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

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A solution of 20 cm3 of 3-ethoxycarbonyl-6,7-difluoro-2-[N-cyclopropyl-N-(b-ethoxycarbonylethyl)amino]quinoline in 20 cm3 of absolute ethanol is added in the course of 60 minutes to a solution of 1.6 g of sodium ethylate brought to reflux in 40 cm3 of absolute ethanol. The solution obtained is stirred under reflux for a further 60 minutes. 2.6 cm3 of glacial acetic acid are then introduced in the course of 10 minutes. The reaction mixture is stirred for a further 15 minutes and, with the mixture still refluxing, 26 cm3 of water are then introduced in the course of 5 minutes. The suspension obtained is cooled to approximately 20° C. The precipitate is drained at approximately 20° C. and washed with twice 10 cm3 of water. The wet product is dried under reduced pressure (20 kPa) at approximately 60° C. 1.25 g of crude 3-ethoxycarbonyl-7,8-difluoro-1-cyclopropyl-4-oxo-1,2,3,4-tetrahydrobenzo[b][1,8]naphthyridine are isolated in the form of a yellow solid, melting point 172° C.

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