Photochemical reactions. 146 th communication . Photochemistry of conjugated methano‐epoxydienes: Participation of the neighboring cyclopropane ring in product formation via carbonyl ylide and carbene intermediates

On singlet excitation (λ = 254 nm, THF, pentane or hexane), the diastereoisomeric methano‐epoxydienes ( E )‐ 6 and ( E )‐ 7 undergo interconversion and yield the products 8 – 11 . The main process is the cleavage of the oxirane ring to the vinyl carbene intermediate e which undergoes addition to the adjacent double bond furnishing the cyclopropene 8 . The alternative carbene intermediate f is evidenced by the formation of the cyclobutene 10 . For the fragmentation leading to 11 , the carbene f as well as the dipolar species h are considered as possible intermediates. On triplet sensitization (acetone, λ > 280 nm), ( E )‐ 7 shows behavior typical of epoxydienes, undergoing fission of the CO bond of the oxirane ring and isomerization to the compounds 13 , 14 and ( E / Z )‐ 15 .