Asymmetric 1,6‐Induction in Hetero‐Diels–Alder Reactions of Chiral Oxabutadienes for a De Novo Synthesis of Enantiomerically Pure Carbohydrates: Lewis Acid Dependent Reversal of Facial Selectivity

Abstract

Excellent endo/exo selectivity and very good induced diastereoselectivity are achieved by hetero-Diels-Alder reactions of oxabutadienes such as 1, accessible from chiral oxazolidinones, with the enol ether 2 in the presence of Lewis acids. The facial selectivity can be reversed by varying the Lewis acid. The endo product is formed almost exclusively from 1 and 2 with Me2AlCl as promotor. In contrast, the other endo product 4 predominates with trimethylsilyltriflate (TMS-OTf) as initiator.