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

Abstract

The asymmetric [2+3] cycloaddition reaction between chiral oxazoline N-oxide 1 and α,β-unsaturated lactone 2 was studied. A double diastereoselection was observed, (1 R)- 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 13C-{ 1H} 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.