Photochemistry of 2-(prop-2-enyl)cyclopent-2-enones

The photochemistry of several substituted 2-(prop-2-enyl)cyclopent-2-enones has been examined. The 2-(prop-2-enyl)cyclopent-2-enones (2a; R = OMe or OAc), (6a), and (21) undergo di-π-methane rearrangement to vinylcyclopropanes [(3a and b), (7), and (29), respectively], whereas the side-chain substituted analogues (2b; R = OMe or OAc), (2c; R = OAc), and (6b), under similar conditions, undergo Z–E isomerisation. Sensitisation and quenching experiments with (2a; R = OAc) suggest that (3) is derived from a triplet excited state of (2a). The reactivity differences between (2a), (6a), and (21), and (2b), (2c), and (6b) are rationalised on the basis of dissipation of triplet energy by ‘free-rotor’ effects in the (prop-2-enyl) side chains of the molecules.Irradiation of cyclopent-2-enones (13) and (22), under conditions similar to those used in the photolyses of (2a), (6a), and (21), gave no products expected from di-π-methane rearrangement.Substitution of the methane portion of the di-π-methane system, as in (33), results in a dramatic increase in efficiency of the di-π-methane rearrangement [to (34)] over that observed for the conversions (2a)→(3), (6a)→(7), and (21)→(29). These data suggest that the latter rearrangements most probably proceed via a radical process involving hydrogen migration.Photolysis of the pyrethrin insecticides (1; R = Me, R′= H) and (1; R = Me, R′= Et) produces the vinyl-cyclopropane (35) and E-alkene (39), respectively, in high yields.