48% hydrobromic acid (2 mL) was added to a solution of the mixture of (5R)-5-[(E)-2-(4-tert-butylphenyl)-2-(6-methoxy-5-phenoxypyridin-2-yl)ethenyl]pyrrolidin-2-one and (5R)-5-[(E)-2-(4-tert-butylphenyl)-2-(6-methoxypyridin-2-yl)ethenyl]pyrrolidin-2-one (61 mg) in 1,4-dioxane (1 mL), and the mixture was stirred at 65° C. for 30 minutes. Water was added to the reaction solution, followed by extraction with ethyl acetate. The organic layer was washed with brine and dried over anhydrous magnesium sulfate, after which the solvent was evaporated under reduced pressure. The residue was purified by preparative HPLC (Waters Sunfire 19×150 mm 5 μm, rate: 20 mL/min, eluent: A=acetonitrile, B=0.1% trifluoroacetic acid solution, gradient: 10 to 90%) and solidified with ethyl acetate-hexane to give the title compound as a colorless amorphous (22 mg, 22% (two steps)). A crude product of (5R)-5-[(E)-2-(4-tert-butylphenyl)-2-(6-methoxypyridin-2-yl)ethenyl]pyrrolidin-2-one (15 mg) was also obtained.
