[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:15])=[C:11]([F:14])[CH:12]=3)[C:7](=[O:16])[C:6]([C:17]([O:19]CC)=[O:18])=[CH:5]2)[CH2:3][CH2:2]1>S(=O)(=O)(O)O>[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:15])=[C:11]([F:14])[CH:12]=3)[C:7](=[O:16])[C:6]([C:17]([OH:19])=[O:18])=[CH:5]2)[CH2:2][CH2:3]1A suspension of 7.85 g of 1-cyclopropyl-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid, ethyl ester in 220 ml of 2N sulfuric acid was heated at 100° C. overnight, then cooled in an ice bath. The solid was collected, washed with water and dried, giving 6.81 g of the desired compound, mp 292°-293° C.
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