Abstract

3,4-Dihydro-2H-pyrans are present in the skeletons of several natural products, and these versatile synthetic intermediates are readily transformed into tetrahydropyrans, pyridines, or 1,5-dicarbonyl units. Among the strategies developed to access 3,4-dihydro-2H-pyrans, the hetero-Diels-Alder reaction between an oxadiene and a dienophile is particularly valuable because up to three contiguous stereogenic centers can be created diastereo- and/or enantioselectively in a single step. This review addresses the mechanism of the reaction and presents methods for preparing the product dihydropyrans enantio- and diastereoselectively. Thermal and Lewis acid promoted cycloadditions are discussed, as is the role of activating groups on the oxadiene.