3-(t-Butoxycarbonylamino)propionic acid (1.0 g, 5.3 mmol) was dissolved in anhydrous tetrahydrofuran (15 ml), N-methylmorpholine (0.6 ml, 5.5 mmol) was added to the solution, the mixture was stirred at −15° C., and isobutyl chlorocarbonate (0.7 ml, 5.4 mmol) was added to the mixture. Then a solution of a free base of 2-(1-adamantyl)-N-pentylethylamine hydrochloride (Intermediate No. 1-1, 1.5 g, 5.3 mmol) in anhydrous tetrahydrofuran (15 ml) was added thereto at −18° C. The whole was stirred at 0° C. for 1.5 hours, then ethyl acetate (100 ml) and a saturated aqueous sodium hydrogencarbonate solution (100 ml) were added to the reaction mixture to distribute it. The organic layer was washed with a 10% aqueous citric acid solution (100 ml), water (100 ml) and a saturated aqueous sodium chloride solution (100 ml) successively, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. Then the obtained concentrate was purified by silica gel column chromatography to give 3-(t-butoxycarbonylamino)propionic acid N-[2-(1-adamantyl)ethyl]-N-pentylamide (1.9 g, 85%) as an oily matter.
