General procedure for the synthesis of 3-bromoindole-5-carboxylic acid from indole-5-carboxylic acid: to a solution of indole-5-carboxylic acid (1.0 g, 6.2 mmol) in N,N-dimethylformamide (DMF, 10 mL) was added bromine (Br2, 334 μL, 1.05 eq.) and the reaction was carried out at room temperature (see Scheme 9). After the reaction was complete (about 5 min), the reaction mixture was poured into ice-cold sodium sulfite (Na2SO3, 1% aqueous solution, 100 mL) to precipitate the product. The precipitate was collected by filtration and dried under high vacuum. The resulting product was a light beige solid (1.46 g, 98% yield) and could be used in subsequent reactions without further purification. The structure of the product was confirmed by 1H NMR (DMSO-d6): δ 7.47 (d, 1H, J = 8.6 Hz), 7.65 (d, 1H, J = 2.5 Hz), 7.76 (dd, 1H, J = 7.3, 1.3 Hz), 8.04 (s, 1H), 11.77 (s, 1H), 12.57 (s, 1H).
