To a solution of (R)-tert-butyl 3-(1-(2-(dimethylamino)ethyl)-2-neopentyl-1H-benzo[d]imidazol-5-ylthio)pyrrolidine-1-carboxylate (STEP A, 802 mg, 1.74 mmol) in methanol (15 mL) was added methane-sulfonic acid (226 microL, 3.46 mmol), 30% hydrogen peroxide solution (622 microL, 6.09 mmol) and sodium tungstate dihydrate (28.7 mg, 0.0870 mmol) in water solution (0.5 mL) at 0° C. The resulting solution was stirred at room temperature for 5 h. The reaction mixture was added sodium thiosulfate aqueous solution and sodium bicarbonate aqueous solution and diluted with dichloromethane to separate. The organic layer was concentrated in vacuo. The residue was purified by amine gel column chromatography (hexane-ethyl acetate, gradient) to give the title compound (612 mg, 71%).
