Photochemische Reaktionen. 99. Mitteilung. Photochemistry of N-Acylimidazoles. IV. Structural Factors Leading to Norrish Type II Elimination and to Cyclobutanol Formation in the Photolysis of Acylimidazoles

Conformational factors leading to Type II elimination and to cyclobutanol formation were studied in the irradiation of N-acylimidazoles with simple acyl groups as well as of their photochemical acyl migration products.     

Photochemische Reaktionen. 53. Mitteilung [1]. Photochemische Addition von Alkylaromaten an Aryl-Carbonylverbindungen

In the preceding paper [1] a novel primary photochemical process of triplet excited α,β-conjugated cycloalkenones in toluene solution has been reported: the abstraction of a benzylic hydrogen from the solvent by the β-carbon (cf. 1 → 2 + 3 + 4 ). The reaction has been attributed to the π,π* triplet. Aromatic aldehydes and ketones ( 5–11a ), the triplet state reactivity of which is known to be mostly π,π* in nature, have now been examined under the same irradiation conditions. However, a reaction similar to that of cycloalkenones — expected to result in the addition of hydrogen to the ortho and para positions of the aryl moiety and the formation of benzylcyclohexa-1,3-and 1,4-diene derivatives — could not been found. Compounds 5 – 10 remained essentially unchanged. 4-Methoxyacetophenone ( 11a ) reacted slowly to form the same type of products [tert-carbinol 12a , pinacol 13a and dibenzyl ( 4 )] as the aromatic carbonyl compounds 11b-d , benzophenone and cyclopropylphenylketone, which exhibit typical n,π* triplet reactivity (hydrogen abstraction by the carbonyl oxygen).     

Photochemische Reaktionen. 62. Mitteilung [1]. Die Photofragmentierung von O-acetyljervin

On ultraviolet irradiation O-acetyljervine ( 1 ) is subjected to several parallel fragmentations. From the complex reaction mixtures obtained in a variety of solvents (dioxan, tetrahydrofuran, acetonitrile, iso-octane, benzene) the major alicyclic products 6 – 8 and the heterocyclic compounds 12 – 16 have been isolated. Products 6 – 8 undergo further photochemical changes, e.g., decarbonylation of 7 to 9 and hydrolytic cleavage of 8 to 10 . These photofragmentations are initiated almost specifically upon selective π → π* excitation at 2537 Å with a quantum yield of Φ²⁵³⁷ = 0.145 for conversion of starting material. Reaction upon irradiation in the long-wavelength n → π* absorption band is very much less efficient (Φ³⁶⁶⁰ = 0.611 · 10^?3, both determinations for O-trimethylsilyl-jervine ( 2 ) in tetrahydrofuran). A high degree of photostability is observed also at 2537 Å on N-protonation of O-acetyljervine ( 1 ) in acetic acid. Furthermore, reactivity is greatly reduced for the N-methyl ( 3 ) and N-acetyl ( 4 ) derivatives in neutral solvents at 2537 Å. N-Chloro-O-acetyljervine ( 5 ) in dioxan at 2537 Å gave preferentially O-acetyljervine hydrochloride.     

Photochemische Reaktionen. 107. Mitteilung. Zur Photochemie offenkettiger 2,6-bzw. 2,7-Dien-Carbonylverbindungen

The Photochemistry of Open-Chained 2,6- or 2,7-Dien-Carbonyl Compounds On ¹n, π*-excitation (λ > 347 nm) citral (5) and the methyl ketone 10 isomerize to compounds A (7, 19) and B (6, 20) , whereas the phenyl ketone 11 changes into the isomer 24 of type E. Evidence is given that the conversions to A and B may arise from the ³n, π*-state of the 2,6-diene-carbonyl compounds. On ¹n, π*-excitation (λ = 254 nm) 5 and 10 yield the isomers A (7, 19) and D (18, 22) , but no products of type B. Furthermore, conversion of 10 to the isomer 21 of type C is observed. Selective ¹n, π*-excitation (λ = 254 nm) as well as selective ¹n, π*-excitation (λ > 347 nm) of the 2,7-diene-carbonyl compounds 12 and 13 give rise to isomerization to the compounds F (25, 28) , exclusively. The intramolecular [2 + 2]-photocycloadditions are shown to be triplet processes. UV.-irradiation (λ > 280 nm) of compounds F (25, 28) furnishes the isomeric products G (26, 29) which photoisomerize to oxetanes of type H (27, 30).     

Photochemische Reaktionen. 98. Mitteilung. Photochemistry of N-Acylimidazoles. III. Rates and Yields of N→C Acyl Migration and of C(α)-C(β) Cleavage of Acyl Side Chains

Relative rates of the photoreactions of 1-( 1 ), 2-( 2 ) and 4(5)-valerylimidazole ( 3 ) as well as the yields of photo-fragmentation product 10 from 1-( 7 ), 2-( 8 ) and 4(5)-stearoylimidazole ( 9 ), respectively, were determined. A reaction path via Norrish Type II fragmentation involving the carbonyl group of 1-acylimidazoles was ruled out.     

Photochemische Reaktionen. 48. Mitteilung [1]. Photolytische Substitutionen von Aromaten mit α-Sulfonyloxyketonen. Vorläufige Mitteilung

Photo-excited arenes have been found to initiate a substitution reaction in which the sulfonyloxy group of α-sulfonyloxyketones is displaced, and the corresponding β-arylketones are formed. Evidence is presented that the electronically excited arene need not be involved directly in the substitution step. It would seem to be more likely that upon interaction with the sulfonyloxyketone a reactive species derived from the latter is produced which is capable of aromatic substitution.     

Photochemische Reaktionen 94. Mitteilung. Vinyloge β-Spaltung bei Epoxy-enonen der Jonon-Reihe

Photoinduced Vinylogous β-Cleavage of Epoxy-enones of the Ionone Series The photochemistry of the α,β-unsaturated γ,δ-epoxy-enones 1–3 is determined by: (i) C(γ)-O-scission of the epoxide (vinylogous β-cleavage of Type A); (ii) C(γ)-C(δ)-cleavage of the oxirane (vinylogous β-cleavage of Type B); (iii) (E/Z)-isomerization of the enone chromophore. In contrast, 4 with tertiary C(β) shows no Type B cleavage. Type A cleavage is induced both by n,π*- and π,π*-excitation and arises probably from the T₁-state, but Type B cleavage is observed only on π,π*-excitation and represents presumably a S₂-reaction. On Type A cleavage 1–4 undergo 1,2-alkyl-shifts to 1,5-dicarbonyl compounds ( 15–18, 25–28, 34 and 35 ) or rearrange to dihydrofuranes ( 7 and 30 ). The isomerization 1→7 proceeds by a stereoselective [1,3]-sigmatropic shift. On Type B cleavage 1–3 isomerize to a bicyclic enol-ether ( 8, 29 ) or to a monocyclic enol-ether ( 9 ; product of a homosigmatropic [1,5]-shift) or undergo fragmentation to isomers such as allenes 10, 22 and 31 or cyclopropenes 11 and 21 . The non-isolated, unstable (Z)-epoxy-enones 14, 19, 24 and 38 isomerize by fragmentation to the furanes 12, 23, 33 and 39 respectively, on contact with traces of acid or by heating. However, for 19 and 4 , Type B cleavage may lead to the furanes 23 and 39 . On UV. irradiation of the epoxy-enone 4 the initially formed (E/Z)-isomers 34 and 35 yield on π,π*-excitation the enones 37 and 40 by a vinylogous β-fragmentation. In addition, on n,π*-excitation 34 isomerizes to 35 , which decarbonylates exclusively to the enone 37 . The reactions of 1–4 with BF₃ · O(C₂H₅)₂ were also studied (see appendix). The epoxy-enones 1 and 2 isomerize by an 1,2-alkyl shift in good yield to the 1,4-dicarbonyl compounds 79 and 81 , whereas 3 gives the 1,4-diketone 83 , and in small amounts the 1,5-diketone 84 . On the other hand, 4 is converted to the fluorohydroxy-enone 85 and to the 1,5-dicarbonyl product 34 , the only isomer in this series which is identical with one of the photoproducts.     

Photochemische Reaktionen. 87. Mitteilung. Zur Photolyse von 9-Oxabicyclo[3.3.1]nonan-2-on

Photolysis of Bicyclo[3.3.1]nonan-2-one. Disproportionations, the secondary processes available to the acyl-alkyl biradical b (X(9) = 0) formed from 9-oxabicyclo[3.3.1]-nonan-2-ones a (X(9) = 0) in a primary photochemical process by α-cleavage (Norrish type I cleavage) were studied. Special attention was paid to the selectivity between the two possible H-abstractions: the one at C(3) (→ ketene c , X(9)= 0) and the other one at C(8) (→ alkenal d , X(9) = 0) and to the selectivity of the H-abstraction at a definite methylene group (C(3) or C(8)). In the case of ketene formation (→ c , X(9) = 0) the specificity of the insertion of the migrating H-atom at C(1) was studied. endo-6-Hydroxy-9-oxabicyclo[3.3.1]nonan-2-one ( 6 ) and derivatives of it ( 7, 8, 16, 17, 19, 21, 30 and 38 ) as well as exo-6-hydroxy-9-oxabicyclo[3.3.1]-nonan-2-one ( 41 ) and its derivative 42 were used as substrates. UV.-irradiation of 6 in benzene yielded 1,5-dioxa-2-cis-decalone ( 44 ) by way of a ketene g (R = H) as demonstrated by the photolysis of 7 (→ 45 ), 8 (→ 43 ), and 17 (→ 47 ). Specific labellings with deuterium proved that H-abstraction occurs intramolecularly at C(3) (e.g. 16 → 54 ; 6 + 16 → 44 + 54 ), that one of the H-atoms at C(3) migrates specifically to C(1) ( 21 → 55 ; 19 → 56 ), endo-H–C(3) being favored by a factor of 6. The abstraction showed an unexpected primary isotope effect of about 2. UV-irradiation of 41 in benzene yielded in addition to the expected 1,5-dioxa-2-trans-clecalone ( 63 ) about 3% of an isomeric compound 67 which probably results from H-abstraction at C(8) (→ alkenal 65) followed by cyclisation.     

Photochemische Reaktionen 114. Mitteilung [1]. Zur Photochemie cyclischer Acetale vom 1,3-Dioxa-4,6-cycloheptadien-Typus

Photochemistry of Cyclic Acetals of the 1,3-Dioxa-4,6-cycloheptadiene Type UV.-irradiation (λ=254 nm) of 3 gives the isomers (E)- 5 (4%), (Z)- 5 (60%) and 6 (3%). On triplet sensitization (acetone; λ ≥ 280 nm) 3 is converted to (E)- 5 (3%), (Z)- 5 (7%) and 7 (9%). ? The ¹π,π*-excitation (λ=254 nm) of 4 yields the isomers 2 (9%), 8 (10%), 9 (34%), 10 (20%) and 11 (3%). On thermolysis (200°) 4 gives 10 (87%) by a Claisen-rearrangement.     

Photochemische Reaktionen. 117. Mitteilung [1] Zur Photochemie von 5,6-Epoxydienen und konjugierten 5,6-Epoxytrienen

Photochemistry of 5,6-Epoxydienes and of Conjugated 5,6-Epoxytrienes On singulet excitation (δ = 254 nm) the 5,6-epoxydiene 6 and the conjugated 5,6-epoxytrienes 7 and 8 exclusively give products arising from cleavage of the C, C-bond of the oxirane (cf. 6 → 9 , 10 , 11 ; 7 → (E)- 15 , 16 , 17 ; 8 → 18 (A+B) , 19 (A+B) , 20 , 21 ). The dihydrofuran compounds 11 and (E/Z)- 15 are formed by cyclization of a ketonium-ylide a and d , respectively. Photolysis of a gives the carbene b which yields the cyclopropene 9 , whereas d forms photochemically the carbenes f and g which yield the methano compounds 16 and 17 . The isomeric cyclopropene derivatives 20 and 21 are products of the intermediates h and i , respectively, which are formed by photolysis of the ylide e . The cyclopropene 21 isomerizes by intramolecular cycloadditions to 18 (A+B) and 19 (A+B) . - On triplet excitation (λ?LD nm; 280 nm; acetone) 6 undergoes cleavage of the C(5), O-bond and isomerizes to 12 and 14 . However, 7 is converted by cleavage of the C, C-bond of the oxirane to yield 15 . On treatment with BF₃O(C₂H₅)₂ 6 gives 14 , whereas 7 yields 22 , and 8 forms 23 and 24 .     

Photochemische reaktionen. 108. Mitteilung. Photochemistry of N-acylazoles. VI. Photoreactivities of 1-acyl-1,2,4-triazoles and of 2-acyltetrazoles

Contrary to the findings in the photolysis of N-acylimidazoles [2] irradiation of 1-acyl-1,2,4-triazoles afforded no photo-Fries product, but instead products formed via the corresponding acyl radicals and aldehydes. Photolysis of 2-acyltetrazoles gave in part the same products as those obtained from the irradiation of the corresponding acyl-triazoles as well as 2-alkyl-1,3,4-oxadiazoles. N-Acyltetrazoles didn't give any photo-Fries product neither.     

Photochemische Reaktionen. 73. Mitteilung. Neuartige Photoreaktionen des (±)-trans-β-Jonon-epoxids

The racemic trans-β-ionone-epoxide ( 7 ) upon irradiation with light of wavelength 254 nm (in pentane solution) gives rise to novel photochemical processes and formation of compounds 12 , 13 , 14 , and 15 .     

Photochemische Reaktionen. 88. Mitteilung [1]. Zur Photochemie des Iso-methyl-β,(E)-jonon-epoxids

Photolysis of iso-methyl-β,(E)-ionone-epoxide On n, π*-excitation (λ ≥ 347 nm) the title compound 7 isomerizes to the (Z)-enone-epoxide 8 , which yields the bicyclic alkohol 9 in a second photochemical step. The photoisomerization 8 → 9 is a further example for the influence of a methyl substituent at C(α) of an enone-chromophore on the nature of the photochemical processes. On UV. irradiation in the presence of traces of hydrochloric acid 7 gives quantitatively the furane 10 .     

Photochemische Reaktionen. 40. Mitteilung. O-Acetyl-6,7-methylen- und O-Acetyl-1-dehydro-6,7-methylentestosterone: Stereochemie und Resultate der UV.-Bestrahlung

The stereochemistry of the isomeric 6, 7-methylene steroids 2 and 3 was established by reduction to the known 7-methyl compounds 6 and 8 , respectively. These results revise previous steric assignments based on n.m.r. data [4].     

Photochemische Reaktionen 130. Mitteilung. Zur Photochemie von 5,6-Epoxy-1,3-dienen: Der Einfluss eines 7-Hydroxysubstituenten auf die Carbenbildung

Photochemistry of 5,6-Epoxy-1,3-dienes: Influence of a 7-Hydroxy Substituent on the Carbene Formation On singlet excitation (λ=254 nm) in MeCN the hydroxy-epoxydiene (E)- 4 undergoes photocleavage to the carbene intermediates d and e as main processes. The carbene d , showing behaviour typical of vinyl carbenes, undergoes addition to the adjacent double bond furnishing the cyclopropenes 5A + B. The carbene e , however, undergoes an insertion reaction into the neighbouring carbinol C,H-bond leading to the enol intermediate 21 , which gives rise to the compounds 6A + B and 7A + B. To a lesser extent the products 8A + B are formed via another enol intermediate (32). On photolysis of (E)- 4 in MeOH instead of MeCN the enol intermediates 21 and 32 undergo rapid tautomerisation to the ketones 9A + B (main products) and 11.     

Photochemische Reaktionen 45. Mitteilung. α-Sulfonyloxyketone: Sterische Voraussetzungen für die photolytische RSO3-Elimination – Nachweis von kationischen Produkten Vorläufige Mitteilung

Upon irradiation in the n→* transition, the 2, 5-cis tosyloxyketone 9 reacted readily to give the cyclohexenone 11 in benzene solution, and a 4:1 mixture of products 11 and 12 in dioxane solution, while the 2, 5- trans isomer 10 remained almost photostable under comparative conditions. Prolonged irradiation of 10 afforded only small amount of products 11 and 12 in an approximate 1:1 ratio. It is concluded that for the photolytic elimination of the tosyloxy group a non-eclipsed conformation of the CO and the C_α O bonds, as already present in the ground state of compound 9 , is required. When 9 was irradiated in aqueous dioxane solution, the β-hydroxy-ketone 13 was formed in addition to 11 . Similarly, the photochemical conversion of the aliphatic tosyloxyketone 5 to the unsaturated ketone 6 was largely suppressed in aqueous dioxane, and the hydroxyketone 15 was formed instead. These results are evidence for the intervention of cationic intermediates in the pathways leading to the rearranged products 11 and 15 , and hence point to a heterolytic mode of photocleavage.     

Photochemische Reaktionen. 85. Mitteilung [1]. Zur Photochemie von α, β-Epoxy-eucarvon

Photochemistry of α,β-epoxy-eucarvone . On π,π^*-excitation (λ = 254 nm) 4 isomerizes to the bicyclic ketoaldehyde 5 ; on n,π^*-excitation (λ ? 280 nm) 4 gives 5 , the β,γ-unsaturated ketone 6 , the enone 7 and the cyclobutanone 8 . Scission of the (C—C)-bond of the oxirane 4 would give the dihydrofurane e , which could isomerize to the ketoaldehyde 5 . On the other hand, 4 is assumed to isomerize to the β,γ-unsaturated aldehyde c , which could yield 6 and 7 by photodecarbonylation. The cyclo-butanone 8 is shown to be a photoisomer of the ketone 6 . Furthermore, eucarvol ( 18 ) rearranges by a thermal [1,5]-H-shift to dihydro-eucarvone ( 20 ); on UV.-irradiation 18 gives the bicyclic isomers 27 and 28 .     

Photochemische Reaktionen. 74. Mitteilung. Notiz zur Photoreaktivität ausgewählter ungesättigter Ketoalkohole der Jononreihe

UV.-irradiation (λ = 254 nm) of the hydroxy-enones 3 and 6 in acetonitrile results in the cyclization to the unstable isomeric allylic alcohols 4 and 7 , which dehydrate easily to the isolated unsaturated ether 5 and 8 . The racemic γ,δ-dihydroxy-enone 9 upon irradiation under the same conditions gives rise to the exclusive formation of the furane-compound 11 .     

Photochemische Reaktionen. 95. Mitteilung. Zur Photochemie konjugierter Epoxy-enone: Die UV.-Bestrahlung von 9-Isopropyl-6-methyl-9, 10-epoxybicyclo [4.4.0] dec-1-en-3-on

The Photochemistry of Conjugated Epoxy-enones: Photolysis of 9-Isopropyl-6-methyl-9,10-epoxybicyclo [4.4.0]dec-l-en-3-on The title compound 1 undergoes on π, π*-excitation vinylogous β-cleavage of the C(γ), O-bond and isomerizes to the unsaturated 1, 5-diketone 8 (58%) and the tricyclic diketone 9 (14%). On n, π*-excitation, however, 1 undergoes only desoxygenation to the dienone 7 (ca. 15%) besides polymerization. UV.-irradiation of 1 (λ ≥ 347 nm) in the presence of tributyl-stannane gives 7 in 85% yield; in the presence of tributylstannane-d₁ 1 yields 13 , the deuterated analogon of 7 (85% yield; d₁ ca. 50%); 7 has been proven to be stable on π, π* - as well as on n, π*-excitation.