Catalyst-Controlled Diastereoselectivity Switch in Formation of Dihydropyrans

摘要

Hetero-Diels-Alder (HDA) reactions provide one of the most direct methods for the formation of partially unsaturated six-membered heterocycles. Stringent diastereoselectivity is dictated by the inherent diastereomeric bias of substrates in most of these reactions, and it is hard to override the intrinsic diastereomeric preference of the substrate. For instance, the chiral amine-catalyzed inverse-electron-demand HDA reaction between β,γ-unsaturated α-keto ester and the E-enamine intermediate generated from an aldehyde is known to give trans-dihydropyran.1