In a mixed solvent (6 ml) of dichloromethane/N,N dimethylformamide (5:1) were dissolved 2-(methoxymethoxy)-3-(6-methoxy-3-oxopyrido[2,3-b]pyrazin-4(3H)-yl)propanal (as 0.217 mmol) and 6-[(4R)-4-(aminomethyl)-2-oxopyrrolidin-1-yl]-2H-pyrido[3,2-b][1,4]oxazin-3(4H)-one (Reference Example 54; 57 mg, 0.217 mmol). Acetic acid (0.025 ml, 0.434 mmol) and anhydrous sodium sulfate (100 mg) were added to the solution and the mixture was stirred at room temperature for 14 hours. Sodium triacetoxyborohydride (92 mg, 0.434 mmol) was added to the reaction solution, the mixture was stirred for 8 hours and then the solvent was removed under reduced pressure. A lower layer solvent of chloroform/methanol/water (7:3:1) was added to the residue, the resultant was alkalized by adding saturated sodium bicarbonate water under cooling on ice and extracted with a lower layer solvent of chloroform/methanol/water (7:3:1). After drying the extract with anhydrous sodium sulfate, the solvent was removed under reduced pressure and the residue was purified by silica gel column chromatography (chloroform/methanol) to yield 41 mg (35%) of the title compound in the form of an orange oily product.
