To a solution of L-proline benzyl ester hydrochloride (20.0 g) in methylene chloride (250 ml) were added triethylamine (11.5 ml), 3-phenylpropionic acid (13.7 g) and DCC (18.8 g) under ice-cooling, and the mixture was stirred for 2 hours. After stirring at room temperature for 14 hours, the insoluble material was filtered off, and the filtrate was concentrated. The residue was dissolved in ethyl acetate, and the solution was washed with 1N hydrochloric acid, saturated aqueous solution of sodium bicarbonate and saturated brine, dried over anhydrous magnesium sulfate, and concentrated. The residue was purified by silica gel column chromatography (eluate: hexane-ethyl acetate). To thus-obtained N-(3-phenylpropionyl)-L-proline benzyl ester (26.1 g) in 1% water-containing methanol (200 ml) was added 10% Pd--C (3.0 g), and the mixture was stirred at room temperature under hydrogen atmosphere for 2.5 hours. The catalyst was filtered off, and the filtrate was concentrated to give 17.4 g of the title compound.
