A study of the photochemistry of benzobicyclo[2.2.2]octan-2-ones

The anti- and syn-3-methyl isomers of 5, 8-dimethoxy-l, 3-dimethyl-3, 4-dihydro-1, 4- ethanonaphthalen-2(lH)-one (2) and (3) and the 1, 3, 3-trimethyl analogue (4) were prepared from the Diels-Alder adduct of benzoquinone and 1, 3-dimethylcyclohexadiene. The photochemistry of each has been studied and the structures of several photoproducts have been assigned. One of the products from irradiation of (3) was found by X-ray analysis to be a 1, 4-disubstituted 1, 2, 3, 4-tetrahydronaphthalene[methyl 2-(5', 8'-dimethoxy-4'-methyl-1', 2', 3', 4'- tetrahydronaphthalen-1'-yl)propanoate] with the unexpected cis-stereochemistry (11). Irradiation of (4) produces, inter alia, a compound containing the unusual tetracyclo[7.4.0.02, 7.04, 8]trideca- l(9), 10, 12-triene skeleton whose structure was determined by n.m.r. spectroscopy.