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
Résumé
The asymmetric [2+3] cycloaddition reaction between chiral oxazoline N-oxide 1 and ,-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 2was 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.