6-(4-Amino-2-oxopyrrolidin-1-yl)-2H-1,4-benzoxazin-3(4H)-one (Reference Example 6; 106 mg, 0.431 mmol) and 3-(7-methoxy-2-oxoquinoxalin-1(2H)-yl)propanal (100 mg, 0.431 mmol) were dissolved in N,N-dimethylformamide (2 ml), and the mixture to which sodium sulfate (400 mg) was added was stirred overnight at room temperature. The reaction solution was diluted with dichloromethane (12 ml), sodium triacetoxyborohydride (110 mg, 0.517 mmol) was added thereto and the mixture was stirred at room temperature for 3 hours. After completion of the reaction, a saturated sodium hydrogen carbonate aqueous solution was added thereto, the organic layer was separated, washed sequentially with saturated sodium chloride solution, water (×3) and saturated sodium chloride solution, dried over anhydrous magnesium sulfate, and the solvent was removed under reduced pressure. The obtained residue was purified by silica gel column chromatography (chloroform/methanol) to yield 193 mg (97%) of the title compound in the form of a light brown solid.
