General procedure for the synthesis of 4-bromo-2-furancarboxylic acid from 4,5-dibromo-2-furoic acid: 10.0 g (37.1 mmol) of 4,5-dibromofuran-2-carboxylic acid was dissolved in a 7.3% aqueous ammonia solution and the reaction system was cooled down to 0°C. The reaction was carried out by adding 2.54 g (38.9 mmol) of zinc powder. Under vigorous stirring conditions, 2.54 g (38.9 mmol) of zinc powder was added in batches and the reaction temperature was controlled not to exceed 7 °C. The reaction mixture was continued to be stirred at 0 °C for 10 min. Subsequently, the pH was adjusted to acidic by dropwise addition of aqueous hydrochloric acid. The resulting suspension was extracted with ethyl acetate, the organic phases were combined and dried over anhydrous sodium sulfate. After filtration, the organic phase was concentrated under reduced pressure to afford 7.27 g (99% yield) of 4-bromo-2-furancarboxylic acid. The structure of the product was confirmed by 1H-NMR (400 MHz, DMSO-d6): δ = 13.5 (s, 1H, COOH), 8.17 (s, 1H, furan cyclic H), 7.40 (s, 1H, furan cyclic H). LC-MS analysis (Method 1): retention time Rt = 1.52 min; mass spectrum (ESI positive ion mode): m/z = 191 [M+ H]+.
