[OH-].[Na+].[F:3][C:4]1[CH:5]=[C:6]2[C:11](=[C:12]([O:15][CH3:16])[C:13]=1[F:14])[N:10]([NH:17][CH2:18]C=O)[CH:9]=[C:8]([C:21]([O:23]CC)=[O:22])[C:7]2=[O:26].C(O)(=O)C>C(O)C>[F:3][C:4]1[CH:5]=[C:6]2[C:11](=[C:12]([O:15][CH3:16])[C:13]=1[F:14])[N:10]([NH:17][CH3:18])[CH:9]=[C:8]([C:21]([OH:23])=[O:22])[C:7]2=[O:26]4.5 ml (0.0045 mole) of a 1N aqueous solution of sodium hydroxide were added to a solution of the whole (0.0015 mole) of the ethyl 6,7-difluoro-1-(N-formylmethylamino)-8-methoxy-1,4-dihydro-4-oxoquinoline-3-carboxylate (XVII, R10 =methyl) [prepared as described in Step (D3) above] dissolved in 10 ml of ethanol, and the mixture was stirred at room temperature for 2 hours. At the end of this time, addition of acetic acid gave rise to crystals, which were collected by filtration to give 0.35 g of 6,7-difluoro-8-methoxy-1-(methylamino)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (IIb, R10 =methyl) as a colorless powder.
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