A solution of 4,4-dimethyl-1-[2-(methylthio)benzyl]-2-(adamantan-2-yl)-1,2-diazetidin-3-one (7.20 mg, 0.0186 mmol) prepared in Example 60 in methanol was added sequentially with tantalum pentachloride (3.30 mg, 0.00930 mmol) and hydrogen peroxide water (5.70 mg, 0.0558 mmol) at room temperature, and the resultant was stirred at the same temperature for 36.5 hours. The reaction solution was added with water and extracted with diethyl ether. The organic layer was dried over anhydrous sodium sulfate and concentrated in vacuo. The obtained residue was purified using Preparative Thin-Layer chromatography (hexane:ethyl acetate=1:5), and the title compound (2.40 mg, 32.0%) was obtained as a white crystalline powder.
