Intermolecular oxonium–ene reaction: a new entry to stereocontrolled synthesis of tetrahydropyrans

Abstract
The intermolecular ene-type reaction with a lactol-derived oxonium ion intermediate as an enophile is shown to exhibit a high level of diastereofacial selectivity to afford predominantly the 2,6-trans-tetrahydropyrans in spite of the diastereoisomeric ratio of the starting δ-lactol derivatives.