Asymmetric Cycloadditions of o-Quinone Methides Employing Chiral Ammonium Fluoride Precatalysts

The catalytic, enantioselective, [4 + 2] cycloaddition reaction of ortho-quinone methides with silyl ketene acetals is described. This mechanistically interesting reaction, initiated by a chiral cinchona alkaloid-derived ammonium fluoride precatalyst complex, affords a variety of alkyl- and aryl-substituted 3,4-dihydrocoumarin products in excellent yield and with good enantioselectivity.