[F:1][C:2]1[CH:3]=[C:4]2[C:9](=[C:10]([O:13][CH3:14])[C:11]=1[F:12])[N:8]([NH:15][CH:16]=O)[CH:7]=[C:6]([C:18]([O:20][CH2:21][CH3:22])=[O:19])[C:5]2=[O:23].[C:24](=O)([O-])[O-:25].[K+].[K+].CI>CN(C)C=O>[F:1][C:2]1[CH:3]=[C:4]2[C:9](=[C:10]([O:13][CH3:14])[C:11]=1[F:12])[N:8]([NH:15][CH2:16][CH:24]=[O:25])[CH:7]=[C:6]([C:18]([O:20][CH2:21][CH3:22])=[O:19])[C:5]2=[O:23]The whole (0.0018 mole) of the ethyl 6,7-difluoro-1-(formylamino)-8-methoxy-1,4-dihydro-4-oxoquinoline-3-carboxylate (XVI) [prepared as described in Step (D2) above] was dissolved in 8 ml of dimethylformamide. 0.51 g (0.0037 mole) of potassium carbonate and 0.78 g (0.0052 mole) of methyl iodide were then added to the solution, and the mixture was stirred at room temperature for 3 hours. At the end of this time, the solvent was distilled from the mixture under reduced pressure. The residue was extracted with chloroform, and the chloroform extract was washed with water, dried and concentrated by evaporation under reduced pressure to give 0.5 g of ethyl 6,7-difluoro-1-(N-formylmethylamino)-8-methoxy-1,4-dihydro-4-oxoquinoline-3-carboxylate (XVII, R10 =methyl) as a colorless powder.
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