To a tetrahydrofuran solution (10 ml) of tert-butyl 4-(7-chloro-2-oxo-3,4-dihydro-1,5-naphthyridin-1(2H)-yl)piperidine-1-carboxylate (Reference Example 9-3) (500 mg) were added at 0° C. sodium hydride (161 mg) and diethyl carbonate (0.66 ml). After the reaction solution was stirred at 70° C. for 4 hours, methyl iodide (0.171 ml) was added at 0° C., and stirred at room temperature for 6 hours. Water was added to the reaction solution at 0° C., and extracted with ethyl acetate. The collected organic layer was washed with saturated sodium chloride solution, dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography to afford ethyl 1-[1-(tert-butoxycarbonyl)piperidin-4-yl]-7-chloro-3-methyl-2-oxo-1,2,3,4-tetrahydro-1,5-naphthyridine-3-carboxylate (286 mg).
