Asymmetric induction in the ene reaction of chloral with (1S,5S)-(–)-β-pinene; control of stereoselectivity by the choice of Lewis acid catalyst

Abstract

The thermal ene addition of chloral to (–)-β-pinene affords a 17 : 83 mixture of the two diastereoisomers (1) and (2), the ratio of which is altered appreciably if a Lewis acid is employed to enhance the rate of the ene addition; with ferric chloride the 97 : 3 ratio of (1) : (2) indicates that the reaction has become almost completely stereoselective, and the results can be rationalised by a simple steric model.