Dioxopyrrolines. LV.₁₎ Stereochemical Pathway of [2 + 2] Photocycloaddition Reaction of 4,5-Diethoxycarbony 1-1H-pyrrole-2,3-dione to Cycloalkadienes and Cycloalkenes

The photocycloaddition reactions of 4,5-diethoxycarbonyl-lH-pyrrole-2,3-dione (6) to cycloalkadienes and cycloalkenes were examined. The addition of cyclopentadiene gave the hydroindole 8a (s + a product) as a major product and the cis-fused cyclobutane 7a (s + s product) as a minor one. In contrast, the addition of cyclohexadiene gave the cyclobutane 7b (s + s product) as a major product and the hydroindole 8b (s + a product) as a minor one. The photocycloaddition of cyclopentene, cyclohexene, and indene proceeded predominantly in an s + s manner to give the cis-syn-cis cyclobutanes, 16, 18, and 19, respectively. The stereochemical results were compared with those of the photocycloaddition reactions of 4-ethoxycarbonyl-5-phenyl-lH-pyrrole-2,3-dione (1) to the corresponding cyclo-olefins, revealing that the steric relationship of the addends plays an important role in determining the stereochemical pathway of the reaction.