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General procedure for the synthesis of 2-bromoquinoline-4-carboxylic acid from 2-quinolone-4-carboxylic acid: a mixture of 2-hydroxyquinoline-4-carboxylic acid (4.0 g, 21.2 mmol) and phosphorus tribromide (25.0 g, 87.2 mmol) in toluene (40 mL) was heated and refluxed for 3 hours at 100°C. Upon completion of the reaction, the mixture was cooled to room temperature and carefully poured into crushed ice. The mixture was extracted with ethyl acetate (2 x 250 mL), the organic phases were combined, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was dissolved in 1N sodium hydroxide solution (150 mL) and extracted with ethyl acetate (2 x 100 mL). The aqueous phase was adjusted to pH 3 with 1N hydrochloric acid and a white solid was precipitated. The solid was collected by filtration, washed with water and dried to give 2-bromoquinoline-4-carboxylic acid (3.0 g, 56% yield) as a white solid.

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[3] Patent: EP2325181, 2011, A1. Location in patent: Page/Page column 61