The general procedure for the synthesis of 5-nitroindazole-3-carboxylic acid from 5-nitroindazole-3-aldehyde was as follows: potassium permanganate (KMnO4, 15 g, 94.91 mmol) was dissolved in 300 mL of water and added slowly and dropwise at room temperature to 1000 mL of acetone solution containing 52.35 mmol of raw material. The reaction mixture was stirred at room temperature for 6 hours. Subsequently, 50 mL of 30% hydrogen peroxide (H2O2) was added dropwise to neutralize the remaining potassium permanganate. The reaction mixture was filtered to remove insoluble matter and then concentrated by distillation under reduced pressure. After acidification of the concentrate, the precipitate was collected by filtration and recrystallized from methanol to give a final 6. yellow solid product; yield 79% (8.56 g); melting point: 191-193°C.
