Lewis Acid Tuned Facial Stereodivergent HDA Reactions Using β-Substituted N-Vinyloxazolidinones

The [4 + 2] acido-catalyzed heterocycloaddition between new β-substituted N-vinyl-1,3-oxazolidin-2-ones (with R‘ = Me, Ar, CH2 Ar) and β,γ-unsaturated α-ketoesters (R = Ar) afforded heteroadducts with high levels of endo and facial selectivities. A complete reversal of facial differentiation was achieved by varying the Lewis acid, leading to the stereoselective formation of either endo-α or endo-β adducts.