6-(4-Hydroxy-3-methoxyphenyl)hex-5-ene-2,4-dione (20 mg, 85 μmol) and boron trioxide (11 mg, 0.16 mmol) was placed in a 20 mL reaction vessel, and dissolved in 0.4 mL of ethyl acetate. To the stirring mixture at 80° C. was added a solution of 3-hydroxy-2-methoxybenzaldehyde (16 mg, 0.11 mmol) and tri-n-butyl borate (25 μL, 93 μmol) in 0.7 mL of ethyl acetate. After the reaction mixture was stirred for 2 h at the same temperature, n-butylamine (10 μL, 0.10 mmol) was added with additional stirring for 1 h. The reaction mixture was treated with a 1:1 solution of 1N HCl and brine, and was stirred at 50° C. for 5 min to 1 h (if necessary, the reaction mixture was neutralized by saturated NaHCO3 aqueous solution). The organic layer was purified directly by silica gel column chromatography (eluting with hexane/ethyl acetate or chloroform/methanol) to obtain the title compound (10.2 mg, 32%) as a solid.
