[NH2:1][C:2]1[C:11]([F:12])=[C:10]([F:13])[CH:9]=[C:8]2[C:3]=1[C:4](=[O:25])[C:5]([C:20]([O:22]CC)=[O:21])=[CH:6][N:7]2[CH:14]1[CH2:19][CH2:18][O:17][CH2:16][CH2:15]1>OS(O)(=O)=O.O.CC(O)=O>[NH2:1][C:2]1[C:11]([F:12])=[C:10]([F:13])[CH:9]=[C:8]2[C:3]=1[C:4](=[O:25])[C:5]([C:20]([OH:22])=[O:21])=[CH:6][N:7]2[CH:14]1[CH2:15][CH2:16][O:17][CH2:18][CH2:19]1A suspension of ethyl 5-amino-6,7-difluoro-1,4-dihydro-4-oxo-1-(tetrahydropyran-4-yl)-3-quinolinecarboxylate (70.0 mg, 0.199 mmol) in a mixture of H2SO4/H2O/AcOH (1:5:6, 3 mL) was heated under reflux for 1 h. The reaction mixture was poured into ice-water and the resulting precipitates were collected by filtration. The filtered precipitates were washed with water, and dried in vacuo to give 5-amino-6,7-difluoro-1,4-dihydro-4-oxo-1-(tetrahydropyran-4-yl)-3-quinolinecarboxylic acid (51.8 mg, 80%) as a yellow powder.
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