[CH2:1]([O:3][C:4]([C:6]1[CH:7]=[N:8][C:9]2[C:14]([C:15]=1[OH:16])=[CH:13][C:12]([F:17])=[C:11]([F:18])[CH:10]=2)=[O:5])[CH3:2].C(=O)([O-])[O-].[K+].[K+].[CH2:25](I)[CH3:26]>CN(C)C=O>[CH2:25]([N:8]1[C:9]2[C:14](=[CH:13][C:12]([F:17])=[C:11]([F:18])[CH:10]=2)[C:15](=[O:16])[C:6]([C:4]([O:3][CH2:1][CH3:2])=[O:5])=[CH:7]1)[CH3:26]A 20 g portion of the above ester was suspended in 400 ml of dimethylformamide, 27.3 g of potassium carbonate was added and this mixture was stirred at 90° C. A 61.8 g portion of ethyl iodide was added and this mixture was stirred at 90° C. for 24 hours and then evaporated. The residue was dissolved in water and then extracted with dichloromethane. The extract was washed with water, dried, filtered and evaporated in vacuo. The residue was recrystallized from ethanol, giving 16.15 g of 1-ethyl-6,7-difluoro-1,4-dihydro-4-oxo-3-quinolinecarboxylic acid, ethyl ester.
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