PURPOSE: To obtain a new compound useful as a synthetic intermediate for pharmaceutical compound having angiotensin converting enzyme inhibiting action.

CONSTITUTION: The compound of formula I (R2 is t-butyl or benzyl; R3 is lower alkyl or aryl), e.g. (4S)-1-methyl-3-[(2R)-2-(p-toluenesulfonyloxy)propionyl]-2- oxomidazolidine-4-carboxylic acid t-butyl ester. The compound of formula I can be produced by reacting an (S)-2-oxoimidazolidine-4-carboxylic acid derivative of formula II with a reactive derivative of an (R)propionic acid compound of formula III at the carboxyl group in the presence of an acid acceptor (e.g. potassium t-butoxide) in a solvent (e.g. THF) under cooling or at room temperature.