The general procedure for the synthesis of 3-formyl-5-carboxyindazole from indole-5-carboxylic acid was as follows: i) Preparation of 3-formyl-1H-indazole-5-carboxylic acid: indole-5-carboxylic acid (5.5 g, 0.0338 mol) was dissolved in water (250 ml), followed by the addition of sodium nitrite (NaNO2, 23.5 g, 0.338 mol) and a hydrochloric acid solution (6 N 42 ml, 0.293 mol). The reaction mixture was stirred at room temperature for 12 hours. Upon completion of the reaction, the precipitate was collected by filtration, washed with water (270 ml) and dried at 50 °C to afford 3-formyl-1H-indazole-5-carboxylic acid (5.36 g, 0.028 mol, 83% yield), which could be used in the subsequent steps without further purification.
