Sodium hydride → 6-bromo-3-methyl-2,3-dihydro-1H-indol-2-one

Sodium hydride (3.9 g) was suspended in dimethyl sulfoxide (24 mL), and diethyl methyl malonate (16 mL) was added at 0° C., and then the reaction mixture was stirred at room temperature for 1.5 hours. The reaction mixture was heated to 100° C., and a solution of 2,5-dibromonitrobenzene (15.3 g) in dimethyl sulfoxide (17 mL) was added at 100° C., and then the reaction mixture was stirred at 100° C. for 5 hours. The reaction mixture was quenched with water, and extracted with ethyl acetate. The obtained organic layer was washed with brine, dried over anhydrous sodium sulfate, filtered, and the filtrate was concentrated under reduced pressure. The residue was dissolved in ethanol (75 mL), and tin (11.5 g) was added at room temperature. Concentrated hydrochloric acid (45 mL) was added at 0° C., and then the reaction mixture was heated at reflux for 2 hours. After cooling to room temperature, the reaction mixture was quenched with water, and extracted with ethyl acetate. The obtained organic layer was washed with a saturated aqueous solution of sodium hydrogen carbonate and brine, dried over anhydrous sodium sulfate, filtered, and the filtrate was concentrated under reduced pressure. The obtained residue was purified by silica gel column chromatography to give the titled compound (4.9 g) as a brown solid.