A solution of 270 mg of N-[3(S)-[[L-asparaginyl]amino]-2(R)-hydroxy-4-phenylbutyl]-L-proline tert.butyl ester and 98 mg of 4-phenylbutyric acid in 10 ml of dry tetrahydrofuran was cooled in an ice/salt mixture. 81 mg of hydroxybenzotriazole, 69 mg of N-ethylmorpholine and 136 mg of dicyclohexylcarbodiimide were added and the mixture was stirred for 64 hours. The mixture was then diluted with ethyl acetate and filtered. The filtrate was washed with aqueous sodium bicarbonate solution and sodium chloride solution. The solvent was removed by evaporation and the residue was chromatographed on silica gel using System G for the elution to give 79 mg of N-[2(R)-hydroxy-4-phenyl-3(S)-[[N-(4-phenylbutyryl)-L-asparaginyl]amino]butyl]-L-proline tert.butyl ester as a white solid; MS: m/e 595 [M+H]+.
