General procedure for the synthesis of 3-bromofuran-2-carboxylic acid from 3-bromofuran-2-carbaldehyde:
A) To a solution of tert-butanol (250 mL) containing 3-bromofuran-2-carboxaldehyde (25.0 g) and 2-methyl-2-butene (45.4 mL) was added slowly and dropwise at room temperature a mixed solution prepared from 80% sodium chlorite (40.5 g) and sodium dihydrogen phosphate (51.5 g) in water (350 mL). The reaction mixture was stirred overnight and acidified by the addition of 1 M hydrochloric acid (300 mL), followed by extraction with ethyl acetate. The organic phase was washed with saturated brine, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was washed with hexane/ethyl acetate (5/1, v/v) and recrystallized from the target product 3-bromofuran-2-carboxylic acid (20.3 g) by hexane/ethyl acetate solvent mixture. The product was characterized by 1H NMR (300 MHz, CDCl3): δ 6.66 (1H, d, J=1.9 Hz), 7.58 (1H, d, J=1.9 Hz).
