Photochemical Reactions. 129th Communication. Photochemistry of a conjugated diepoxydiene: Product formation via carbonyl ylide and carbene intermediates with participation of neighbouring epoxy functions

Abstract

AbstractOn singlet excitation (λ=254 nm, acetonitrile) the diepoxydiene (E)‐7 undergoes photocleavage to the carbonyl ylide VII and the carbenes X and XI. The carbonyl ylide VII rearranges to the thermally labile dioxabicyclo [3.2.1]octene 20 or fragments via VIII to the aldehyde 9 and propyne. The carbene X, showing behaviour typical of vinyl carbenes, undergoes addition to the adjacent double bond furnishing the cyclopropene 11. The carbene XI, however, undergoes an insertion reaction into the neighbouring oxirane C,C‐bond leading to the oxetene (E)‐21 which can be isolated at −78°, but at room temperature is rapidly transformed to the aldehyde 10. On triplet excitation (acetone, λ>280 nm), however, (E)‐7 shows the typical behaviour of epoxydienes, undergoing C, O‐cleavage of the oxirane and isomerization to 22, 23 and (E/Z)‐24.