Chiral Brønsted acid-catalyzed enantioselective ionic [2+4] cycloadditions

The first asymmetric chiral N-triflylphosphoramide-catalyzed ionic [2+4] cycloaddition reaction of unsaturated acetals is described. This reaction proceeds through the intermediacy of a vinyl oxocarbenium/chiral anion pair, and the chiral N-triflylphosphoramide anion controls the stereoselectivity of the cycloaddition step. Moderate enantioselectivities (up to 80:20 e.r.) have been obtained when α,β-unsaturated dioxolanes were employed as the dienophiles. These reactions demonstrate strong dependence on the counterion coordinating properties and solvent polarity, a behavior characteristic of oxocarbenium ions.