196 mg of cesium carbonate (0.6 mmol) was added to a solution of 345 mg of (2S,4S,5S)-6-[2,2-dimethyl-4-(2-methylphenyl)-5-oxopiperazin-1-yl]-4-hydroxy-2-isopropyl-5-(2-nitrobenzenesulfonylamino)hexanoic acid (2-carbamoyl-2-methylpropyl)amide obtained in the above reaction (0.5 mmol) and 0.15 ml of thiophenol (content: 95%) (1.5 mmol) in N,N-dimethylformamide (5 ml) under a nitrogen atmosphere at room temperature, and the mixture was stirred at the same temperature for four hours. Brine was added to the reaction mixture, followed by extraction with methylene chloride. Then, the organic layer was dried over anhydrous sodium 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/triethylamine=20/1/0-10/1/0-100/10/1). 38.7 mg of fumaric acid (0.33 mmol) was added to a solution of 168 mg of (2S,4S,5S)-5-amino-6-(2,2-dimethyl-4-(2-methylphenyl)-5-oxopiperazin-1-yl)-4-hydroxy-2-isopropylhexanoic acid (2-carbamoyl-2-methylpropyl)amide obtained above (0.33 mmol) in methanol (3 ml), and the mixture was stirred at room temperature for five minutes. The reaction mixture was concentrated under reduced pressure, and methylene chloride (2 ml) was added to the residue. Diethyl ether (5 ml) was further added and the solid was collected by filtration to obtain 132 mg of the title compound (yield: 67%).
