Oxonium, trimethyl-, tetrafluoroborate(1-) (1:1) → methyl 5-bromo-2-methyl-2H-indazole-4-carboxylate

To a solution of methyl 5-bromo-1H-indazole-4-carboxylate (4.50 g, 17.6 mmol) in ethyl acetate (176 mL) was added trimethyloxonium tetrafluoroborate (3.38 g, 22.9 mmol) at room temperature, and the mixture was stirred for 2.5 hr. The reaction solution was diluted with saturated aqueous sodium hydrogen carbonate solution, and the mixture was extracted with ethyl acetate. The extract was washed with saturated brine, and dried over anhydrous sodium sulfate. The solvent was evaporated under reduced pressure, and the residue was purified by silica gel column chromatography (ethyl acetate/hexane=5/95→30/70) to give the title compound (3.42 g, yield 73%).