Under nitrogen atmosphere, methyl 4-acetylamino-5-bromo-2-tert-butyl-1-benzofuran-7-carboxylate (I-252) (147 mg, 0.4 mmol), phenylboronic acid (61 mg, 0.5 mmol), aqueous 2 M cesium carbonate solution (0.5 ml, 1.0 mmol) and tetrakistriphenylphosphine palladium(0) (23 mg, 0.02 mmol) were added, followed by stirring at 100° C. for 6 hours. After cooling to room temperature, ethyl acetate was added to the reaction liquid, followed by washing with saturated sodium bicarbonate water and saturated brine and drying over anhydrous magnesium sulfate. After filtration and concentration under reduced pressure, the resulting residue was subjected to column chromatography, and the eluate with n-hexane/ethyl acetate (2:1) gave the entitled compound (117 mg, 80%) as a white solid.
