2-(Methoxymethoxy)-3-(7-methoxy-2-oxoquinoxalin-1(2H)-yl)propanal (0.340 mmol) and 6-(4-amino-2-oxopyrrolidin-1-yl)-2H-1,4-benzoxazin-3(4H)-one (Reference Example 6; 84 mg, 0.340 mmol) were dissolved in a mixed solvent of dichloromethane/N,N-dimethylformamide (10:1,11 ml). Acetic acid (0.039 ml, 0.680 mmol) and anhydrous sodium sulfate (200 mg) were added to this solution, and the mixture was stirred at room temperature for two and a half hours. Sodium triacetoxyborohydride (144 mg, 0.680 mmol) was added thereto, the mixture was stirred for 14 hours and then the solvent was removed under reduced pressure. Chloroform was added to the residue, the resultant was alkalized by adding saturated sodium bicarbonate water under cooling on ice and extracted with chloroform. After drying over anhydrous sodium sulfate, the solvent was removed under reduced pressure. The residue was purified by silica gel column chromatography (chloroform/methanol) to yield 115 mg (65%) of the title compound in the form of a colorless clear oily product.
