Abstract: Process for preparing optically active 2-piperazine-carboxylic acid derivatives, particularly S-enantiomer thereof, in high yield and high optical purity on industrial scale. As the optical resolving agents, easily accessible sulfonamides derived from selected optically active amino acids, such as N-tosyl-L-phenylalanine, N-tosyl-D-phenylglycine, N-tosyl-L-alanine or N-tosyl-L-valine, give excellent results. These resolving agents are stable and easily recovered from the reaction mixture and reused. Resolved 2-piperazinecarboxylic acid derivatives are preferably isolated as 4-t-butoxycarbonyl (Boc) derivatives. Diastereomeric salts (an example being shown below) formed as the intermediates of resolution are novel. ##STR1## ?In the formula "Ar" stands for a phenyl or naphthyl group which may be substituted with one to three C1-C6 alkyl groups, halogen atoms, nitro or alkoxy groups; and n=0 or 1.