Abstract Quantum mechanics calculations have been performed to study the stereoselectivities in the direct anti- and syn-aldol reactions catalyzed by different amino acid derivatives. The effects of two kinds of catalysts, l-proline amino alcohol amides and l-leucine amino alcohol amides, on the diastereoselectivity and enantioselectivity of the direct intermolecular aldol reactions between α-substituted ketones and 4-nitrobenzaldehyde have been studied. Transition states of the crucial C–C bond-forming step with the enamine intermediate addition to the aldehyde for the subject reactions are reported. These theoretical calculations provide a good explanation for the opposite syn versus anti diastereoselectivities of these two kinds of catalysts ( anti-selectivity for the proline derivative, syn-selectivity for the leucine derivative). Calculated and observed diastereomeric ratios and enantiomeric excess values are in good agreement.