Facial diastereoselectivity in the photocycloaddition of chiral N-acyl enamines to benzaldehyde

The facial diastereoselectivity in the photocycloaddition of the chiral N-acyl enamines 5, 6, and 7 to benzaldehyde has been studied. The enamide 5 derived from alpha-phenylethylamine (8) gave the corresponding oxetanes in good total yield (74%) but the diastereofacial selection was unsatisfactory (32% de). The N-vinyl-oxazolidinones 6 and 7 were prepared from the parent chiral 4-phenyl- (11) or 4-benzyloxazolidinone (15) by vinylation. Their photocycloadditons to benzaldehyde proceeded smoothly and yielded the corresponding oxetanes 12 and 16 (70-80%). A remarkable discrepancy in the course of the reaction was observed. Whereas oxetane formation from 6 proceeded with low diastereoselectivity (30% de) the Paterno-Buchi reaction of compound 7 gave predominantly one diastereoisomer 16a (62% de) the relative configuration of which was opposite to the major diastereomer 12a obtained from oxazolidinone 6. (C) 1998 Elsevier Science Ltd. All rights reserved.