(2R,4S)-4-{[3,5-Bis(trifluoromethyl)benzyl]-(5-carboxypyrimidin-2-yl)}amino-2-ethyl-6-methoxy-3,4-dihydro-2H-[1,5]naphthyridine-1-carboxylic acid ethyl ester (200 mg) is dissolved in N,N-dimethylformamide (2 ml), and thereto are added triethylamine (53 μl), 3-aminopropionic acid methyl ester hydrochloride (67 mg), 1-hydroxybenzotriazole hydrate (65 mg) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (92 mg) under ice-cooling. The mixture is stirred for 21 hours. The reaction solution is concentrated under reduced pressure, then the residue is partitioned by adding a saturated aqueous sodium bicarbonate solution and ethyl acetate, and the organic layer is concentrated under reduced pressure. The resulting residue is dissolved in methanol (2 ml), and thereto is added 5N aqueous NaOH solution (1 ml), and the mixture is stirred at room temperature for 2 hours. To the reaction solution is added 6N HCl (0.85 ml), and then the mixture is extracted with ethyl acetate and the organic layer is concentrated under reduced pressure. The resulting residue is purified by column chromatography (silica gel; chloroform:methanol=1:0→19: 1) to give (2R,4S)-4-([3,5-bis(trifluoromethyl)benzyl]-{5-[(2-carboxyethyl)carbamoyl]pyrimidin-2-yl})amino-2-ethyl-6-methoxy-3,4-dihydro-2H-[1,5]naphthyridine-1-carboxylic acid ethyl ester (190 mg). MS (m/z): 699 [M+H]+.
