(2S,4S) -2-(dimethylaminocarbonyl)-4-mercapto-1- (p-nitrobenzyloxycarbonyl)-1-pyrrolidine. or(2S-cis)- 2-[(dimethylamino)carbonyl]-4-mercapto-1-pyrrolidinecarboxylic acid, (4-nitrophenyl) methyl ester.