Dehydrative Re2O7-Catalyzed Approach to Dihydropyran Synthesis.

Monoallylic 1,3- and 1,5-diols undergo Re2O7-mediated ionization to form allylic cations that engage in cyclization reactions to form dihydropyran products. The reactions give the 2,6-trans-stereoisomer as the major products as a result of minimizing steric interactions in a boat-like transition state. The results of these studies are consistent with cationic intermediates, with an intriguing observation of stereochemical retention in one example.