STEREOCHEMISTRY OF [2+2] PHOTOCYCLOADDITION OF CYCLIC ENONES TO ALKENES - STRUCTURAL AND MECHANISTIC CONSIDERATIONS IN FORMATION OF TRANS-FUSED CYCLOADDUCTS

Photocycloaddition of 2-cyclohexenone to cyclopentene gives four [2 + 2] adducts, rather than only two as reported previously. Differentiation between the cis-anti-cis and cis-syn-cis structures was made on the basis of differences in C-13 chemical shifts in the two isomers. The structures of the two trans-fused adducts, which were not isolated, derive from their base-catalyzed epimerization to the corresponding cis-fused isomers. One trans-fused and one cis-fused cycloadduct are formed initially in a ratio of 2:1, respectively, upon photoaddition of testosterone acetate to cyclopentene, but this ratio changes in favor of the cis-fused isomer as the irradiation progresses due to secondary reactions. These adducts were identified by X-ray crystallography as the trans-fused (4alpha,5beta) and cis-fused (4alpha,5alpha) structures. Epimerization of the trans-fused adduct in base led to isolation of the alternative cis-fused adduct (4beta,5beta) whose X-ray crystal structure was also determined. The X-ray structure of the oxime of the trans-fused photoadduct of 2-cyclohexenone and tetramethylethylene was also obtained for structural comparison with the steroid enone adducts. These are believed to be the first definitive structure determinations of trans-fused bicyclo[4.2.0]octan-2-ones. The assignments of H-1 NMR coupling constants to vicinal cis and trans protons on the cyclobutane ring are based on the relevant dihedral angles obtained from the X-ray structures and from molecular modeling. Mechanisms for the formation of trans-fused adducts from both planar and twisted cyclohexenone triplets are discussed.