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. 44. Mitteilung [1]. Zur Photochemie von gesättigten β-Ketosulfiden

Ultraviolet irradiation of the saturated β-ketosulfides 6 and 7 (direct n → π* excitation) in benzene and methanol solutions resulted mainly in the selective α-cleavage of the bond between C?O and C_α-S, producing ketenes by the well known hydrogen transfer as secondary reactions. Depending upon the availability of internal and/or external nucleophiles, the compounds 8 , 9 , 10 and 11 were formed as major products. The high yields in some of these light-induced transformations open an attractive synthetic approach to new heterocompounds.     

Photochemische Reaktionen. 97 Mitteilung [1]. Zur Photochemie konjugierter Epoxy-diene II. Versuche mit (E)-β-Jonyliden-epoxiden

Photolysis of Conjugated Epoxy-dienes Direct and sensitized excitation of the (E)-β-ionylidene-epoxides 1 and 4 leads to different types of isomerizations. Thus photocycloelimination to the cyclopropene-ketones 2 and 6 is only achieved by ¹(π, π*)-excitation (λ=254 nm), whereas ³(π, π*)-excitation (λ > 280 nm, acetone) gives selective C(1′), O-cleavage of the oxirane ( 1 → 7 – 10 and 4 → 11 – 13 ). In contrast to 1 the twofold methylsubstituted epoxy-diene 4 shows mainly (E/Z)-isomerization ( 4 → 5 ) on both ¹(π, π*)- and ³(π, π*)-excitation while the isomerizations 4 → 6 and 4 → 11 – 13 are minor processes, only.     

Photochemische Reaktionen. 110. Mitteilung [1]. Zur Photochemie γ,δ-Methano-α-enone

Photochemistry of γ,δ-Methano-α-enones Direct excitation (λ = 254 or ≥ 347 nm) converts the γ,δ-methano-α-enone (E)- 10 into the isomeric ether 23 and the isomeric diene-ketone 24 . Furthermore, on ¹π,π*-excitation (λ = 254 nm) (E)- 10 undergoes an 1,3-homosigmatropic rearrangement yielding the enone (E)- 25 . In addition (E → Z)-isomerization of (E)- 10 and conversion of 10 to the isomeric furan 28 is observed. The isomerization (E)- 10 → 23 , 24 and (E)- 25 proceeds by photocleavage of the C(γ), C(δ)-bond, whereas the formation of 28 occurs by photocleavage the C(γ), C(δ)-bond together with that of the C(γ), C(δ′)-bond of 10 . On direct excitation the bicyclic diene-ether 23 yields the methano-enone 10 , the dieneketone 24 and the tricyclic ether 29 . Evidence is given, that the conversion 23 → 10 is a singulet process. On the other hand, the isomerization 23 → 24 and the intramolecular [2 + 2]-photocycloaddition 23 → 29 are shown to be triplet reactions. Irradiation (λ = 254 nm) of the homoconjugated ketone 24 yields the isomeric ketone 27 by an 1,3-acyl shift. The excitation of the (E)-enone 25 induces (E → Z)-isomerization and photoenolization to give the homoconjugated ketone 26 .     

Photochemische Reaktionen. 89. Mitteilung [1]. Zur Photochemie konjugierter Epoxy-diene I. Versuche mit (E),β-Jonyliden-epoxiden

Photolysis of conjugated epoxy-dienes UV.-irradiation (λ = 254 nm) of (E),β-ionylidene-epoxide ( 3 ) in n-pentane gives the isomeric cyclopropene-ketone 7 (90%) in a hitherto unreported type of photoreaction. The methylsubstituted (E),β-ionylidene-epoxide 6 , however, undergoes (E/Z)-photoisomerization to the (Z),β-ionylidene-epoxide 8 (91%).     

Zur Photochemie des Iso-methyl-α, (E)-jonons

UV.-irradiation of iso-methyl-α, (E)-ionone ( 4 ) in neutral solvents yields iso-methyl-α, (Z)-ionone ( 5 ), the bicyclic ether 6 and the epoxide 7 by a sequence of successive photoisomerizations. The steps leading to des-methyl homologues of 6 and 7 do not occur on irradiation of α, (E)-ionone ( 1 ) [10]. The reversible isomerization 4 ? 5 is followed by the irreversible photoprocess 5 → 6 and the final transformation 6 → 7 . Irradiation of iso-methyl-α, (E)-ionone ( 4 ) in acidic or basic solvents leads to a deep change in the type of products and gives the isomeric ketones 9 and 10 in high yields. A tentative mechanism for the photoisomerization steps 5 → 6, 6 → 7 and 5 → 9 + 10 is proposed.     

Photochemische Reaktionen. 106. Mitteilung. Zur Photochemie tetraalkylsubstituierter γ-Keto-olefine

The Photochemistry of Tetraalkyl Substituted γ-Keto-olefines The photochemistry of 7,8-dihydro-β-ionone ( 1 ) in solution is shown to depend on temperature, polarity and viscosity of the solvent. UV. irradiation (λ ≥ 245 nm) in pentane at +25° converts 1 to the isomeric ethers 3 (16%), 5A (48%) and 5B (22%), whereas at ?65° 7,8-dihydro-γ-ionone ( 26 ) is obtained in 12% yield together with 13% of 3 , 12% of 5A and 9% of 5B . The ¹n,π*-excitation of 1 in acetonitrile gives similar results. In the more viscous 1,2,3-triacetoxypropane the photoisomerization 1 → 26 takes place even at + 60° (10% yield, cf. 40% at ?15°). In alcoholic solvents, however, no formation of 26 is detected, but the hitherto unknown [2+2]-photocycloaddition 1 → 11 can be observed (4% at ?7°, 15% at ?65S° in 2-propanol). An intermediate e may be involved (Scheme 14). In addition to the photoreactions 1 → 3, 5A, 5B and 11 the isomerization of 1 to the novel spirocyclic ketone 28 takes place in alcoholic solvents only. Photoisomerization 1 → 3 is presumably a photo-ene process involving a stereoselective intramolecular H-transfer. This type of photoisomerization is restricted to cyclic γ-keto-olefines. The tetraalkylated acyclic γ-keto-olefines 14 and 15 photoisomerize exclusively by [2+2]-cycloaddition, independent of the solvent. On ¹n,π*-excitation the δ,?-unsaturated bicyclic ketone 44 undergoes Norrish-Type-II photofragmentation to the diene 45 or isomerizes to the γ, ?-unsaturated ketone 17 . Competition between these two reactions is strongly temperature dependent: photolysis in pentane at ?72° yields quantitatively 45 , whereas at + 35° only 30% of 45 and 68% of 17 are obtained. UV. irradiation of the novel spirocyclic ketone 28 gives as primary photoproduct the isomeric aldehyde 29 , and in a secondary photoreaction the isomeric oxetanes 30A and 30B . Experiments with deuteriated substrates show that the isomerization of type 28 → 29 is stereocontrolled.     

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. 50. Mitteilung [1]. Zur Photochemie von gesättigten β-Ketosulfiden II. Synthese von 6-Hydroxy-2-thia-7-oxa-isotwistan

The results of the ultraviolet irradiation of the saturated β-ketosulfide 2 are discussed. The photochemistry of 2 is characterized by the occurrence of two primary photoprocesses. Their efficiences depend markedly on the excited transition, (charge-transfer)- or (n → π*)-excitation, respectively. In methanol solution (charge-transfer)-excitation leads almost exclusively to product 7 , due to (C_α – S)-fission, and (n → π*)-excitation to nearly equal amounts of 7 and of products 4 and 6 which result from α-cleavage. On solvent sensibilization in benzene products 4, 6, 7 and the still unidentified product 5 ² are formed. Compounds 4 and 6 have been described earlier [2]. The structure elucidation of 7 is reported in this paper. Acid-induced transformation of 7 yields the dihetero-isotwistane 15 .     

Photochemische Reaktionen. 51. Mitteilung [1]. Zur Photochemie von gesättigten β-Ketosulfiden III. Strukturaufklärung eines weiteren Photoproduktes Zusammenfassende Beurteilung des photochemischen Reaktionsbildes

Ultraviolet irradiation of the saturated β-ketosulfide 2 [(charge-transfer)- or (n → π*)-excitation] in methanol or benzene solutions resulted in the formation of the products 4, 5a, 6 (due to α-cleavage) and 7 [due to (C_α–S)-fission]. Compounds 4, 6 and 7 have been described earlier [1] [2]. The structure elucidation of 5a is reported in this paper. The photochemistry of the β-ketosulfide 2 is summarized.     

Photochemische Reaktionen 122. Mitteilung [1]. Zur vinylogen β-spaltung von epoxy-enonen: Photoisomerisierung des 3,4:5,6-diepoxy-5,6-dihydro-β-jonons

Vinylogous β-Cleavage of Epoxy-enones: Photoisomerization of 3,4: 5,6-Diepoxy-5,6-dihydro-β-ionone On ¹n,π*-excitation (λ>347 nm), 3,4:5,6-diepoxy-5,6-dihydro-β-ionone ((E)- 3 ) shows the typical behaviour of α,β-unsaturated γ,δ-epoxy ketones furnishing the (Z)-enone 3 and by C(γ),O cleavage of the oxirane the dihydrofuryl ketone 10 and the cyclohexanones (E/Z)- 11 . However, on ¹π,π*-excitation an unexpected type of transformation is observed: (E)- 3 is isomerized to the four aliphatic triketones 5 – 8 as the main products. To a smaller extent the allene diketone 9 is formed by a known type of isomerization as well as (Z)- 3 . As the starting material for the preparation of (E)- 3 , the known epidioxy-enone (E)- 4 was used. In addition to (E)- 3 , (E)- 4 gives the aliphatic triketone 6 and the hydroxyenone 15 by thermal or catalytic isomerization.     

Photochemische Reaktionen 115. Mitteilung [1]. Zur Photochemie konjugierter γ,δ-Epoxyenone: Der Einfluss eines Hydroxylsubstituenten in ϵ-Stellung

Photochemistry of Conjugated γ,δ-Epoxyenones: The Influence of a Hydroxy Substituent in ?-Position On ¹n, π*- or ¹π,π*-excitation (λ ≥ 347 or λ=254 nm), the ?-hydroxy-γ;,δ-epoxyenone 8 undergoes fission of the C(γ)? O bond followed by the cleavage of the C(δ)-C(?) bond. This hitherto unknown sequence of reactions is evidenced by the structure determination of the new type products 10–17 and 25 , including a synthetic proof for 12 and the X-ray analysis of 11 (X-ray data: triclinic P1; a=7,386(2), b=8,904(4), c=9,684(5)Å; α=82,29(4)°, β=74,46(3)°, γ=82,29(3)°; Z=2). The selective ¹π,π*-excitation also induces competitive C(γ)-C(δ) bond cleavage to yield the bicyclic acetal 18 and a ketonium-ylide intermediate a , which photochemically forms a carbene b giving the allene 19 and the cyclopropene 20 . On ¹n,π*-excitation of the acetate 9 the initial C(γ)-O bond fission is, in contrast to the behaviour of the corresponding alcohol 8 , followed by a 1,2-methyl shift affording (E/Z)- 28 or by a cyclization-autoxidation process yielding the lactone 29 .     

Photochemische Reaktionen. 82. Mitteilung. Zur Photochemie der α,β-Epoxylketone: γ-Wasserstoffabstraktion versus Epoxyketon-Umlagerung

Photochemistry of α,β-epoxyketones: γ-H-abstraction versus epoxyketone-rearrangement. The photochemical behaviour of conformationally mobile α,β-epoxyketones that could undergo competing reactions has been studied. The UV.-irradiation of 5 yields the stereoisomeric cyclobutanols 9 and 10 as well as the fragmentation product 11 . Irradiation of 6 gives only the cyclobutanols 12 and 13 , whereas 4 and 8 in inert solvents yielded only intractable gums. The non-occurrence of the typical isomerization of α,β-epoxylketones to the corresponding β-diketones is attributed to steric factors.     

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. 109. Mitteilung [1]. Zur Photochemie konjugierter δ-Keto-enone und β,γ,δ,ϵ-ungesättigter Ketone

Photochemistry of Conjugated δ-Keto-enones and β,γ,δ,?-Unsaturated Ketones On ¹π,π*-excitation the δ-keto-enones 5–8 are isomerized to compounds B ( 18 , 22 , 26 , 28 ) via 1,3-acyl shift and to compounds C ( 19 , 23 , 27 , 29 ) via 1,2-acyl shift, whereas the β,γ,δ,?-unsaturated ketone 9 gives the isomers 32 and 33 by 1,2-and 1,5-acyl shift, respectively. Furthermore, isomerization of 6 to 24 , dimerization of 8 to 30 and addition of methanol to 8 ( 8 → 31 ) is observed. Unlike 7 and 8 the acyclic ketones 5 , 6 and 9 undergo photodecarbonylation on ¹π,π*-excitation ( 5 → 20 , 21 ; 6 → 20 , 25 ; (E)- 9 → 35–38 ). Evidence is given, that the conversion to B as well as the photodecarbonylation of 5,6 and 9 arise from an excited singulet state, but the conversion to C as well as the dimerization of 8 from the T₁-state.     

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. 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).