Photochemische Reaktionen. 76.Mitteilung [1]. UV.-Bestrahlung von 11-Oxo-Steroiden V Die Photoisomerisierung von 3,20-Diäthylendioxy-5α,14β,17α-pregnan

The UV.-isomerisation of 11-oxo-14β,17α-pregnane 9 to the 11,19-cyclo-derivative 11 is described. In addition the Pb(OAc)₄-fragmentation of photoproduct 11 was investigated. 11 yielded besides the expected 11-oxo-19-hydroxy-pregnane 18 the novel 9,11-seco-11,19-cyclosteroid 19 . The structure of 19 was established by chemical transformations and subsequently confirmed by X-ray analysis [2].     

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. 68. Mitteilung. Die Photoisomerisierung von α,β-ungesättigten γ,δ-Epoxyketonen: 9α, 10α- und 9β, 10β-Oxido-3-oxo-17β-acetoxy-Δ4-östren

Irradiation in the n→π* absorption band of the α,β-unsaturated γ,δ-epoxyketone 5 in ethanol at ?65° exclusively afforded the rearranged ene-dione 13 , whereas at + 24° under otherwise unchanged reaction conditions or upon triplet sensitization with Michler's ketone and with acetophenone at + 24° essentially identical mixtures of 13 (major product), 14 , and 15 were obtained. Selective π→π* excitation of 5 at ?78° and + 24° led to similar product patterns. The 9β,10β-epimeric epoxyketone 7 selectively isomerized to 14 and 15 at + 24° and n → π* or π → π* excitation. Neither the epoxyketones 5 and 7 nor the photoproducts 13–15 were photochemically interconverted. In separate photolyses each of the latter gave the double bond isomers 16 , 18 , and 19 , respectively. Cleavage of 13 to the dienone aldehyde 17 competed with the double bond shift ( → 16 ) when photolyzed in alcoholic solvents instead of benzene. The selective transformations 5 → 13 (at ?65° and n → π* excitation) and 7 → 14 + 15 are attributed to stereoelectronic factors facilitating the skeletal rearrangements of the diradicals 53 and 55 , the likely primary photoproducts resulting from epoxide cleavage in the triplet-excited compounds 5 and 7 , via the transition states 54 , 56 , and 57 . The loss of selectivity in product formation from 5 at higher temperature and n → π* excitation or triplet sensitization is explicable in terms of radical dissociation into 58 and 59 increasingly participating at the secondary thermal transformations of 53 . The similar effect of π → π* excitation even at ?78° indicates that some of the π,π* singlet energy may become available as thermal activation energy. It is further suggested that the considerably lesser ring strain in 14 and 15 , as compared with 13 , is responsible that selectivity in product formation from 7 is maintained also at +24° and at π → π* excitation.     

Photochemische Reaktionen 42. Mitteilung [1]. Photoisomerisierung von α, β-Epoxyketonen II Der sterische Verlauf der Umlagerung von 3-Oxo-4, 5-oxido-Steroiden

The photorearrangement previously described [3] of saturated and Δ¹-unsaturated 3-oxo-4,5-epoxy-10β-steroids to 3,5-dioxo-10(5 →4)-abeo compounds proceeds most likely via a radical 1,2-alkyl shift (Chart 1). The similar rearrangements of the related 10α-epoxyketone 10 and the 4-methyl-epoxyketones 13 , 15 , 16 , 20 and 21 to the corresponding 3,5-diketones occurred without epimerization at the migrating carbon atom (C-10) and the site of substitution (C-4) (Chart 3). The stereochemical control of the rearrangement is in agreement with the earlier proposed mechanism of a concerted alkyl radical shift in these alicyclic systems.     

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. 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. 56. Mitteilung [1]. Die Photoisomerisierung des α,β-ungesättigten δ-Diketons 3, 7-Dioxo-4, 4-dimethyl-17 β-acetoxy-δ5-androsten und dessen 3-Ketalderivates

On irradiation with light of wavelengths 2537 or > 3400 Å 4,4-dimethyl-17β-acetoxy-androst-5-ene-3, 7-dione ( 8 ) rearranges to the two diastereoisomeric products 9 and 10 . This isomerization is the only detectable photochemical reaction of 8 in a variety of solvents, including p-dioxane. Complete quenching with 0.5M naphthalene (on irradiation with > 3400 Å) indicates a triplet reaction. The photochemistry of the 3-ethylene ketal derivative 29 differs completely. Irradiation in p-dioxane solution leads exclusively to photoreduction and formation of the four diastereoisomeric dioxanyl allyl tert.-carbinols 30a – d .     

Photochemische Reaktionen. 116. Mitteilung [1]. Notiz zur 1γ, δ*-induzierten Photospaltung von γ, δ-Epoxy-eucarvon

π, π*-Induced Photocleavage of γ, δ-Epoxy-eucarvone . On ¹π, π*-excitation 1 undergoes cleavage of the C, C-oxirane bond ( 1 → c ) and isomerizes to the bicyclic dihydrofurane compound 5 . In addition, 1 shows photocleavage of the C (γ), O-oxirane bond ( 1 → d ) and gives the isomers 2, 3, 6, 7 and 8. Furthermore, the cyclohexenone 9 and the cyclohexene-1, 4-dione 10 are formed presumably via an intermediate 13 , which could also arise from d. Besides these products the compounds 11 and 12 are obtained, which are photoproducts of 2 .     

Photochemische Reaktionen. 121. Mitteilung. Zur Photochemie von ε,ζ-Methano-α,γ-dienonen und 7,8-Methano-1,3,5-trienen

Photochemistry of ε,ζ-Methano-α,γ-dienones and 7,8-Methano-1,3,5-trienes Irradiation of the δ-cyclopropyl-dienone (E)- 6 (λ ≥ 347 nm) gives (Z)- 6, 10 (1,5-sigmatropic H-shift), (E/Z)- 9 (electrocyclic process involving C(ε), C(ζ)-cleavage) and 11 (ring opening). The corresponding 6-cyclopropyl-triene (E)- 7 gives on singlet excitation (δ > 280 nm) 14 (1,5-sigmatropic H-shift) and, to a smaller extent, the bicyclo [3.2.0] heptenyl-dienes (E/Z)- 13 . However, on triplet excitation (λ ≥ 347 nm, benzophenone) (E)- 7 gives (E/Z)- 13 as the main products. On both ¹π,π*- and ³π,π*-excitation, (Z)- 7 and 15 are formed in small amounts.     

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 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 111. Mitteilung [1] Zur Photochemie α, β-ungesättigter γ, δ-Epoxyester I: Singulett- versus Triplettreaktivität

On triplet excitation (E)- 2 isomerizes to (Z)- 2 and reacts by cleavage of the C(γ), O-bond to isomeric δ-ketoester compounds ( 3 and 4 ) and 2,5-dihydrofuran compounds ( 5 and 19 , s. Scheme 1). - On singulet excitation (E)- 2 gives mainly isomers formed by cleavage of the C(γ), C(δ)-bond ( 6–14 , s. Scheme 1). However, the products 3–5 of the triplet induced cleavage of the C(γ), O-bond are obtained in small amounts, too. The conversion of (E)- 2 to an intermediate ketonium-ylide b (s. Scheme 5) is proven by the isolation of its cyclization product 13 and of the acetals 16 and 17 , the products of solvent addition to b . - Excitation (λ = 254 nm) of the enol ether (E/Z)- 6 yields the isomeric α, β-unsaturated ε-ketoesters (E/Z)- 8 and 9 , which undergo photodeconjugation to give the isomeric γ, δ-unsaturated ε-ketoesters (E/Z)- 10 . - On treatment with BF₃O(C₂H₅)₂ (E)- 2 isomerizes by cleavage of the C(δ), O-bond to the γ-ketoester (E)- 20 (s. Scheme 2). Conversion of (Z)- 2 with FeCl₃ gives the isomeric furan compound 21 exclusively.     

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. 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. 112. Mitteilung [1] Zur Photochemie α, β-ungesättigter γ, δ-Epoxyester. III. Ergänzende Untersuchungen zur Triplettreaktivität

On triplet sensitization (E)- 5 gives (Z)- 5 and isomerizes via C(δ), O-bond cleavage to the cyclobutanone 6 and the conjugated γ-ketoester 7 . - On singulet excitation 6 undergoes decarbonylation and yields the bicyclo [4.1.0]heptane 8 . However, on triplet sensitization 6 is converted to the isomeric tricyclononane 9 by a stereospecific oxa-di-π-methane rearrangement. The structure of 9 is determined by X-ray analysis of the p-nitrobenzoate 15: a = 10.573, b = 14.707, c = 13.494 Å, β = 112.40°, P2₁/n, Z, = 4.     

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. 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.