To pyridine (20 ml) containing L-valyl-N-[(1S)-2-(2-benzoxazolyl)-1-isopropyl-2-hydroxyethyl]-L-prolinamide (1.0 g) are added 2-(3-tert-butoxycarbonylmethyl-2,4-dioxo-1-pyrimidinyl)acetic acid (0.7 g) obtained in Example 4 and 1-ethyl-3-dimethylaminopropyl)carbodiimide hydrochloride (0.6 g), and the mixture is stirred at room temperature for 15 hours. The reaction solution is poured into water, and the mixture is extracted with ethyl acetate. The extract is washed successively with 1 mol/liter hydrochloric acid, a saturated aqueous sodium hydrogen carbonate solution, and a saturated brine, and dried over anhydrous magnesium sulfate. The solvent is removed by evaporation under reduced pressure to give the desired 2-(3-tert-butoxycarbonylmethyl-2,4-dioxo-1-pyrimidinyl)acetyl-L-valyl-N-[(1S)-2-(2-benzoxazolyl)-1-isopropyl-2-hydroxyethyl]-L-prolinamide (1.5 g) as powders.
