6-[(4R)-4-Amino-2-oxopyrrolidin-1-yl]-2H-1,4-benzoxazin-3(4H)-one (23.9 mg, 0.0967 mmol) and 3-(7-methoxy-2-oxoquinoxalin-1(2H)-yl)propanal (Reference Example 8; 22.4 mg, 0.0967 mmol) were dissolved in N,N-dimethylfomamide (0.5 ml). Sodium sulfate (100 mg) was added to the obtained solution, and the mixture was then stirred at room temperature overnight. Thereafter, sodium triacetoxy borohydride (30.7 mg, 0.145 mmol) was added to the reaction solution, and the obtained mixture was then stirred at room temperature for 2 days. After completion of the reaction, dichloromethane and a saturated sodium hydrogen carbonate aqueous solution were added to the reaction solution, and the organic layer was separated. The organic layer was washed with water (4×) and saturated sodium chloride solution, and was then dried over anhydrous magnesium sulfate. Thereafter, the solvent was removed under reduced pressure. The obtained residue was purified by preparative thin layer chromatography (chloroform/methanol) to yield 18.4 mg (41%) of the above captioned compound in the form of a light brown solid.
