Photochemische Reaktionen. 90. Mitteilung. Die UV.‐Bestrahlung von ( E )‐5‐Isopropyl‐6‐methyl‐5,6‐epoxy‐hept‐3‐en‐2‐on

Photolysis of ( E )‐5‐Isopropyl‐6‐methyl‐5,6‐epoxy‐hept‐3‐en‐2‐on. This paper continues the series of investigations of the photochemistry of α, β‐unsaturated γ, δ‐epoxy‐ketones, by examinating the photochemical behaviour of the aliphatic vinylogous epoxy‐ketone 1 , the chromophore of which is structurally similar to that of γ, δ‐epoxy‐(E),β‐ionone ( 44 ). On π, π*‐excitation (λ = 254 nm) 1 isomerizes mainly to the enol‐ether 2 and gives as minor products the isomeric dihydrofurane 3 , the 1,5‐diketones 4 and 5 and the 1,3‐diketone 6 . To a smaller extent, 1 also undergoes photofragmentation to the furane 7 , the allenyl‐ketone 8 and the cyclopropenyl‐ketone 9 . On n,π*‐excitation (λ ≥ = 347 nm) 1 yields the photoisomers 3 , 4 , 5 and in traces the hydroxyallenyl‐ketone 14 , but no fragmentation products. It is shown that on irradiation at λ ≥ = 254 nm the 1,5‐diketone 4 isomerizes to 5 , 6 and 15 and photodecarbonylates to the β, γ‐unsaturated ketone 16 . The isomers 3 , 4 and 5 , obtained both from n, π*‐ and π,π*‐excitation, represent products of cleavage of the C(γ)O‐bond in 1 . The enolether 2 , on the other hand, formed only by π,π*‐excitation, results from cleavage of the C(γ)C(δ)‐bond. Finally, the fragmentation products 7 , 8 and 9 , which could be detected only on π,π*‐excitation, may arise from a common intermediate g ↔ h .