Under nitrogen atmosphere, trimethylaluminium (1.03 M n-hexane solution, 1.80 ml, 1.85 mmol) was dropwise added at room temperature to a dichloromethane (3 ml) solution of N-methylphenethylamine (269 μl, 1.85 mmol), and stirred for 25 minutes. Subsequently, a dichloromethane (3 ml) solution of ethyl 4-cyano-7-fluoro-5-methyl-6-phenyl-1,3-benzoxazole-2-carboxylate (I-111) (300 mg, 925 μl) was dropwise added and stirred for 14.5 hours. After the reaction, aqueous 1 N hydrochloric acid solution was added to the reaction liquid with cooling with ice, followed by stirring at room temperature, and the reaction liquid was extracted with chloroform.
