C(O[C:4]([C:6]1[C:7]([N:18]([CH3:23])[CH2:19][CH2:20][C:21]#[N:22])=[N:8][C:9]2[C:14]([CH:15]=1)=[CH:13][C:12]([F:16])=[C:11]([F:17])[CH:10]=2)=[O:5])C.CC([O-])(C)C.[K+].C(O)(=O)C.C(O)C.O>O1CCCC1>[C:21]([CH:20]1[CH2:19][N:18]([CH3:23])[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:4]1=[O:5])#[N:22]A solution of 19.17 g of 3-ethoxycarbonyl-6,7-difluoro-2-[N-methyl-N-(β-cyanoethyl)amino]quinoline in 50 cm3 of tetrahydrofuran is introduced in the course of 60 minutes into a solution, cooled to -10° C., of 8.74 g of potassium tert-butylate in 200 cm 3 of tetrahydrofuran. The suspension obtained is stirred, still at -10° C., for a further 30 minutes. 4 cm3 of glacial acetic acid are then .introduced. The tetrahydrofuran is evaporated off under reduced pressure (20 kPa). The crude reaction mixture is taken up with 200 cm3 of an aqueous-alcoholic ethanol/water (70/30 vol/vol) mixture. The precipitate obtained is filtered off, washed with twice 50 cm3 of water and then dried under reduced pressure (20 kPa). 16.1 g of 3-cyano-7,8-difluoro-1-methyl-4-oxo-1,2,3,4tetrahydrobenzo[b][1,8]naphthyridine are isolated in the form of a golden-yellow solid, melting point 144° C.
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