Photochemical reactions. 134 th Communication . Photochemistry of 5,6‐epoxy‐5,6‐dihydro‐3,4‐methano‐β‐ionone: Influence of a cyclopropane ring on the reactivity of an ylide intermediate

On 1 n,π*‐excitation(λ > 347 nm), the diastereomeric methanoepoxyenones ( E )‐ 6 undergo isomerization via C,O‐cleavage of the oxirane leading to diastereomeric photoproducts (( E )‐ 5 →( E / Z )‐ 13 and 14 ; ( E )‐ 6 →( E / Z )‐ 16 and 17 ). On 1 π,π*‐excitation (λ = 254 nm) of either ( E )‐ 5 ) or ( E ‐ 6 the photoproducts 9, 10 and 11 are formed. By laser flash photolysis (λ = 265 nm) the ylide intermediate 3 was detected, with a lifetime of 10 μs in MeCN at ambient temperature. Stern‐Volmer analysis of the ylide quenching by MeOH disclosed that compounds 9 and 10 , but not 11 , arise from the ylide intermediate e .