Benzopyranoisoxazolidines as Chiral Auxiliaries for Asymmetric Synthesis

The asymmetric alkylation of N-propionylisoxazolidines (2a and 2b), which are prepared by treatment of 1a and 1b with propionyl chloride and triethylamine, proceeds with high diastereoselectivity, even with β-branched alkyl triflates. The alkylated products 3 can be converted into the alcohols 4, aldehydes 5, and ketones 6 in high yields in one step with recovery of the auxiliary 1.