N-t-butyloxycarbonyl-L-alanyl-L-proline (4.3 g, 15 m mole), L-proline benzyl ester hydrochloride (3.7 g, 15.3 m mole) and HOBt (2.0 g, 15 m mole) were dissolved in methylene dichloride (40 ml). WSC (2.8 ml) was added dropwise to the above mixture while cooling to -15° C. and stirring. The reaction was carried out for 3 hours at a temperature of not more than 0° C., and then overnight at room temperature. The solvent was removed by distillation under reduced pressure. The residue was dissolved in ethyl acetate, and washed with 1N hydrochloric acid, water, 5% sodium bicarbonate and water, in order. The mixture was dried over anhydrous sodium sulfate. The solvent was distilled off under reduced pressure. The residue was crystallized with a mixture of ethyl acetate and n-hexan to give a crystal of N-t-butyloxycarbonyl-L-alanyl-L-prolyl-L-proline benzyl ester (6.3 g, 88.7%) having melting point of 143° to 145° C. and specific rotatory power of [α]D25 =-131.0° (C=1, chloroform).
