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

On 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 .