[F:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][C:11]=1[F:12])[N:8]([C:13]1[CH:18]=[CH:17][C:16]([O:19][CH3:20])=[CH:15][CH:14]=1)[CH:7]=[C:6]([C:21]([O:23]CC)=[O:22])[C:5]2=[O:26].Cl>C(O)(=O)C>[F:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][C:11]=1[F:12])[N:8]([C:13]1[CH:14]=[CH:15][C:16]([O:19][CH3:20])=[CH:17][CH:18]=1)[CH:7]=[C:6]([C:21]([OH:23])=[O:22])[C:5]2=[O:26]To ethyl 6,7-difluoro-1-(4 -methoxyphenyl)-1,4-dihydro-4-oxoquinoline-3-carboxylate (0.61 g) are added 90% acetic acid (8.0 ml) and conc. hydrochloric acid (2.0 ml), and the mixture is heated with stirring at 110° C. for 2 hours. After the reaction, the solvent is distilled off under reduced pressure. The resulting residue is washed well with water to give 6,7-difluoro-1-(4-methoxyphenyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (0.54 g) as colorless needles, m.p. 256°-258° C.
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