GENERAL PHOTOISOMERIZATION APPROACH TO TRANS-BENZOBICYCLO[5.1.0]OCTENES - SYNTHETIC AND MECHANISTIC STUDIES

The preparation and photoisomerization of cis-bicyclo[5.1.0]octenes ()-3, 16, and 21-23 are described. For the benzannulated substrates, th e primary photochemical events established photostationary states betw een the cis isomers and the trans-fused cyclopropanes (+)-9, 32, 37, a nd 44. Studies employing enantioenriched 21, 32, and 44 demonstrated t hat the reactions occurred primarily but not exclusively via cleavage of the peripheral benzylic cyclopropane bonds. Utilization of the trip let quencher piperylene revealed a predominance of triplet diradical i ntermediates in the trans-to-cis conversions and afforded an improved protocol for preparative isomerizations.