Methyl (2S,3S)-7-fluoro-2-(4-fluorophenyl)-3-(1-methyl-1H-1,2,4-triazol-5-yl)-4-oxo-1,2,3,4-tetrahydroquinoline-5-carboxylate (36 g, 90.37 mmol) was used as a raw material, which was added with ethanol (450 mL) and 50% hydrazine hydrate (28.96 g, 452 mmol) to the reactor and heated to reflux for 3 hours. After completion of the reaction, the solvent was removed by distillation under reduced pressure. The crude product was sequentially washed with water, filtered, and then washed with ethanol, and finally purified by recrystallization to afford the target compound (8S,9R)-5-fluoro-8-(4-fluorophenyl)-9-(1-methyl-1H-1,2,4-triazol-5-yl)-8,9-dihydro-2H-pyrido[4,3,2-de]phthalazin-3(7H)-one (tarazolba, white solid, 31 g , 90.2% yield). The product was 99.5% pure by HPLC. m/z LC-MS (ESI): 381 (M + 1)+. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 3.68 (s, 3H), 4.99-5.06 (m, 2H), 6.92-6.96 (m, 1H), 7.08-7.11 (m, 1H). 7.16-7.20 (t, J = 8.8Hz, 2H), 7.49-7.53 (m, 2H), 7.75 (s, 1H), 7.83 (s, 1H), 12.35 (s, 1H).