[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:16])=[C:11]([CH3:15])[C:12]=3[F:14])[C:7](=[O:17])[C:6]([C:18]([O:20]CC)=[O:19])=[CH:5]2)[CH2:3][CH2:2]1.C(O)(=O)C.S(=O)(=O)(O)O>O>[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:16])=[C:11]([CH3:15])[C:12]=3[F:14])[C:7](=[O:17])[C:6]([C:18]([OH:20])=[O:19])=[CH:5]2)[CH2:2][CH2:3]1A mixture of 30 g of ethyl 1-cyclopropyl-6,8-difluoro-7-methyl-1,4-dihydro-4-oxo-3-quinolinecarboxylate, 210 ml of glacial acetic acid, 160 ml of water and 23.5 ml of concentrated sulphuric acid is heated under reflux for 2 hours. The hot suspension is then poured onto ice and the precipitate is filtered off with suction, rinsed thoroughly with water and dried in vacuo at 100° C. 25.9 g of pure 1-cyclopropyl-6,8-difluoro-7-methyl-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid of melting point 227°-229° C. are obtained in this manner.
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