b) Synthesis of 1-methyl-1H-indazole-3-carboxaldehyde: 1-methyl-1H-indazole-3-methanol (0.320 g, 1.97 mmol, prepared from step a) was dissolved in 25 mL of dichloromethane (DCM), followed by the addition of Dess-Martin periodontane (0.920 g, 2.17 mmol). The reaction mixture was stirred at room temperature for 30 minutes. Upon completion of the reaction, 150 mL of diethyl ether was added to dilute, and 50 mL of 2 M sodium hydroxide solution was added to hydrolyze the suspension, and stirring was continued for 10 min. The organic layer was separated, washed sequentially with 1 M sodium hydroxide solution and water, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The crude product was purified by a pre-populated silica gel column (Isolute, 10 g) with the eluent dichloromethane: methanol (98:2, v/v). The target product 1-methyl-1H-indazole-3-carbaldehyde was obtained in a yield of 0.271 g and 86%. The structure of the product was confirmed by 1H NMR (300 MHz, CDCl3): δ 10.21 (s, 1H), 8.29 (m, 1H), 7.50-7.43 (m, 2H), 7.36 (m, 1H), 4.18 (s, 3H).
