Photochemical ring opening of 7-benzoyl- and 7-methoxycarbonyldibenzonorcaradienes. Competing 1,2-hydrogen shift and cyclization reactions of 1,3-diradicals

[GRAPHICS] The UV irradiation of dibenzonorcaradienes bearing an acyl or alkoxycarbony l substituent in the 7-position results in formation of substituted phenant hrenes, as well as cis-trans isomerization of the starting material, This r eaction apparently proceeds via intermediate formation of a short-lived (ta u = 1-20 ns) 1,3-diradical, which is produced by photochemical cleavage of one cyclopropane bond, while no evidence of alpha -carbonylcarbene formatio n was found.