A) Preparation of intermediate 1H-indazole-5-carboxaldehyde: 2.60 g (65.00 mmol, 60% dispersed in mineral oil) of sodium hydride was added batchwise to 150 mL of tetrahydrofuran (THF) solution containing 11.64 g (59.08 mmol) of 5-bromo-1H-indazole under argon gas protection, and the addition process lasted for 10 minutes. After the addition was completed, the reaction mixture was stirred for 15 min at room temperature. Subsequently, the reaction mixture was cooled to -70°C and 100.00 mL (130.00 mmol, 1.3 M in cyclohexane) of sec-butyllithium solution was slowly added dropwise over 30 min, keeping the reaction temperature below -60°C. After the dropwise addition was completed, stirring was continued for 2 h at -70°C. The reaction mixture was then stirred for 10 min at room temperature. Then, 20.00 mL (0.260 mol) of N,N-dimethylformamide (DMF) dissolved in 20 mL of THF was added dropwise, keeping the reaction temperature below -50°C. The reaction mixture was then slowly warmed to room temperature and stirred for 16 hr. Upon completion of the reaction, the mixture was cooled to 0°C and 180 mL of aqueous 2N hydrochloric acid was slowly added dropwise and stirring was continued for 15 min. The pH was adjusted to 9-10 with saturated aqueous sodium bicarbonate. the aqueous phase was fully extracted with ethyl acetate (EtOAc) and the organic phases were combined. The organic phase was dried with magnesium sulfate and concentrated under reduced pressure. The product was purified by column chromatography (silica gel, petroleum ether/EtOAc 1:1 v/v), ground with hexane and dried. Drying under reduced pressure at 50°C gave the target product 1H-indazole-5-carbaldehyde. Yield: 4.40 g (51% yield). Thin layer chromatography (TLC) Rf = 0.37 (silica gel, petroleum ether/EtOAc 1:1 v/v). Electrospray ionization mass spectrometry (ESI-MS): (M + H)+ = 147.
