The proline-catalyzed double Mannich reaction of acetaldehyde with N-Boc imines.

Double-cross: Proline catalyzes the double Mannich reaction of acetaldehyde with N-Boc imines in excellent yields (up to 99 %; Boc = tert-butoxycarbonyl) and close to perfect diastereo- and enantioselectivities. Depending on the choice of catalysts, both the chiral, pseudo-C(2)-symmetric diastereomer and the corresponding meso compound can be prepared. Cross double Mannich reactions of acetaldehyde with two different imines are also demonstrated.