In nitrogen atmosphere, trimethylsilyle iodide (0.188 mL) was added to a methylene chloride solution (6 mL) of (4R,9aS)-4-(3,4,5-trifluorophenyl)hexahydropyrido[2,1-c][1,4]oxazin-6-one (252 mg) and N,N,N′,N′-tetramethylethylenediamine (0.466 mL) at 0° C. The resulting reaction solution was stirred under ice-cooling for 30 min, and iodine (336 mg) was added thereto under ice-cooling. The resulting reaction solution was stirred under ice-cooling for 1 hr, and ethyl acetate and a saturated sodium thiosulfate aqueous solution were added thereto. The organic layer was separated, washed with saturated saline, dried over magnesium sulfate, and concentrated under reduced pressure to give a crude iodinated substance. To the given crude iodinated substance, triethyl phosphite (3 mL) was added. The resulting mixture was stirred at 120° C. for 2 hr, allowed to cool to room temperature, and then concentrated under reduced pressure to give 372 mg of the title compound. The physical property values of this compound were as follows:
