A solution of 650 mg of N-tert.butyl-1-[3(S)-[(L-cysteinyl)amino]-2(R)-hydroxy-4-phenylbutyl]-2(S)-piperidinecarboxamide and 242 mg of quinaldic acid was cooled in an ice/salt mixture. 189 mg of hydroxybenzotriazole, 161 mg of N-ethylmorpholine and 317 mg of dicyclohexylcarbodiimide were added and the mixture was stirred for 64 hours. The mixture was diluted with ethyl acetate and filtered, and the filtrate was then evaporated. The residue was partitioned between dichloromethane and aqueous sodium bicarbonate solution. The organic phase was washed with sodium chloride solution and then evaporated. The residue was chromatographed on silica gel using dichloromethane/methanol (19:1) for the elution to give 350 mg of N-tert.butyl-1-[2(R)-hydroxy-4-phenyl-3(S)-[[N-(2-quinolylcarbonyl)-L-cysteinyl]amino]butyl]-2(S)-piperidinecarboxamide as a white solid; MS: m/e 620 [M+H]+.
