Methyl 7-methoxy-4-oxo-1,4-dihydroquinoline-6-carboxylate (3.00 g, 12.86 mmol) was added to thionyl chloride (30.00 mL). N,N-dimethylformamide (93.99 mg, 1.29 mmol) was then added to the reaction system. The reaction solution was heated to an external temperature of 90 °C under nitrogen protection and reacted under reflux conditions for 1 hour. The reaction process was monitored by thin layer chromatography (TLC). Upon completion of the reaction, the aqueous phase was combined and concentrated to dryness. The residue was dissolved in ice water (50 mL) and extracted with ethyl acetate (20 mL x 2). The aqueous phase was then extracted with dichloromethane (30 mL x 5). The dichloromethane phases were combined, washed with sodium chloride solution (20 mL × 2), dried over anhydrous sodium sulfate, and subsequently concentrated under reduced pressure by pumping to afford methyl 4-chloro-7-methoxyquinoline-6-carboxylate (2.60 g, 9.81 mmol) in 76.32% yield and 95% purity as a mixed solid. The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 3.87 (s, 3H), 3.98 (s, 3H), 7.60 (s, 1H), 7.66 (d, J = 4.77 Hz, 1H), 8.41 (s, 1H), 8.83 (d, J = 4.77 Hz, 1H).
