SINGLET OXYGEN PHOTOOXYGENATION OF TRICYCLO[4.3.0.0(1,3)]NONA-6,8-DIENES AND BICYCLO[4.3.0]NONA-1(6),7-DIENES - CHARACTERIZATION OF THE PRIMARY REACTION-PRODUCTS AND SUBSEQUENT TRANSFORMATIONS

Ring expansion of 3-methyl-1-phenyl-1,2-dihydropentalene 1 using a seq uence of cyclopropanation and acid-catalyzed vinylcyclopropane rearran gement in the presence of nucleophilic trapping reagents led to bicycl o[4.3.0]nona-1(6),7-dienes 8 and 9 in good yields and with high diaste reoselectivity. Photooxygenation of these compounds, as well as the se condary product tricyclo[4.3.0.0(1,3)]nona-6,8-diene 2, afforded the u nstable endoperoxides 3 and 11 both with high (but completely inverse) diastereofacial selectivity. Both endoperoxides underwent Kornblum-De La Mare reaction in the presence of triethylamine to give the hydroxy enones 4 and 15. X-ray structure analyses of 4a and 15 confirmed the proposed reaction topicities. The thermal rearrangement of 11 afforded a series of products (bis-epoxide 12, epoxy enone 13, and epoxy enal 14) following the well-known peroxide homolysis protocol, whereas 3 th ermally rearranged into the cyclooctene dione 5 and the cycloheptenone aldehyde 6, by cleavage of the central bicyclo[3.3.0]octane bond and competing 1,2-hydrogen and 1,2-alkyl shifts, respectively.