Formation of bicyclic pyrroles and furans through an enone allene photocycloaddition and fragmentation sequence

The [2+2] photocycloaddition of allenes 15-18, 23 and 26 was studied. Irradiation of a solution of these substrates in acetonitrile at 300 nm resulted in the clean conversion of the starting materials into a mixture of photoproducts. The major product in all cases was a bicyclic pyrrole or furan fused to an eight membered ring (43-70 % yield). The formation of these products is thought to be a result of a heteroatom-induced fragmentation of the straight adduct (7). This is supported by irradiation of the carbon analogue 32 which allowed the isolation of straight adduct 45 after catalytic hydrogenation in 27 %. The minor crossed photoproducts were isolated in 10-20 % yield. The observed major/minor ratio of 4:1 was not affected by the variation of substituents on the cyclohexene ring. Introduction of a substituent on the allene had a more significant effect on the ratio which changed to 2:1.