Photochemische Reaktionen. 118. Mitteilung [1] Zur vinylogen β‐Spaltung von Enonen. UV.‐Bestrahlung von 4‐(3',7',7'‐Trimethyl‐2'‐oxabicyclo [3.2.0]hept‐3'‐en‐1'‐yl)but‐3‐en‐2‐on

Vinylogous β‐Cleavage of Enones: UV.‐irradiation of 4‐(3′,7′,7′‐trimethyl‐2′‐oxabicyclo[3.2.0]hept‐3′‐ene‐1′‐yl)but‐3‐ene‐2‐on On 1 π,π*‐excitation (λ = 254 nm) in acetonitrile ( E/Z )‐ 2 is converted into the isomers 4–9 and undergoes fragmentation yielding 10 ; in methanol ( E/Z )‐ 2 gives 7–10 and is transformed into 11 by incorporation of the solvent. On 1 π,π*‐excitation (λ λ⩾347 nm; benzene‐d 6 ) ( E )‐ 2 is isomerized into ( Z )‐ 2 , which is converted into the isomers 3 and 4 by further irradiation. 1 π,π*‐Excitation (λ = 254 nm; acetonitrile) of 4 gives 6 and ( E )‐ 9 , whereas UV.‐irradiation (λ = 254 nm; acetonitrile‐d 3 ) of 5 yields ( E )‐ 7 and 8 . On 1 π,π*‐excitation (λ = 254 nm; acetonitrile) of ( E/Z )‐ 12 the compounds ( E )‐ 14 and ( E )‐ 15 are obtained.