127199-25-1

127199-27-3

Ethyl 8-chloro-6,7-difluoro-1-((1R,2S)-2-fluorocyclopropyl)-4-ethyl-1,4-dihydroquinoline-3-carboxylate (Compound 3, 12.4 g, 34.3 mmol) was used as raw material. Compound 3 was dissolved in a mixture of hydrochloric acid (125 mL) and acetic acid (125 mL). The reaction mixture was stirred at 120-130°C for 2 hours. After completion of the reaction, the reaction solution was diluted with deionized water (500 mL) and the precipitate was collected by filtration. The precipitate was washed sequentially with deionized water and ether and then dried under reduced pressure to afford the target product 8-chloro-6,7-difluoro-1-((1R,2S)-2-fluorocyclopropyl)-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (Compound 4, 8.72 g, 76.2% yield) as a colorless solid.