206 mg of cesium carbonate (0.63 mmol) was added to a solution of 326 mg of N-{(S)-1-[(2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl]-2-[4-(2-methoxymethoxyphenyl)-2,2-dimethyl-5-oxopiperazin-1-yl]ethyl}-2-nitrobenzenesulfonamide obtained in the above reaction (0.53 mmol) and 0.12 ml of thiophenol (content: 95%) (1.1 mmol) in N,N-dimethylformamide (5 ml) under a nitrogen atmosphere at room temperature, and the mixture was stirred at the same temperature for two hours. Brine was added to the reaction mixture, followed by extraction with methylene chloride. Then, the organic layer was dried over anhydrous magnesium sulfate. After filtration, the solvent was evaporated under reduced pressure, and the residue was purified by silica gel column chromatography (elution solvent:methylene chloride/methanol=20/1-10/1). 49 mg of sodium bicarbonate (0.57 mmol) and 78 μl of benzyloxycarbonyl chloride (0.53 mmol) were added to a solution of 209 mg of 4-{(S)-2-amino-2-[(2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl]ethyl}1-(2-methoxymethoxyphenyl)-5,5-dimethylpiperazin-2-one obtained above (0.48 mmol) in a mixed solvent of ethyl acetate (3 ml) and water (3 ml) under ice-cooling, and the mixture was stirred at the same temperature for 30 minutes. Brine was added to the reaction mixture, followed by extraction with ethyl acetate. Then, the organic layer was dried over anhydrous sodium sulfate. After filtration, the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (elution solvent:methylene chloride/methanol=20/1) to obtain 263 mg of {(S)-1-[(2S,4S)-4-isopropyl-5-oxotetrahydrofuran-2-yl]-2-[4-(2-methoxymethoxyphenyl)-2,2-dimethyl-5-oxopiperazin-1-yl]ethyl}carbamic acid benzyl ester (yield: 87%).
