C([O:4][CH2:5][C:6](=O)[CH2:7][S:8][C:9]1[C:18]([C:19]([O:21]CC)=[O:20])=[C:17]([OH:24])[C:16]2[C:11](=[C:12]([F:27])[C:13]([F:26])=[C:14]([F:25])[CH:15]=2)[N:10]=1)(=O)C>S(=O)(=O)(O)O>[F:25][C:14]1[CH:15]=[C:16]2[C:11](=[C:12]([F:27])[C:13]=1[F:26])[N:10]1[C:6]([CH2:5][OH:4])=[CH:7][S:8][C:9]1=[C:18]([C:19]([OH:21])=[O:20])[C:17]2=[O:24]Concd. sulfuric acid (300 ml) was added to ethyl 2-(3-acetoxy-2-oxopropylthio)-6,7,8-trifluoro-4-hydroxyquinoline-3-carboxylate (36.0 g) and the mixture was stirred at room temperature for 45 minutes and then at 85° C. for 19 hours. After adding ice-water (250 g) with cooling, the mixture was stirred at room temperature for 3 hours and the reaction mixture was then poured into ice-water. After precipitated crystal was filtered and washed with water, the resultant was recrystallized from a mixed solvent of chloroform-methanol to give the title compound (14.2 g) as light brown crystals, m.p. 220° C. (dec.).
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