The thus-obtained 2-{4-[2-(2,2,2-trifluoroethoxy) ethoxy]pyridin-2-yl-methylthio}-1H-benzimidazole (6.5 g) was dissolved in toluene (160 mL). Molecular Sieve 4A (6.5 g) and water (310 μL) were added, and the mixture was stirred for 15 minutes at room temperature. Subsequently, diethyl (+)-tartrate (7.0 g) and tetraisopropyl titanate (4.8 g) were added, and the mixture was stirred for one hour at 50° C. The temperature of the mixture was returned to room temperature. N,N-Diisopropylethylamine (3.0 mL) and 88% cumene hydroxyperoxide (2.8 mL) were added, and the mixture was stirred for three hours at room temperature. A saturated sodium bicarbonate solution was added to the reaction mixture, and the mixture was stirred for 10 minutes, followed by filtration through the Celite. The filtrate was extracted with ethyl acetate. The organic layer was washed sequentially with water and brine, and brought to dryness over sodium sulfate anhydrate. The solvent was removed under reduced pressure. The residue was subjected to silica gel column chromatography (silica gel: NH-DM1020 (product of Fuji Silicia), chloroform:methanol=10:1), whereby (−)-2-{4-[2-(2,2,2-trifluoroethoxy) ethoxy]pyridin-2-yl-methanesulfinyl}-1H-benzimidazole was obtained (5.4 g).
