Double diastereoselection in asymmetric [2+3] cycloaddition of chiral oxazoline N-oxides:: application to the kinetic resolution of a racemic α,β-unsaturated δ-lactone

The asymmetric [2+3] cycloaddition reaction between chiral oxazoline N-oxide 1 and alpha,beta-unsaturated lactone 2 was studied. A double diastereoselection was observed, (1R)-1 and (R)-2 gave a mismatched pair with almost no cycloadduct obtained. A transition state model is proposed, accounting for the destabilization of transition state in the cycloaddition reaction. This result has led to kinetic resolution studies, in which both enantiomers of 1 were reacted with racemic lactone 2. The enantiomeric excess of the recovered lactone 2 was determined to be up to 70% ee, by C-13-{H-1} NMR analysis in a chiral liquid crystalline solvent. The experimental results are in agreement with predicted enantiomeric excesses and consistent with the transition state models. (C) 1999 Elsevier Science Ltd. All rights reserved.