To a solution of N-{(2S)-2-(5-chloro-2-methoxybenzyl)-3-[(2,4,6-trimethoxybenzyl)amino]propanoyl}glycine (1g) in acetonitrile (30 ml), 1-hydroxy benzotriazole (421 mg) was added at room temperature. The mixture was stirred at 40° C. for 20 minutes, then N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (598 mg) was added to the reaction solution, and the mixture was stirred for 2 hours. After the reaction solution was concentrated in vacuo, toluene was added to the obtained residue, and the resulting mixture was washed with 6% aqueous sodium bicarbonate and saturated brine. The organic layer was concentrated in vacuo, then the obtained residue was purified by silica gel column chromatography to obtain the title compound (894 mg) as a colorless crystal.
