[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:17])=[C:11]([F:16])[C:12]=3[C:14]#[CH:15])[C:7](=[O:18])[C:6]([C:19]([O:21]CC)=[O:20])=[CH:5]2)[CH2:3][CH2:2]1.O.S(=O)(=O)(O)O>C(O)(=O)C>[CH:1]1([N:4]2[C:13]3[C:8](=[CH:9][C:10]([F:17])=[C:11]([F:16])[C:12]=3[C:14]#[CH:15])[C:7](=[O:18])[C:6]([C:19]([OH:21])=[O:20])=[CH:5]2)[CH2:3][CH2:2]110.3 g of ethyl 1-cyclopropyl-8-ethinyl-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylate are refluxed for 4 hours in a mixture of 100 ml of glacial acetic acid, 8 ml of water and 3 ml of concentrated sulphuric acid. After cooling to room temperature, the solid is filtered off with suction, washed with water and dried. In this way, 5.7 g of 1-cyclopropyl-8-ethinyl-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid (62% of theory) are obtained.
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