Anisole (0.25 mL) was added to a solution of 6-{(E)-1-(4-tert-butylphenyl)-2-[(2R)-1-(2,4-dimethoxybenzyl)-5-oxopyrrolidin-2-yl]ethenyl}-3-ethylpyridin-2(1H)-one (49 mg) in trifluoroacetic acid (0.5 mL), and the mixture was stirred at 65° C. for seven hours. Trifluoroacetic acid (0.5 mL) was added and the mixture was stirred for further five hours, after which the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (chloroform:methanol=1:0→9:1). The resulting crystals (25 mg) were recrystallized from ethyl acetate-hexane to give the title compound as a colorless powder (20 mg).
