C(OC([C:6]1[C:7]([N:18]([CH3:26])[CH2:19][CH2:20][C:21]([O:23]CC)=O)=[N:8][C:9]2[C:14]([CH:15]=1)=[CH:13][C:12]([F:16])=[C:11]([F:17])[CH:10]=2)=O)C.[CH3:27][CH2:28][O-:29].[Na+].[C:31](O)(=[O:33])C.O>C(O)C>[CH2:28]([O:29][CH:20]1[C:19](=[C:31]=[O:33])[N:18]([CH3:26])[C:7]2[N:8]=[C:9]3[CH:10]=[C:11]([F:17])[C:12]([F:16])=[CH:13][C:14]3=[CH:15][C:6]=2[C:21]1=[O:23])[CH3:27]A solution of 94 g of 3-ethoxycarbonyl-6,7-difluoro-2-[N-methyl-N-(β-ethoxycarbonylethyl)amino]quinoline in 300 cm3 of absolute ethanol is added in the course of 80 minutes to a solution of 26.6 g of sodium ethylate brought to reflux in 900 cm3 of absolute ethanol. The suspension obtained, still refluxing, is stirred for a further 15 minutes. 38 cm3 of glacial acetic acid are then introduced in the course of 30 minutes. The reaction mixture is stirred for a further 15 minutes and, with the mixture still refluxing, 500 cm3 of water are then introduced in the course of 45 minutes. The suspension obtained is cooled to approximately 20° C. The precipitate is drained at approximately 20° C. and washed with twice 300 cm3 of water. The wet product is dried under reduced pressure (20 kPa) at approximately 60° C. 71.5 g of 3-ethoxy-carbonyl-7,8-difluoro-1-methyl-4-oxo-1,2,3,4-tetrahydrobenzo[b][1,8]naphthyridine are isolated in the form of a yellow solid, melting point 188° C.
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