Acetic acid (32 mg, 0.58 mmol) was added to a mixture of 3-(6,7-difluoro-2-oxoquinoxalin-1(2H)-yl)propanal (0.1 g, 0.45 mmol), 6-[(4R)-4-(aminomethyl)-2-oxopyrrolidin-1-yl]-2H-pyrido[3,2-b][1,4]oxazin-3(4H)-one (Reference Examples 54, 0.11 g, 0.45 mmol), molecular sieve (4 A, 0.1 g) and methanol (4 ml). The obtained mixture was stirred for 20 minutes, cooled on ice and sodium triacetoxyborohydride (0.11 g, 0.58 mmol) was gradually added thereto. Then, the mixture was stirred at room temperature for 4 hours, an aqueous solution of sodium hydrogen carbonate was added thereto and the mixture was extracted with ethyl acetate. The obtained crude product was purified by silica gel column chromatography (dichloromethane/methanol) to yield 15 mg (7%) of the title compound.
