A process for preparing [(1R,2R)-4-oxo-1,2-cyclopentanedicarboxylic acid II, by the resolution of racemic 4-oxo-1,2-cyclopentanedicarboxylic acid (V), said process comprising: (a) reacting 4-oxo-1,2-cyclopentanedicarboxylic acid (V) with brucine or (1R,2S)-()-ephedrine, thus preparing the bis-brucine or bis-(1R,2S)-()-ephedrine salt of (V), and (b) precipitating selectively the bis-brucine or bis-(1R,2S)-()-ephedrine salt of (1R,2R)-4-oxo-1,2-cyclopentanedicarboxylic acid II, while the bis-brucine or bis-(1R,2S)-()-ephedrine salt of [(1S,2S)-4-oxo-1,2-cyclopentanedicarboxylic acid stays in solution; (c) liberating the acid II by removal of brucine or (1R,2S)-()-ephedrine from the precipitated salt obtained in step (b).