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

The photocycladdition reaction of the dioxopyrroline 1 to cyclic trimethylsilyloxy olefins was examined. The addition of 3-trimethylsilyloxy-lH-indene and 4-trimethylsilyloxy-l,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 a favored O-endo-π-complex transition. On the other hand, the addition of 1-trimethylsilyloxycyclopentene and l-trimethylsily!oxycyclohexene 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 increased steric hindrance around the olefin moiety affects the polarity of enone-olefin pair in the transition state.