photoaddition of enones to alkenes is that they are nearly a l l non-stereospecific t r i p l e t state reactions. For example 55 Corey has shown that photoaddition of cyclohexenone to either cis or trans-2-butene gives essentially the same product mixture (three isomers) in each case (Scheme 25). This was explained by postulating the intermediacy of t r i p l e t diradicals 128 and/or 129 in which bond rotation can compete with spin inversion and closure. Several other similar studies have been reported. - 50 -Scheme 25 or or 128 129 (three isomers) 130 It seems l i k e l y that adduct t-59 i s also reacting from i t s 59 t r i p l e t state. For example, evidence has been presented which indicates that the unsubstituted cyclohex-2-ene-l,4-dione chromophore (as in t-59) reacts intermolecularly (2+2 cycloaddition) via a ir-ir* t r i p l e t excited state of E,j,~59 kcal/mole. Similarly, t r i p l e t excited states are involved in the 2+2 cycloaddition reactions of a wide variety of p-quinones Thus the concerted mechanism is provisionally ruled out in the photoconversion of t-59 to 127. A diradical process is proposed for the formation of 127. Actually bonding between the excited ene-dione chromophore and the side-chain vinyl group can give rise to four possible diradical intermediates 131, 132, 133 and 134 (Scheme 26). Of these four, only two 131 and 132, are capable of giving the observed photoproduct 127, while the other two lead to another isomer 126. - 51 -- 5.2 -It is d i f f i c u l t to rationalize the exclusive formation of photoproduct 127 especially since i t appears to incorporate a greater degree of ring strain than i t s counterpart 126. One possible explanation involves the diradical s t a b i l i t i e s . Of the four possible diradical intermediates 131-134, 131 and 133, the two more highly substituted species, are assumed to be formed preferentially. The exclusive formation of photo-product 127 can then be rationalized i f we can explain that diradical 131 might be favoured over 133. One such reason involves conformational analysis and non-bonded interaction arguments. Diels-Alder adduct t-59 has six more or less well-defined conformers A-F shown in Scheme 24. However, of these six conformers, only B and F bring the ene-dione double bond and the side-chain v i n y l group into the required proximity for 2+2 cycloaddition. It is found from the examination of the molecular model study that from either conformation Bor F, formation of diradical 133 involves far greater unfavourable non-bonded interactions than the formation of 131. These are the carbonyl-vinyl interaction in conformer F and Cg axial hydrogen-vinyl interaction in B* In another way of expressing the same thing, for both B and F the vinyl group prefers to rotate so as to occupy the "outer" or exo position (closer to C^) as opposed to the more congested "inside" or endo (closer to C 9 ) position. F B - 53 -Several previous studies on the regiospecificity of intermole-cular 2+2 enone-olefin photocycloaddition have implicated the existence 55 56 58 of exciplex intermediates prior to carbon-carbon bond formation. ' ' ' For example, Corey 5 5 found that photoaddition of cyclohex-2-ene-l-one to electron-rich olefins gave dimers as major products, in which the electron-donating substituents were located in a head to t a i l fashion with respect to the carbonyl group (structure 137, Scheme 27). Exactly the reverse regiospecificity was found with electron-deficient olefins. Corey explained this by suggesting that in i t s excited state, cyclohex-2-ene-l-one has the polarity shown in structure 135, the reverse of i t s ground state polarity, and that electrostatic attraction between the olefin and excited state enone leads regiospecifically to the exciplex 136 and hence to the observed photoproduct 137. Scheme 27 0 137 - 54 -There exists in the literature one example in which the regio-specif i c i t y of 2+2 photocycloaddition between unsymmetrical olefins and the 2-ene-l,4-dione chromophore (such as in adduct t-59) has 64 been studied. S t i l l , Kwan, and Palmer reported that 3-carene-2,5-dione (139) undergoes photochemical addition to a wide variety of olefins and alkynes to give the corresponding cyclobutane and cyclo-butene derivatives. However, in the case of unsymmetrical alkenes (Scheme 28), the assignment of either structure 140 or 141 to the major photoproduct i n each case was very tenuous at best and was based in large part on arguments involving formation of the expected more stable exciplex intermediate. This study should obviously not be cited as evidence for the involvement of such intermediates, but i t is interesting in that i t demonstrates that such photoadditions can be regiospecific even i f the direction of the regiospecificity i s unknown. Scheme 28 0 R 1 R 2 + 0 138 hv 139 R 1 R 2 140 141 - 55 -It seems l i k e l y that an intramolecular exciplex intermediate may also be formed in the t-59 127 reaction described in this thesis, although we have no evidence for or against such a species at this time. The n-ir* ultraviolet spectrum of Diels-Alder adduct t-59 , which i s essentially the same as the uv spectrum of the butadiene-p-benzoquinone Diels-Alder adduct, reveals no ground state interaction between the side chain vinyl group and the ene-dione chromophore,but as many studies have shown,6"* this does not necessarily mean that such an interaction w i l l not be present in the excited state. However, even i f such an exciplex were formed, i t is d i f f i c u l t to see how i t could account for the regiospecificity of our intramolecular process since the ene-dione chromophore of adduct t-59 i s , to a f i r s t approximation, symmetrical. Finally, the poss i b i l i t y that the double bond of the ene-dione chromophore of t-59 may be twisted in the excited state and hence favour the formation of photoproduct 127 over that of 126 in some obscure way should not be overlooked. The c i s , trans isomerization of cycloheptenone and higher cyclic enones>66 the observation that trans-fused cycloadducts are the major products in cc c~l CO the photoaddition of cyclohexenones to olefins, ' ' , and the theoretical calculations of McCullough 6^ and de Mayo^ a l l indicate the likelihood of such a poss i b i l i t y . Again i t is not at a l l clear how such twisting can affect the regiospecificity of the reaction under consideration, and we must be content at this time with the ground state conformation / avoidance of non-bonded interactions argument previously described. - 56 -In general, the correlation of photochemical reactivity with ground state conformation has recently been shown to be valid provided that conformational equilibrium is not rapid in relation to the excited state lifetimes involved.^ A possible example in the case of intra-molecular 2+2 cycloaddition has recently been provided by Meinwald 72 and Young. These authors showed (Scheme 29) that irradiation of the divinyl naphthalenes 142a and 142b give r i s e predominantly to the "crossed" intramolecular dimer 143 rather than to 144. This result was explained on the basis of the ground state preference for conformer 145 (less strained) which has the double bonds ideally situated for "crossed" bonding. Scheme 29 R R R R - 57 -D. Conclusions The faci l e photochemical transformation of Diels-Alder adduct t-59 into the tetracyclic diketone 127 is a novel and interesting process. The ring system of 127 is unusual and unknown and warrants further study. There is no reason to believe that such a cycloaddition w i l l not occur for other substituted hexatriene-p-quinone Diels-Alder adducts. The ready synthesis of a wide variety of substituted tetracyclo['1"''0^,^]dodecane derivatives may thus be anticipated. - 58 -EXPERIMENTAL General Infrared (ir) spectra were obtained using a Perkin-Elmer 137 spectrophotometer. Nuclear magnetic resonance (nmr) spectra were recorded by Miss P. Watson and Mr. B. Lee of this department on the following spectrometers: Varian Model A-60, T-60 and HA-100, and Jeolco Model C-60H. Tetramethylsilane (TMS) was used as an internal standard. An AEI-MS-9 spectrometer was used for mass spectra, recorded by Mr. G.D. Gunn of this department. Ultraviolet (uv) spectra were measured on a Unicam Model SP 800 B spectrophotometer. Elemental analyses were performed by Mr. P. Borda of this department. Melting points were determined on a Fisher-Johns melting point block and are uncorrected. For gas liquid partition chromatography (glpc), Varian Aerograph Model 90 P and Varian Aerograph Autoprep Model A 700 instruments were used with helium as the carrier gas. Both were connected to Honeywell Electronik 15 strip chart recorders. The following analytical columns were used: 5'x 1/4" , 20% DEGS on 60/80 Chromosorb W; 20' x 3/8", 20% Carbowax on 45/60 Chromosorb W and 5* x 1/4", 5% QF-1 on 60/80 Chromosorb W. The column temperature (°C) and the helium flow rate (ml/min) are given in parenthesis after the column specifications. A l l photolysis solvents were d i s t i l l e d - 59 -before use. A l l photolysis solutions were thoroughly deoxygenated prior to irradiation with either Canadian Liquid Air argon (<5 ppm oxygen) or with L grade high purity nitrogen. Photolyses were performed by means of a 450 watt medium pressure Hanovia lamp either placed in a water-cooled quartz immersion well or placed at a distance of 6 inches from the sample. Thin layer chromatography (tic) plates were prepared either with Aluminum Oxide G Type E of EM Reagents or S i l i c a Gel G and G F ^ for TLC acc. to Stahl (10-40 y) and developed in iodine chambers. For column chromatography, S i l i c a Gel (<0.08 mm) from E. Merck AG under 5-10 psi nitrogen pressure or Aluminum Oxide ( Woelm neutral activity grade 1 ) was used. A l l organic reagents used were reagent grade. 24 3-Dimethylaminopropiophenone Hydrochloride (67) Acetophenone ( 30 g, 0.25 mol ), paraformaldehyde ( 9.9 g, 0.11 mol ), dimethylamine hydrochloride ( 26.35 g, 0.32 mol ) and concentrated hydrochloric acid ( 1 ml ) were placed i n a 500 ml round bottomed flask equipped with a reflux condenser. Ethanol ( 100 ml ) was added to the mixture which was then refluxed over a steam bath. After 2 hrs refluxing, the solution was f i l t e r e d while hot and the f i l t r a t e diluted with 200 ml of acetone. The f i l t r a t e was cooled slowly at room temperature and then was placed i n the refrigerator overnight. The resulting crystals were separated by suction f i l t r a t i o n and washed with 15 ml of acetone. Recrystallization from ethanol furnished 17 g (88%) of colourless crystals of 67, mp 156-157 °C ( L i t . 2 4 mp 156 °C ). - 60 -Phenyl Vinyl Ketone (66) 2 5 $-Dimethylaminopropiophenone hydrochloride ( 20 g, 0.09 mol ) was dissolved in 100 ml water and the resulting solution subjected to steam d i s t i l l a t i o n for approximately 3 hrs. The milky d i s t i l l a t e was extracted with chloroform ( 3 x 50 ml ), and the chloroform layer dried over sodium sulfate. Evaporation of chloroform in vacuo l e f t a yellowish pungent smelling liquid which was d i s t i l l e d under reduced pressure ( 48°C/0.35 mm Hg ) to give 4.2 g of 66_ as a colourless liquid ( 35% ), i r (film) 6.03 (s, C=0) y; nmr (CDC13) T 2.10 (m, 2, phenyl a-protons), 2.51 (m, 3, phenyl 3- and Y ~ P r o t o n s ) > 2.85 (m, 1, vinyl a-protons), 3.68 (dd, 1, J=16 Hz, J=3 Hz, vinyl 3-trans proton) and 4.21 (dd, 1, J=ll Hz, J=3 Hz, vinyl 3~cis proton). 23 Formation of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone (47) A mixture of phenyl v i n y l ketone ( 4.2 g, 0.0033 mol ) and 2,3-dimethyl-l,3-butadiene ( 3.0 g, 0.0032 mol ) was heated at 100 °C-105 °C in a sealed tube f o r 21 hrs. The yellowish liquid was then poured into a 50 ml round bottomed flask and excess 2,3-dimethyl-l,3-butadiene removed on a rotary evaporator at 35-40 °C. The residual liquid was d i s t i l l e d under reduced pressure ( 130 °C/ 0.75 mm Hg ) to give 5.5 g ( 80% ) of colourless liquid Diels-Alder adduct 47. Uv (ethanol) max 320 nm (n-ir*, e=104); i r (film) 6.0 (s, C=0) y; nmr (CDC13) T 1.95-2.20 (m, 2, phenyl a-protons), 2.40-2.75 (m, 3, phenyl 3 and Y~P r o t o n s)> 6.30-6.80 (m, 1, methine), 7.50-8.50 (m, 6, methylene protons), 8.40 (s, 6, methyl protons); mass spectrum (70 eV) - 61 -m/e parent 214. Direct Photolysis of 3,4-Dimethyl-3-cyclohexenyl Phenyl Ketone (47) A solution of 600 mg (2.8 mmol ) of 3,4-dimethyl-3-cyclohexenyl penyl ketone 47 in 200 ml of d i s t i l l e d benzene was thoroughly degassed with argon prior to photolysis. The solution was irradiated through a Pyrex f i l t e r ( X>290 nm ) with an internal Hanovia lamp and the reaction followed by glpc ( 5'-DEGS, 20%, 160 °C, 150 ml/min ). A new peak (E) appeared in the early and middle stages of photolysis. This peak gradually diminished as the photolysis was further continued, and in the meantime another new peak (D) increased steadily. After 2 hrs of irradiation, the peak of the starting material had disappeared. Three minor photoproducts (A, B and C) and one major product (D) were found in a ratio of 1 : 1 : 1 : 10 at the end of the photolysis. Only the major product D was present in amounts sufficient to be collected as a colourless o i l on glpc using the same column as above (collected yield ~5%). The intermediate E could also be isolated by glpc during the middle stages of photolysis. As outlined i n the text these photoproducts were not completely characterized. The spectral characteristics for major product D: uv (ethanol) max 320 nm (n-ir*, e=29); i r (film) 6.0 (s, C=0) u; nmr (CDC13) x 1.95-2.18 (m, 2, phenyl a-protons), 2.35-2.70 (m, 3, phenyl 3 and y-protons), 6.60 (m, 1, methine), 5.20 (d, 2, J=6 Hz, exo-methylene), 8.85 (d, 3, J=7 Hz methyl) and 7.30-8.60 (m, 7); mass spectrum (70 eV) m/e parent 214. For intermediate E: uv (ethanol) max 320 nm (n-Tr*, e=20); ir(film) 6.0 - 62 -(s, C=0) y; nmr (CDC±3) T 1.95-2.10 (m, 2, phenyl a-protons), 2.30-2.80 (m, 3, phenyl 3 and y-protons), 4.65 (m, 1, viny l ) , 6.05 (m, 1, methine), 8.38 (t, 3, J=4 Hz methyl), 9.03 (m, 3, methyl) and 7.50-9.00 (m, 5); mass spectrum (70 eV) m/e parent 214. 30 31 3,4-Dimethyl-2, 5-dihydro-thiophene-l,l-dioxide (79) ' Sulfur dioxide was l i q u i f i e d using a Dry Ice-acetone bath (-77 °C). A mixture of 4.5 g ( 0.07 mol) of l i q u i f i e d sulfur dioxide, 4.1 g ( 0.05 mol ) of 2,3-dimethyl-l,3-butadiene and 40 mg ( 0.36 mmol ) of hydroquinone was heated in a sealed ampoule at 65-70 °C for 12 hrs. The colour of the mixture turned from yellowish to slightly brown after heating. The contents were then poured into a round bottomed flask and chloroform was used to extract the white solid in the ampoule. The chloroform solution upon evaporation in vacuo l e f t a brown solid residue which was recrystallized from water to give 6.5 g (82%) of white crystals of 29 , mp 134 °C ( L i t . 135 °C); nmr (CDC13) T 6.30 (s, 4, methylene), 8.30 (s, 6, methyl protons); mass spectrum (70 eV) m/e parent 146. 32 3,4-Dimethyl-2,5-dihydro-thiophene-l,l-dioxide-2,2,5,5-d^ (78) A mixture of sulfolene 79_ ( 5.0 g, 0.034 mol ), dioxane ( 50 ml ), deuterium oxide (12 ml) and potassium tert-butoxide ( 0.65 g, 0.006 mol ) was refluxed in a 250 ml round bottomed flask for 20 hrs. The reaction mixture was cooled to room temperature and acidified with hydrochloric acid. The solution was concentrated to about 10 ml by - 63 -evaporation in vacuo, and then the mixture was extracted with chloroform (3 x 30 ml). The chloroform extracts were dried over anhydrous sodium sulfate for two hours and the solution evaporated to dryness to give a white solid residue. The crude white solid was recrystallized from water to yield 3.0 g of pure white crystals (50%), mp 130 °C. Nmr (CDC13) T 6.30 (s, 0.5, unexchanged methylene residue) and 8.25 (s, 6, methyl groups); mass spectrum (70 eV) m/e parent 147, 148, 149 33 and 150; total deuterium content (based on mass spectrum) 88%. 2,3-Dimethyl-l,3-butadiene-l,1,4,4-d^ (77) 3 2 The tetradeuterio-sulfolene 78_ (5.0 g, 0.033 mol, deuterium content 75%) was placed in a long necked round bottom flask which was connected to a U tube immersed in an ice bath. The round bottom flask with i t s long neck was heated to 170 °C in a Kugelrohr oven. The other end of the U tube was joined to a drying tube. Decomposition occurred in the round bottom flask to give sulfur dioxide and the desired product. Sulfur dioxide escaped through the U tube while the desired product was trapped in the U tube in the form of a colourless liquid. In this way 2 g of the crude product 77 was obtained (70%). This material was directly subjected to the desired Diels-Alder reaction. 2,3-Dimethyl-3-cyclohexenyl-2,2,5,5-d4-phenyl ketone (76) Phenyl v i n y l ketone (2 g, 0.016 mol) and 2,3-dimethyl-l,3-butadierie-l,l , 4,4-d 4 (2 g, 0.023 mol, deuterium content 75%) were heated at 105 °C for 20 hours in a sealed tube. A sticky brown liquid was obtained. This material was d i s t i l l e d twice at 142 °C and 0.45 mm Hg - 64 -to give 500 mg 0-5%) of yellowish Diels-Alder adduct 76_. The total 33 deuterium content was 68.7% based on the mass spectrum. Ir (film) 6.0 (s, C=0) p; nmr (CDC13) T 1.95-2.10 (m, 2, phenyl a-protons), 2.38-2.70 (m, 3, phenyl 3,y-protons), 6.30-6.70 (m,l, methine), 8.40 (s, 6, methyls) and 7.50-8.30 (m, ~3, unsubstituted methylene). Photolysis of 2,3-Dimethyl-3-cyclohexenyl-2,2,5,5-d^-phenyl ketone (76) Tetradeuterated Diels-Alder adduct _76 (500 mg, 0.0023 mol, deuterium content 68.7%) was dissolved in 200 ml of d i s t i l l e d benzene. After the solution had been thoroughly degassed with nitrogen, the solution was irradiated for one hour using an internal 450 W Hanovia lamp and a Pyrex f i l t e r sleeve. The reaction was followed by glpc (5'-DEGS, 20%, 150 °C, 150 ml/min). As i n the case of the photolysis of the undeuterated adduct 47_, one major product D, three minor products A, B, C, and an intermediate E were detected. Since the yields were even lower than those previously obtained, only compound D could be obtained with d i f f i c u l t y . For speculations concerning i t s possible structure, see the text of the thesis. The project was eventually discontinued due to inconclusive results and lack of material. 23 Diels-Alder Adducts 2,5-Dimethyl-3-cyclohexenyl Methyl Ketone 48 and _56 The Diels-Alder reaction was conducted using d i s t i l l e d methyl vinyl ketone (79-80 °C/760 mm Hg) and trans-2-trans-4-hexadiene (Aldrich Chemical Company). Methyl vinyl ketone (1.9 g, 0.027 mol) and trans-2-trans-4-hexadiene (2.2 g, 0.027 mol) were sealed in a glass ampoule and heated at 105-110°C for 20 hrs. The mixture was cooled, hexane - 65 -added, and the resulting white precipitate (presumably a polymerization by-product) removed by f i l t r a t i o n . Evaporation on the rotovap at 35-40 °C gave 1.3 g (32%) of yellowish liquid. Two products, cis and trans-2,5-dimethyl-3-cyclohexenyl methyl ketone (48 and 56), could be separated in a ratio of cis (48) : trans (56) = 2 : 1 on glpc (20'-Carbowax, 20%, 135°C, 150 ml/min). The cis-isomer 48 was a colour-less, v o l a t i l e liquid at room temperature while the trans-isomer 56 was isolated as colourless crystals with mp of 24-25 °C. Spectral characteristics for 48: i r (film) 5.80 (s, C=0), 6.25 (w, vinyl) y; nmr (CDC13) T 9.00 (d, 3, J=7 Hz, CH 3), 9.10 (d, 3, J=6 Hz CH 3), 7.83 (s, 3, CH 3), 7.10-8.80 (m,5, methine and methylene) and 4.50 (m, 2, vin y l ) ; uv (hexane) max 285 nm (e=22, n - i r * ) ; mass spectrum (70 eV) m/e parent 152. Anal. Calcd. for C^H^O: C, 78.90; H, 10.59; Found: C, 78.79; H, 10.50. For compound 56_: i r (film) 5.80 (s, C=0), 6.25 (w, vinyl) y; nmr (CDC13) T 9.00 (d, 3, J=6 Hz, CH 3), 9.10 (d, 3, J=6 Hz, CH 3), 7.82 (s, 3, CH 3), 7.40-8.40 (m, 5, methine and methylene), 4.50 (m, 2, vi n y l ) ; mass spectrum (70 eV) m/e parent 152. Anal. Calcd. for C^H^O: C, 78.90; H, 10.59; Found: C, 78.90; H, 10.90. Photolysis of cis-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone(48) A solution of 400 mg (0.0026 mol) of cis-isomer 48 i n 200 ml of hexane (spectranalyzed) was degassed thoroughly with nitrogen prior to photolysis. The solution was irradiated through a Corex f i l t e r (X>260 nm) with an internal medium-pressure (450 W) Hanovia lamp. - 66 -The reaction was monitored by glpc ( 20'-Carbowax, 20%, 135°C, 150 ml/min ). Three photoproducts were produced. After four hours, at which time the starting material had been consumed, the reaction was terminated and the solution concentrated in vacuo. Three photoproducts, A, B, and C, were isolated in the ratio 6.7 : 1 : 3.3 on glpc. A and C were collected as colourless v o l a t i l e liquids while B could not be rigorously purified because of i t s low yield and i t s glpc overlap. The total yield was 10%. A and B were oxetanes having structure 80A or 81B. Product C was an alcohol having structure 82C. Compound A (structure 80A or 81B): i r (film) no OH or C=0 absorption; nmr (CDCl^ x 5.80 (d, 1, J=4 Hz, methine), 7.50-8.40 (m, 6, methines at C, , C«, C, and C , plus methylene at C c), 8.53 (s, 3, C_ methyl), 9.00 (d, 3, J=ll Hz, C_ or C, methyl), and 9.10 (d, 3, J=10 Hz, C ' or C c methyl); mass spectrum (70 eV) m/e parent 152. 3 o Anal. Calcd. for C 1 0 H 1 6 ° : C ' 7 8 ' 9 0 » H, 10.59; Found: C, 78.65; H, 10.66. Compound C (structure 82C): i r (film) 2.90 (broad, OH) y, no C=0 absorption; nmr (CDC13) x 4.40 (m,2, vinyls), 6.80-8.40 (m, 7), 8.80 (s, 3, C 7 methyl), and 9.00 (d, 3, J=7 Hz, C^, methyl); mass spectum (70 eV) m/e parent 152. Photolysis of trans-2,5-Dimethyl-3-cyclohexenyl Methyl Ketone (56) Irradiations were carried out in dilute hexane solutions at a concentration of 1 mg/ml (200 mg of 5j5 in 200 ml of spectranalyzed hexane). The solution was thoroughly degassed with nitrogen prior - 67 -to and during photolysis. A Corex f i l t e r (A>260 nm) was used with an internal medium-pressure (450 W) Hanovia lamp. The reaction was followed by glpc ( 20'-Carbowax, 20%, 135°C, 150 ml/min ). After four hours, three photoproducts (A, B and C) were detected, but only two products B and C could be isolated (ratio 2 : 1) as colourless v o l a t i l e liquids in a total yield of 1% using the same column as above. Com-pound B was assigned the structure 83B, while C was not completely characterized, l i k e l y having either structure 84C or 85C. Compound B (structure 83B): i r (film) no C=0 absorption, 6.08 (w, vinyl) y; nmr (CDC13) T 4.45 (m, 1, v i n y l proton), 6.05 (m, 2, C^ and C^ methines), 7.70-8.30 (m, 4, C,. methylene and C^ and Cg methines), 8.35 (s, 3, vinyl methyl), 8.80 (d, 3, J=6 Hz, C 3 or C g methyl), and 9.00 (d, 3, J=6 Hz, C_ or C„ methyl); mass spectrum (70 eV) m/e parent 152. Compound C (structure 84C or 85C): i r (film) 5.80 (s, C=0) y; nmr (CDCl^ x 4.65 (m, 1, v i n y l ) , 7.90 (s, 3, methyl), 8.40 (m, 3, methyl), 7.70-9.00 (m, 6), and 9.10 (m, 3, methyl); mass spectrum (70 eV) m/e parent 152. Thermolysis of the Diels-Alder Adducts 4_8 and 56 Small amounts of adducts 48_ and 5_6_ were sealed in separate tubes and were heated at 180 °C for 21 hours. Glpc ( 20'-Carbowax, 20%, 135°C, 150 ml/min ) showed two peaks indicating an equilibrium mixture of 48 : 56^  = 1 : 2 in both instances. - 68 -46 l,5-Hexadien-3-ol (116) Magnesium turnings (30.6 g, 1.25 g-atoms), 72 ml of anhydrous ether (reagent grade), and a few crystals of iodine were placed in a 500 ml three-necked flask equipped with a magnetic s t i r r e r , a dropping funnel and a condenser. A solution of 70.5 g (0.58 mol) of a l l y l bromide in 500 ml of ether was added at such a rate as to maintain gentle refluxing of the ether. After the addition of a l l y l bromide, the reaction mixture was refluxed on a steam bath for another hour. Acrolein (21.0 g, 0.35 mol) was added through the dropping funnel during one hour, and this caused gentle refluxing. After an additional hour at room temperature, the reaction mixture was poured slowly into 400 ml of ice water. A white precipitate was formed. A solution of 24 ml of concentrated sulfuric acid in 80 ml of water was added to dissolve the precipitate. The ether layer was separated and three 40 ml portions of ether were used to extract the water layer. The ether layers were combined and dried over 2-4 g of anhydrous magnessium sulfate. After removal of ether, the residue was subjected to d i s t i l l a t i o n on a Vigreux column to give 17 g (50%) of colourless liquid 116, bp 72-76 °C/110 mm Hg ( L i t . 4 6 62-65 °C/50 mm Hg). Nmr (CDC13) x 3.65-5.05 (m, 6, vinyl protons), 5.80 (q, 1, J=6 Hz, C 3 proton), 7.50-7.80 (t, 2, J=7 Hz, methylene) and 7.85 (s, 1, OH). 46 1,3,5-Hexatriene (115) Phosphorus tribromide 20 g (0.082 mol, practical grade) and two drops of 48% hydrobromic acid were placed in a 100 ml three-necked, round-bottomed flask equipped with a mechanical s t i r r e r , a - 69 -thermometer and a dropping funnel. l,5-Hexadien-3-ol 116_ ( 15 g, 0.15 mol ) w a s added through the dropping funnel while the contents of the flask were stirred and maintained at 10-15 °C by an ice-water bath. After the addition, the mixture was stirred at 10-15 °C for another hour, and then was set aside at room temperature overnight. The flask was cooled in an ice-salt bath for 30 minutes, and the upper organic layer separated from the residue. The organic layer was washed successively with three 40 ml portions each of ice water, 5% sodium bicarbonate and water. The organic layer (crude bromohexadiene) weighed 21 g. Dimethyl benzylamine (18 g, 0.12 mol) which was freshly d i s t i l l e d , 0.026 g of hydroquinone, and 100 ml of water were placed in a three necked, round-bottomed flask. The crude bromohexadiene was added during 40 minutes, and the reaction mixture stirred and heated at 50 °C for three hours. The water was removed from the reaction mixture by reduced pressure d i s t i l l a t i o n (40-50 °C/30 mm Hg). An aqueous sodium hydroxide solution (21 g, 0.5 moles in 110 ml of water) was placed in a 1 l i t e r flask equipped with a magnetic st i r r e r and an outlet arranged for downward d i s t i l l a t i o n into an ice cooled receiver. The aqueous solution of the quaternary bromide obtained from the last procedure was added dropwise to the boiling solution of sodium hydroxide during a period of 1 hour. The reaction mixture was stirred at 50 °C for three hours. The hexatriene and dimethylbenzylamine which formed were d i s t i l l e d with the water during the reaction. D i s t i l l a t i o n was continued for 10-15 minutes after the f i n a l addition of quaternary bromide solution. The clear upper layer - 70 -(hexatriene) was separated, cooled to 5-10 °C, washed with three 30 ml portions each of cold 2 N hydrochloric acid and water, and dried over anhydrous sodium sulfate. After f i l t r a t i o n , colourless, v o l a t i l e 1,3,5-hexatriene ( 5 g, 50% ) was obtained. Without further pur i f i c a -tion, the crude product was subjected to Diels-Alder reaction. Nmr (CDC13) T 3.00-4.00 ( m, 4, C 2,C 3,C 4 and C 5 v i n y l protons ) and 4.50-5.00 ( m, 4, and vinyl protons). p-Benzoquinone-l,3,5-Hexatriene Diels-Alder Adduct t-59 In a 50 ml round-bottom flask equipped with a reflux condenser and drying tube were placed p-benzoquinone (2 g, 0.02 moles, recrystal-lized from petroleum ether, mp 108-110 °C) and 1,3,5-hexatriene (4 g, 0.05 moles). This mixture was stirred at 60 °C for two hours. At the end of this time, the p-benzoquinone had completely dissolved. The mixture was stirred at room temperature for another three hours. After the excess 1,3,5-hexatriene was removed, crude yellow crystals of t-59 were obtained. Recrystallization from petroleum ether (bp 68 ° C ) gave 2 g (60%) of slightly yellow crystals of t-59, mp 58-60 °C. Tic ( s i l i c a gel or alumina plate, eluting solvent: ethyl acetate) indicated a single product. Ir (CHC13) 5.95 (s, C=0), 6.20 (w, vinyl) y; nmr (CDC13) T 3.30 (d,2, J=2 Hz, vi n y l protons at and C^), 4.10-4.55 (m, 3, vinyl protons at C^, and C^), 4.85-5.10 (m, 2, v i n y l protons at C ^ Q ) , 6.50-7.05 (m, 3, methine protons at C^, C g a and C^), 7.10-8.00 (m, 2, methylene protons at C Q); uv (ethanol) max 355 nm (E=75, n-ir*); o mass spectrum (70 eV) m/e parent 188. - 71 -Anal. Calcd. for C 1 2 H 1 2 ° 2 : C ' 7 6 , 5 9 ; H ' 6- 3 95 Found: C, 76.51; H, 6.30. Photolysis of the p-Benzoquinone-1,3,5-Hexatriene Diels-Alder Adduct  t-59 Adduct t-59 (200 mg, 5 mmol) was dissolved in 200 ml of d i s t i l l e d benzene.. The solution was thoroughly degassed with Argon prior to photolysis. The sample was situated 6 inches from the 450 watt medium-pressure Hanovia lamp and irradiated through a Corning 7380 plate (X> 340 nm) for 2.5 hrs. The photolysis was followed by t i c (alumina plate or s i l i c a gel, eluting solvent: ethyl acetate) which showed a single product was produced. Glpc (5'-QF-l, 5%, 150 °C, 150 ml/min, the peak due to starting material i s not eluted) also indicated a single photoproduct formed. The photoproduct was f i r s t isolated by column chromatography on alumina using ethyl acetate as eluent (200 mg of starting material/20 g of alumina). This gave 122 mg (60%) of crude solid photoproduct, mp 80-84 °C. Recrystallization (twice) from petroleum ether : ethyl ether = 9 : 1 afforded 78 mg (40%) of 2 11 A 9 analytically pure tetracyclo[ ' 0 ' ]dodec-6-ene-3,10-dione (127), mp 91.5-92.5 °C. Ir (CC14) 5.82 (s, C=0) y; nmr (CDC13) T 4.25 (m,2, vinyl) 6.40-9.00 (m, 10); uv (ethanol) max 283 nm (e=30, n - i r * ) ; mass spectrum (70 eV) m/e parent 188. Anal. Calcd. for C 1 2 H 1 2 ° 2 : C ' 7 6 ' 5 9 ; H> 6 - 3 9 5 F°und: C, 76.28; H, 6.40. -. 72 -BIBLIOGRAPHY 1. Estimated from the number of references in Journal of Chemical Society special "Photochemistry" publication. 2. W. L. D i l l i n g , Chem Rev.,66, 373 (1966). 3. (a) P. G. Bauslaugh, Synthesis, 2, 287 (1970). (b) P. G. Sammes, Quart.Rev., 24, 37 (1970). 4. R. 0. Kan, "Organic Photochemistry, P.2 and 8, McGraw-Hill series in Advanced Chemistry, New York, 1966. 5. D. C. Neckers, "Mechanistic Organic Photochemistry", Reinhold Publishing Corporation, New York, 1967, P.28. 6. H. E. Zimmerman, Angew. Chem., Int.Ed. Engl., 18, 1 (1969). 7. C. Walling and M. J. Gibian, J. Amer. Chem. Soc, 87, 3361 (1965); A. Padwa, Tetrahedron Lett., 3465 (1964); P. J. Wagner and R. W. Spoeike, J. Amer. Chem. Soc., 9^,4437 (1969). 8. R. 0. Kan "Organic Photochemistry" P.12, McGraw-Hill Series in Advanced Chemistry, New York, 1966. 9. Calvert and P i t t s , "Photochemistry",P.284 and P.175, John Wiley and Sons Inc., 1966. 10. P. J. Wagner,Accts. Chem. Res., 4_, 168 (1971) 11. J. C. Dalton, N. J. Turro, Ann. Rev. Phys. Chem., 21, 499 (1970) and references given therein. 12. P. J. Wagner and A. E. Kemppainen, J. Amer. Chem. Soc, 9i0,5898 (1968), 13. J. R. Scheffer, J. Trotter, R. A. Wostradowskd , C. S. Gibbons and K. S. Bhandari, J. Amer. Chem. Soc., 93, 3813 (1971). 14. R. C. Cookson, E. Crundwell, R. R. H i l l and J. Hudec, J. Chem. Soc., 3062 (1964 ). - 73 -15. J. R. Scheffer, K. S. Bhandari, R. E. Gayler and R. H. Wiekenkamp, J. Amer. Chem. Soc, 94,285 (1972). 16. (a) A. Padwa and W. Eisenhardt, J. Amer. Chem. Soc., 90, 2442 (1968). (b) A. Padwa and W. Eisenhardt, i b i d , 93, 1400. (1971). 17. Rudolf Erich Gayler, Ph. D. Thesis, University of British Columbia, 1973. 18. K. S. Bhandari, unpublished results. 19. P. Gull, H. Wehrle and 0. Jeger, Helv. Chim. Acta., 54, 2158 (1971). 20. J. R. Scheffer, et.al., Tetrahedron Lett., 1413 (1973) 21. The nmr spectrum of compound 4_6 showed no vinyl hydrogen signal, no quartet due to the C^ methine, and a t r i p l e t (J=l ti.%) due to the C.j methyl. 22. P. de Mayo, Accts. Chem. Res. , 4_, 41 (1971).. 23. The procedure followed was based on a synthesis reported by Pottov and Sopov., A. A. Pottov, N. P. Sopov, J. Gen. Chem. (Russia), 22, 653 (1952). 24. C. E. Maxwell,' "Org. Syn." C o l l . , Vol. 2> 305, John Wiley and Sons Inc., New york, 1955. 25. G. C. Mannich and G. Heilner, Chem. Ber., 356 (1922). 26. C. F. H. Allen, A. C. B e l l , A. Bell and J. Van. Allan, J. Amer. Chem. Soc., 62, 656 (1940). 27. Based on the ratio of the peak areas which were measured by triangulation. 28. R. M. Silverstein and G. C. Bassler, "Spectrometric Identification  of Organic Compounds", John Wiley and Sons, Inc., New york, London, Sydney, 1967, P. 138. - 74 -29. N. P. Peet, R. L. Ca r g i l l and J. W. Crawford, J. Org. Chem., 38, 1222 (1973). 30. 0. Grummit, A. E. Ardis, J. Amer. Chem. Soc. , 72, 5167 (1956). 31. H. J. Backer and J. A. Bottema, Rec. Trav. Chem., 294 (1932). 32. A. C. Cope, G. A. Berchtold and D. L. Ross, J. Amer. Chem. Soc, 83, 3859 (1961). 33. The extent of deuterium incorporation was calculated as described by K. Biemann, "Mass Spectrometry", McGraw-Hill Book Co., Inc., New York, N. Y. 1962, Chapter 5. 34. J. Sauer, Angew. Chem., Internat. Edn., \6, 16 (1967). 35. The nmr spectrum of oxetane i showed a multiplet of two protons adjacent to the oxygen at x 5.58. R. R. Saners and J. A. whittle, J. Org. Chem., 34, 3579 (1969). 36. D. H. Williams and I. Fleming, "Spectroscopic Methods in Organic Chemistry", McGraw-Hill Publishing Co. Ltd., London, 1966, P. 126. 37. The nmr (CDCl^) spectrum for compound 43 is T 4.7 (m, 1, vinyl proton 8 i at C f i), 6.1 (q, 1, J=6 Hz, C 3 methine), 7.9 ^/ I i 0 2 (m> 3, C,. methylene and C^ methine), 8.3 (m, J 4 ^ 5, vi n y l methyl and Cg methylene), 8.6 (s, 3, 43 C 1 methyl) and 8.9 (d, 3, J=6 Hz, C 3 methyl). 38. The stereochemistry follows from the analogous compounds 4_3 and 46_ which were assigned on the basis of the lack of coupling between the C_ and C. methines. See reference 21. For a discussion of the nmr 3 4 of a similar system, see W. C. Agosta and A. B. Smith, J. Amer. Chem. Soc. , 93, 5513 (1971). - 75 -39. For reviews see (a) L. L. Muller and J. Hamer, "1, 2-Cycloaddition  Reaction", Interscience Publishers, New York, N. Y., 1967, p. I l l ; (b) D. R. Arnold, "Advan. Photochem.", 6, 301 (1968). 40. N. J. Turro and P. A. Wriede, J. Amer. Chem. Soc, 92, 320 (1970). 41. S. H. Schroeter and C. M. Orlando, J . Org. Chem., 34, 1181 (1969). 42. H. S. Ryang, K. Shima and H. Sakurai, Tetrahedron Lett., 1091 (1970). 43. C. C. Liao and P. De Mayo, Chem. Commun., 1525 (1971). 44. T. S. Cantrell, J. Amer. Chem. Soc, 95, 2714 (1973). 45. J. M. Schepherd, D. Singh and P. Wilder, J r . , Tetrahedron Lett., 2743 (1974). 46. J. C. H. Hwa and H. Sims, Org. Syn., 41, 49 (1961). 47. C. A. Stewart, J. Org. Chem. , 28, 3320 (1963). 48. R. B. Woodward and R. Hoffman, "The Conservation of Orbital Symmetry", Verlag Chemie GmbH, Academic Press Inc., 1971, P. 145. 49. K. Alder, H. V. Brachel and K. Kaiser, Arm., 608, 195-215 (1957). 50. F. Kasler, "Quantitative Analysis by NMR Spectroscopy", Academic Press Inc. (London and New York) LTD., 1973, P. 30. 51. For the shift reagent Eu(fod) 3, see R. E. Rondeau and R. E. Sievers, J. Amer. Chem. Soc, 93, 1522 (1971). 52. The author would li k e to thank Mr. B i l l Lee of the department for recording the nmr spectrum. 53. K. Nakanishi,"Infrared Absorption Spectroscopy", Holden-Day, Inc., San Fracisco and Nankodo Company Limited, Tokyo, 1962, P. 42. o 54. Measured on a Dreiding Model, 1 cm = 0.4 A. 55. E. J. Corey, J. D. Bass, R. LeMahieu and R. B. Mitra, J. Amer.  Chem. Soc., 86, 5570 (1964). - 76 -56. P. de Mayo, J. P. Pete and M. F. Tchir, Can. J. Chem., 46, 2536 (1968). 57. W. L. D i l l i n g , T. E. Tabor, F. P. Boer, and P. P. North, J. Amer.  Chem. Soc, 92, 1399 (1970). 58. P. E. Eaton, Accounts Chem. Res., 1, 50 (1968). 59. J. A. Barltrop and D. Giles, J. Chem. Soc, C, 105 (1969). 60. J. M. Bruce, Chem. Soc Quart. Rev.(London), 21, 405 (1967). 61. (a) P. E. Eaton, and W. S. Hurt, J. Amer. Chem. Soc, 88, 5038 (1966). (b) J. L. Ruhlen and P. A. Leermakers, ibid, 89, 4944 (1967). 62. P. J. Wagner and D. J. Bucheck, Can. J. Chem., 47, 713 (1969). 63. B. D. Challand, and P. de Mayo, Chem. Commun., 982 (1968). 64. I. W. J. S t i l l , M-H Kwan and G. E. Palmer, Can. J. Chem., 46, 3731 (1968) . 65. Th. Forster, Angew. Chem., internat. edit., 8_, 333 (1969). 66. (a) E. J. Corey, M. Tada, R. LeMahieu and L. L i b i t , J. Amer. Chem.  Soc., 87, 2051 (1965). (b) P. E. Eaton and K. Lin, i b i d , 86, 2087 (1964). 67. P. J. Nelson, D. Ostrem, J. D. Lassila and 0. L. Chapman, J. Org. Chem., 34, 811 (1969). 68. R. M. Bowman, C. Calvo, J. J. McCullough, P. W. Rasmussen, and F. F. Snyder, J. Org. Chem., 37, 2084 (1972). 69. J. J. McLullough, H, Ohorodnyk and D. P. Sandy, Chem. Commun., 570 (1969) . 70. P. de Mayo, A. A. Nicholson and M. F. Tchir, Can• J. Chem., 48, 225 (1970) . - 77 -71. F. D. Lewis, R. W. Johnson and D. E. Johnson, J. Amer. Chem. Soc, 96, 6090 (1974). 72. J. Meinwald and J. W. Young, ib i d , 93, 725 (1971). 


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