N-Methylmorpholine (525 μl), hydroxybenzotriazole (647 mg) and L-phenylalaninol (491 mg) are added to a solution of {(3RS)-4-acetyl-3-isopropyl-2-oxo-6-phenyl-1,2,3,4-tetrahydropyrazin-1-yl}acetic acid (1.01 g, Reference compound No. 51-1) in methylene chloride (25 ml). The mixture is cooled with ice, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is added to the mixture, and the whole is stirred for three days. Ethyl acetate is added to the reaction mixture, and the whole is washed with a 0.1 N aqueous sodium hydroxide solution, saturated brine, 1 N hydrochloric acid and saturated brine successively. The organic layer is dried over anhydrous magnesium sulfate and concentrated under reduced pressure, and the resulting residue is purified by silica gel column chromatography to give the titled compound (1.37 g).
