[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:16])=[C:11]([F: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]([F: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 94 g of ethyl 1-cyclopropyl-6,7,8-trifluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylate, 600 ml of glacial acetic acid, 450 ml of water and 70 ml of concentrated sulphuric acid is heated to reflux for 1.5 hours. The hot suspension is then poured onto ice, and the precipitate is filtered off with suction, thoroughly washed with water, and dried in vacuo at 100° C. In this manner, 88.9 g of pure 1-cyclopropyl-6,7,8-trifluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid of melting point 228°-230° C. (decomposition) are obtained.
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