Under nitrogen, to dimethylsulfoxide (3 ml) was added potassium hydroxide (31 mg) at room temperature, and the suspension was stirred at the same temperature for 1 hour. To this one were added N-(3-hydroxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-6-yl)-N-[(2S)-3-phenoxy-2-(triethylsilyloxy)propyl]-tert-butoxycarbonylamine (150 mg) and isopropyl bromide (78 μl), and the mixture was stirred at room temperature for 3 hours. The resulting mixture was poured into aqueous 10% sodium hydrogencarbonate, and extracted with ethyl acetate. The organic layer was washed with brine, dried over anhydrous magnesium sulfate, and evaporated in vacuo. The residue was purified by column chromatography on silica gel (hexane:ethyl acetate=20:1) to give N-(3-isopropoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-6-yl)-N-[(2S)-3-phenoxy-2-(triethylsilyloxy)propyl]-tert-butoxycarbonylamine (96 mg).
