4.27 g of N-(tert-butoxycarbonyl)aspartic acid-4-benzyl ester (manufactured by PEPTIDE INSTITUTE, INC.) and 2.1 mL of 1-phenylbutyl amine were dissolved in 40 mL of DMF, and then 2.70 g of WSC hydrochloride and 1.93 g of HOBt were added thereto, and the reaction solution was stirred at room temperature for 6 hours. The reaction solution was added with water, and extracted with ethyl acetate. The organic layer was washed with a saturated aqueous solution of sodium hydrogen carbonate, dried with magnesium sulfate, and then distilled under reduced pressure to remove ethyl acetate, and then dried under vacuum, thereby to give 3.94 g of Compound 1.
