Quantum mechanics calculations have been performed to study the stereoselectivities in the direct Mannich reactions catalyzed by different chiral secondary airline catalysts. The effects of two kinds of catalysts, (S)-1-(2-pyrrolidinylmethyl)pyrrolidine and proline on the diastereoselectivities of the direct Mannich reactions between ketimine and aldehyde have been studied with the aid of the BH and HLYP method. Transition states of the stereochemistry-determining C-C bond-forming step with the enamine intermediate addition to the ketimine for the subject reactions are reported. These theoretical calculations provide a good explanation for the opposite diastereoselectivities of these two different kinds of catalysts. calculated and observed diastereomeric ratio and enantiomeric excess values are in good agreement. (C) 2009 Elsevier B.V. All rights reserved