6-Bromo-2,3-dihydro-1H-inden-1-one (2.0 g, 9.5 mmol) was used as starting material and dissolved in chloroform (30 mL). Under stirring, methanesulfonic acid (6 mL) was slowly added dropwise to the reaction system. Subsequently, sodium azide (1.90 g, 9.5 mmol) was added in batches, and after addition, the reaction mixture was heated to reflux for 3 hours. After completion of the reaction, the mixture was cooled to room temperature and carefully poured into ice water. The aqueous phase was extracted with dichloromethane, the organic phases were combined and dried over anhydrous sodium sulfate. After filtration, the solvent was removed by concentration under reduced pressure to give the crude product. The crude product was purified by column chromatography (eluent: petroleum ether/ethyl acetate=2/1) to afford the target compound 7-bromo-3,4-dihydroquinolin-2(1H)-one (0.90 g) in 42% yield.
