[Br:1][C:2]1[C:11]([CH2:12]O)=[C:10]2[C:5]([C:6](=[O:22])[C:7]([C:17]([O:19][CH2:20][CH3:21])=[O:18])=[CH:8][N:9]2[CH:14]2[CH2:16][CH2:15]2)=[CH:4][C:3]=1[F:23].C(N(S(F)(F)[F:30])CC)C.C(=O)([O-])O.[Na+]>C(Cl)Cl>[Br:1][C:2]1[C:11]([CH2:12][F:30])=[C:10]2[C:5]([C:6](=[O:22])[C:7]([C:17]([O:19][CH2:20][CH3:21])=[O:18])=[CH:8][N:9]2[CH:14]2[CH2:16][CH2:15]2)=[CH:4][C:3]=1[F:23]In 127 ml of methylene chloride was suspended 1.27 g of ethyl 7-bromo-1-cyclopropyl-6-fluoro-8-hydroxymethyl-1,4-dihydro-4-oxoquinoline-3-carboxylate, and 1.33 g of diethylaminosulfur trifluoride was dropwise added to the suspension at -40° C., after which the resulting mixture was stirred at -20° C. for one hour and then at 0° C. for three hours. The reaction mixture was added to 120 ml of ice water and the pH was adjusted to 8 with a saturated aqueous sodium hydrogencarbonate solution. Thereafter, the organic layer formed was separated, washed with a saturated saline solution and then dried over anhydrous magnesium sulfate. The solvent was then removed by distillation under reduced pressure. The residue thus obtained was purified by a column chromatography (eluent: chloroform:ethanol=100:1), to obtain 1.05 g of colorless, crystalline ethyl 7-bromo-1-cyclopropyl-6-fluoro-8-fluoromethyl-1,4-dihydro-4-oxoquinoline-3-carboxylate.
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