Under a nitrogen atmosphere, to a suspension of 2.28 g of magnesium chloride in 50 mL of tetrahydrofuran was added 3.80 g of tert-butyl 3-oxobutanoate at room temperature. The mixture was cooled to −8° C., and 3.9 mL of pyridine was added thereto, followed by stirring at the same temperature for 30 minutes, warming to room temperature, and further stirring for 30 minutes. The mixture was cooled to −8° C., and a solution of 5.3 g of 4-bromo-2-nitrobenzoyl chloride in 20 mL of tetrahydrofuran was added thereto, followed by stirring at the same temperature for 1 hour, warming to room temperature, and further stirring for 1.5 hours. The mixture was poured into a mixture of water and ethyl acetate, and adjusted to pH 3 with concentrated hydrochloric acid. The aqueous layer was separated, and then the organic layer was washed with a saturated aqueous sodium hydrogen carbonate solution and saturated brine, and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure to obtain 4.95 g of tert-butyl 2-(4-bromo-2-nitrobenzoyl)-3 -oxobutanoate.
