To a mixture of 2-ethyl-1-(5-methoxy-3-methyl-1-benzofuran-2-yl)butan-1-one (800 mg) synthesized above, methyl 4-aminobenzoate (464 mg), triethylamine (3.43 mL) and methylene chloride (10 mL) was added titanium (IV) chloride (404 μL), and the mixture was stirred overnight at room temperature under argon atmosphere. Saturated aqueous sodium hydrogen carbonate solution was added to quench the reaction, methylene chloride was evaporated in an evaporator, and the residue was extracted with ethyl acetate. The extract was washed with saturated brine, dried over magnesium sulfate, and concentrated under reduced pressure to give a dark brown oil. To a solution (20 mL) of the obtained oil in tetrahydrofuran were added acetic acid (882 μL) and sodium cyanoborohydride (386 mg), and the mixture was stirred at room temperature for 1.5 hr. Saturated aqueous sodium hydrogen carbonate solution was added to quench the reaction, and the reaction mixture was extracted with ethyl acetate. The extract was washed with saturated brine, dried over magnesium sulfate, and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (20% ethyl acetate/hexane) to give the title object compound (718 mg, 59%) as a brown oil.
