Acetic Acid → 3-Ethoxycarbonyl-7,8-difluoro-1-ethyl-4-oxo-1,2,3,4-tetrahydrobenzo[b][1,8]naphthyridine

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A solution of 68 g of 3-ethoxycarbonyl-6,7-difluoro-2-[N-ethyl N-(b-ethoxycarbonylethyl)amino]quinoline in 200 cm3 of absolute ethanol is introduced in the course of 60 minutes into a solution of 16.1 g of sodium ethylate brought to reflux in 600 cm3 of absolute ethanol. The suspension obtained, still refluxing, is stirred for a further 60 minutes. 20 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, 400 cm3 of water are then introduced in the course of 45 minutes. The suspension obtained is cooled to approximately 20° C. The precipitate obtained is drained at approximately 20° C. and washed with twice 200 cm3 of water. The wet product is dried under reduced pressure (20 kPa) at approximately 50° C. 52.4 g of 3-ethoxycarbonyl -7,8-difluoro-1-ethyl-4-oxo-1,2,3,4-tetrahydro-benzo[b][1,8]naphthyridine are isolated in the form of a golden-yellow solid, melting point 152° C.

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