Methyl 5-bromo-1H-indazole-3-carboxylate → methyl 5-formyl-1H-indazole-3-carboxylate

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A mixture of 5-bromo-1H-indazole-3-carboxylic acid methyl ester (1 g, 3.92 mmol), HCOONa (400 mg, 5.88 mmol), and PdCl2(PPh3)2 (138 mg, 0.2 mmol) in DMF (10 mL) was put under vacuum, and charged with CO. This process was repeated three times, and the mixture was kept at 110° C. for 6 hr. The reaction mixture was cooled to rt, diluted with EtOAc and water, and extracted. The organic phase was dried and concentrated. Purification via flash chromatography afforded 5-formyl-1H-indazole-3-carboxylic acid methyl ester (430 mg, 54%).

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