A carbon tetrachloride solution (25 ml) of tert-butyl 4-(7-chloro-3,3-dimethyl-2-oxo-3,4-dihydro-1,5-naphthyridin-1(2H)-yl)piperidine-1-carboxylate (Reference Example 10-2) (1.0 g), N-bromosuccinimide (903 mg) and 2,2′-azobisisobutyronitrile (43.2 mg) was heated at reflux for 40 minutes. After the reaction solution was cooled to 0° C., diethylether was added, and filtered through Celite. The filtrate was concentrated under reduced pressure, and the resulting residue was purified by silica gel column chromatography to afford tert-butyl 4-(4-bromo-7-chloro-3,3-dimethyl-2-oxo-3,4-dihydro-1,5-naphthyridin-1(2H)-yl)piperidine-1-carboxylate (492 mg).
