[structure: see text] An organocatalytic asymmetric alpha-alkylation of aldehydes has recently been shown to provide cyclic aldehydes in high yields and enantioselectivities upon treating substituted acyclic halo-aldehydes with a catalytic amount of 2-methylproline in the presence of 1 equiv of triethylamine. Here, we report a density functional study on the mechanism of this reaction. The crucial step is an intramolecular nucleophilic substitution in the enamine intermediate. The added base accelerates the reaction through the electrostatic activation of the leaving group and affects the stereoselectivity by stabilizing anti and syn transition states to a different extent. On the basis of the computed barriers and transition states, we provide an explanation for the remarkable and unexpected increase in enantioselectivity that is observed when using 2-methylproline instead of proline as the catalyst. Calculated and observed enantiomeric excess values are in good agreement.