Direct Catalytic Asymmetric Mannich-Type Reactions of γ-Butenolides: Effectiveness of Brønsted Acid in Chiral Metal Catalysis

Abstract
Direct catalytic asymmetric Mannich-type reactions of γ-butenolides are described. A chiral Lewis acid/amine base/Brønsted acid combination was used to catalyze a γ-addition of γ-butenolides to N-diphenylphosphinoyl imines, affording the products in up to >99% yield, anti/syn = >97:3, and 84% ee. The use of a catalytic amount of TfOH in addition to La(OTf)3/Me-PyBox/TMEDA was important for improving yield and stereoselectivity.