N,N'-dicyclohexylcarbodiimide (18.77 g) was added to a methylene chloride solution (250 ml) containing L-proline benzylester hydrochloride (20.00 g), triethylamine (11.53 ml), and 3-phenylpropionic acid (13.67 g), and the mixture was stirred at 0° C. for 20 hours. After the precipitated dicyclohexylurea was filtered off, the filtrate was concentrated. The residue was dissolved in ethyl acetate, and the resultant solution was successively washed with 1N hydrochloric acid, saturated sodium hydrogencarbonate, and brine, dried over magnesium sulfate, and concentrated. The residue was purified by silica gel column chromatography to obtain 26.13 g of N-(3-phenylpropionyl)-L-proline benzyl ester.
