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 Paternò-Büchi reaction of compound 7 gave predominantly one diastereoisomer 16a (62% de) the