Origins of opposite absolute stereoselectivities in proline-catalyzed direct Mannich and aldol reactions.

[structure: see text] The first computational studies to elucidate the stereoselectivity of the proline-catalyzed direct Mannich reaction have been performed using density functional theory (B3LYP/6-31G*). The transition states for the proline-catalyzed direct Mannich reaction of the proline enamine of acetone with the N-phenyl imine of acetaldehyde are reported here. The computed transition states provide a general model that explains the origin of the opposite stereoselectivities of proline-catalyzed Mannich and aldol reactions.