Dihydropyrans by Cycloadditions of Oxadienes
Résumé
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.