C(OC([CH:6]1[CH2:15][N:14]([CH3:16])[C:13]2[N:12]=[C:11]3[CH:17]=[C:18]([F:22])[C:19]([F:21])=[CH:20][C:10]3=[CH:9][C:8]=2[C:7]1=[O:23])=O)C.OO.S([O-])([O-])(=O)=S.[Na+].[Na+].[CH2:33]([OH:35])[CH3:34]>O.[I-].[K+]>[CH2:33]([O:35][C:6]1[C:7](=[O:23])[C:8]2[CH:9]=[C:10]3[CH:20]=[C:19]([F:21])[C:18]([F:22])=[CH:17][C:11]3=[N:12][C:13]=2[N:14]([CH3:16])[CH:15]=1)[CH3:34]A solution of 3.78 g of potassium iodide in 20 cm3 of water is added with stirring at approximately 20° C. to a suspension of 71 g of 3-ethoxycarbonyl-7,8-difluoro-1-methyl-4-oxo-1,2,3,4-tetrahydrobenzo[b][1,8]naphthyridine in 1,000 cm3 of ethanol. The suspension is heated to 77° C. and 30 cm3 33% by weight hydrogen peroxide are added in the course of 60 minutes at this temperature. The reaction mixture is kept refluxing for a further 30 minutes and is then cooled to approximately 20° C. A solution of 11.4 g of sodium thiosulphate in 50 cm3 of water is introduced in the course of 5 minutes at this temperature. The precipitate obtained is drained at approximately 20° C. and washed with twice 300 cm3 of water. The wet product obtained is dried under reduced pressure (20 kPa) at approximately 60° C. 73 g of 3-ethoxy-7,8-difluoro-1-methyl-4-oxo-1,4-dihydrobenzo[b][1,8]naphthyridine are isolated in the form of a white solid, melting point above 270° C.
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