Methyl 3,5-diallyloxy-2-ethylphenylacetate (0.31 g, 1.3 mmol) obtained in Example 5, Step 3 was dissolved in trifluoroacetic acid (2.0 mL). After the solution was cooled to 4° C., benzoic acid (0.40 g, 3.3 mmol) and trifluoroacetic anhydride (1.0 mL) were added thereto, followed by stirring for 18 hours, while the temperature of the reaction mixture was raised to room temperature. The reaction mixture was gradually added to a saturated aqueous solution of sodium hydrogencarbonate for neutralization, followed by extraction with ethyl acetate. The organic layer was dried over anhydrous sodium sulfate and then concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (ethyl acetate/hexane=1/9-1/4) to obtain methyl 3,5-diallyloxy-2-benzoyl-6-ethylphenylacetate (0.29 g, 55%).
