Sodium Dithionite → 2,7-Dichloro-6-fluoroquinoline-3-carboxylic acid

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Reaction Steps

A solution of 89.3 g of potassium permanganate in 1.4 liters of water is added in the course of 1 hour and while keeping the temperature between 10° and 14° C. to a stirred suspension, cooled to 10° C., of 69.5 g of 2,7-dichloro-6-fluoro-3-formyl-1,4-dihydroquinoline in 282 cm3 of 2 N aqueous potassium hydroxide solution and 282 cm3 of water. The temperature is allowed to rise to about 20° C. and the mixture is stirred for a further 30 minutes at this temperature. 26 g of sodium dithionite are added, the mixture is stirred for 10 minutes at a temperature close to 20° C. and filtered through diatomaceous silica for filtration and the filter cake is washed with twice 250 cm3 of water. The filtrate and the aqueous washing phases are combined and 90 cm3 of a 35% aqueous solution of hydrochloric acid are added. The precipitate formed is extracted with 4 times 500 cm3 of ethyl acetate. The combined organic extracts are washed with 3 times 500 cm3 of water, dried over magnesium sulphate, filtered and the filtrate is concentrated under reduced pressure (20 kPa) at 50° C. The residue is taken up in 350 cm3 of diethyl ether, drained and washed with twice 200 cm3 of diethyl ether. 45 g of 2,7-dichloro-6-fluoroquinoline-3-carboxylic acid are obtained in the form of a beige solid melting at 230° C. which is used for the subsequent steps without further purification.

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