Trifluoromethanesulfonic acid (1.00 mL) was added to a solution of 2-[2-(3-nitrophenyl)cyclohex-1-enylmethyl]isothiourea hydrochloride obtained in Preparation Example 4-(4) (2.04 g) in TFA (10.0 mL) in an ice bath. The reaction solution was warmed to room temperature, followed by stirring overnight. Trifluoromethanesulfonic acid (1.00 mL) was further added to the reaction solution, followed by stirring for two days. After confirming completion of the reaction, the reaction mixture was carefully poured into a mixed solution of a saturated sodium bicarbonate solution and ether in an ice bath. The aqueous layer was extracted with ethyl acetate, and the organic layer was washed with a saturated sodium bicarbonate solution and a saturated sodium chloride solution. The organic layer was dried over anhydrous magnesium sulfate. The drying agent was removed by filtration and the filtrate was concentrated under reduced pressure to obtain a crude product. The resulting crude product was purified by NH-silica gel column chromatography to obtain the title compound (1.62 g).
