Diethylaminosulfur trifluoride (13.2 μL) was added to a solution of (E)-(6S,8R,9aR)-6-(3,4,5-trifluorophenyl)-8-hydroxy-3-[3-methoxy-4-(4-methyl-1H-imidazol-1-yl)benzylidene]octahydroquinolizin-4-one that is an optically active compound obtained above with a retention time of 4.8 minutes (10 mg) in dichloromethane (2.0 mL) under ice-cooling, and the reaction solution was stirred for two hours. Crushed ice, water, and ethyl acetate were added to the reaction solution, and the organic layer was separated. The resulting organic layer was washed with brine, dried over anhydrous magnesium sulfate, and then concentrated under reduced pressure. The residue was purified by silica gel column chromatography (elution solvent: heptane-ethyl acetate system->ethyl acetate-methanol system) to obtain 5.9 mg of the title optically active compound. The property values of the optically active compound are as follows.
