A crossover from S(N)2 to general base catalysed nucleophilic substitution can account for the dichotomous diastereoselectivity observed in DKR reactions of alpha-haloacyl imidazolidinones. Aprotic nucleophiles (Nu(-)) react preferentially with the 5S*,2'R* diastereomer via an S(N)2 mechanism. Conversely, amines (R2NH) generally react preferentially with the 5S*,2'S* diastereomer. General base catalysis via a bifurcated hydrogen bonded assembly accounts for this anomalous stereoselectivity. (C) 1998 Elsevier Science Ltd. All rights reserved.