Dioxopyrrolines. LIII.Stereochemical Pathway of [2+2] Photocycloaddition Reaction of 4-Ethoxycarbonyl-5-phenyl-1H-pyrrole-2, 3-dione to Cyclic Enol Trimethylsilyl Ethers

The photocycloaddition reaction of the dioxopyrroline 1 to cyclic trimethylsilyloxy olefins was examined. The addition of 3-trimethylsilyloxy-1H-indene and 4-trimethylsilyloxy-1, 2-dihydronaphthalene occurred in a regio- and stereoselective manner to give the cis-anti-cis cyclobutanes 3 and 4, respectively. The stereochemical result supported the stereo-selection rule which states that the photocycloaddition of a very polar dioxopyrroline-olefin pair proceeds in an s+s manner via f favored O-endo-π-complex transition. On the other hand, the addition of 1-trimethylsilyloxy-cyclopentene and 1-trimethylsilyloxycyclohexene occurred less stereoselectively, proceeding predominantly in an s+a manner to give the adducts 5a, b, 6a, b, 7a, b, and 12. The results are consistent with the prediction of the stereo-selection rule obtained by assuming that these dioxopyrroline-olefin pairs are polar. The latter cases demonstrate that an incresed steric hindrance around the olefin moiety affects the polarity of enone-olefin pair in the transition state.