The compound (1.26 g) obtained in (1) was dissolved in THF (50 mL), and cooled to −78° C. while stirring under nitrogen stream. To the present solution was added dropwise n-butyl lithium (2.76 M hexane solution, 1.82 mL), followed by stirring at the same temperature for 30 minutes. Then, N-methoxy-N-methylacetamide (1.05 mL) was added, and stirring was continued for 30 minutes. A saturated aqueous ammonium chloride solution was added to the reaction solution to terminate the reaction, followed by extraction with ethyl acetate. The extract was washed with saturated brine, dried over anhydrous sodium sulfate, and subsequently the solvent was distilled off under reduced pressure. The resulting residue was purified by flash chromatography (ethyl acetate/hexane=5%→70%) using silica gel column (product name: Hi-Flash Column 3 L, manufactured by Yamazen Corporation) to afford the title compound (0.75 g).
