1987 Volume 35 Issue 1 Pages 23-34
The photocycloaddition of 4-ethoxycarbonyl-5-phenyl-2H-pyrrole-2, 3-dione 1 to (Z) -and (E) -d-styrenes proceeded in a stereoselective manner to give the compounds in which the styrene component added to 1 antarafacially as the major products. Including these results, the observed exo-or endo-stereoselectivity in the photocycloaddition of 1 to various acyclic olefins is well explained by assuming plural transition states, stepwise C-C bond formation, and a stereoselection rule which may be stated as follows : 1) the first C-C bond formation always occurs suprafacially; 2) the second C-C bond formation in a polar enone-olefin pair (an enone and an olefin with a weak electron donating substituent) occurs antara-selectively from the favored π-complex, while it occurs supra-selectively from the less favored transition state; and 3) in a very polar enone-olefin pair (an enone and an olefin with a strong electron donating group), the second C-C bond formation occurs supra-selectively from the favored π-complex and antara-selectively from the less-favored transition state. Many other stereochemical results hitherto reported can be rationalized in terms of this stereo-selection rule.
