Photochemistry of 7,7-disubstituted 3,4,4a,5,6,7-hexahydro-4a-methyl-1(2H)-naphthalenones

The photochemistry of (+/-)-3,4,4a,5,6,7-hexahydro-4 alpha-methyl-7,7-diphenyl-1(2H)-naphthalenone (1) and (+/-)-3,4,4a,5,6,7-hexahydro-4a,7,7-trimethyl-1(2H)-naphthalenone (2) is presented. Both molecules incorporate a rigid s-cis enone extended over two rings. In benzene, substrates 1 and 2 were photoinert. In alcohol solvents, hydrogen bonded enols resulting from solvent addition to the enone double bond were isolated as the only photoproducts. The rearrangements anticipated for gamma,gamma-disubstituted enones, aryl migration for 1 and Type A rearrangement for 2, were not observed. Spectroscopic and X-ray structure elucidation showed that solvent addition is highly regio-and stereospecific. Ketonization of the less hindered enols derived from 2 was observed upon heating or exposure to silica gel; enols produced from 1 were stable. Photolysis of 1 and 2 in aqueous dioxane led exclusively to beta-hydroxy ketones. The mechanism leading to product formation was rationalized in terms of a stepwise addition of solvent to a triplet-derived ground state trans enone. A mechanism involving direct addition of solvent to a polarized excited state was also considered. (C) 1997 Elsevier Science S.A.