Asymmetric Catalytic Mannich Reactions Catalyzed by Urea Derivatives:  Enantioselective Synthesis of β-Aryl-β-Amino Acids

Highly enantioselective addition reactions between silyl ketene acetals and N-Boc aldimines are catalyzed by the thiourea-based catalyst 1c. Extraordinary scope is observed in this methodology with regard to the imine substrate, with aryl and heteroaromatic derivatives generally affording nearly quantitative yields of beta-amino ester product in up to 98% enantioselectivity.