Photochemical reactions, 136th communication. Photochemistry of conjugated epoxyenones and dienes of the ionone series: Influence of a neighboring spiro‐oxirane function

The preparation and photolyses of the diepoxyenones ( E )‐ 8 and ( E )‐ 9 as well as the diepoxydiene ( E )‐ 10 are described. On 1 π,π * ‐excitation (λ = 254 nm), the diastereoisomeric diepoxyenones ( E )‐ 8 and ( E )‐ 9 undergo isomerization via the ylide intermediate f and the carbene intermediate g leading to the primary photoproducts 17A and 18–21 ( Scheme 8 ). On 1 n, π * ‐excitation (λ > 347 nm), ( E )‐ 8 shows behaviour typical of epoxyenones undergoing C(γ), O‐bond cleavage of the oxirane and isomerization to compounds 22 , ( E/Z )‐ 23 and ( E )‐ 24 ( Scheme 10 ). On singlet excitation, the diepoxydiene ( E )‐ 10 , is cleaved to the carbonyl ylide j and the carbenes 1 and m ( Scheme 11 ). The carbonyl ylide j fragments via the dipolar intermediate k to the acetylenic dienone ( E )‐ 31 . The carbene 1 , showing behaviour typical of vinyl carbenes, furnishes the cyclopropene 30 . The alternative carbene m , however, undergoes an insertion reaction into the neighboring oxirane C,C‐bond leading to the proposed but not isolated oxetene 43 , which is further transformed to the products 33A _ B by an intramolecular cycloaddition.