To a solution of di-tert-butyl (4S)-4-(3-hydroxypropyl)-N-trityl-L-glutamate (439 mg, 0.78 mmol) and triethylamine (330 μL, 2.35 mmol) in dichloromethane (7.0 mL) was added 4-nitro-3-(trifluoromethyl)benzenesulfonyl chloride (340.7 mg, 1.18 mmol) at 0° C. The reaction mixture was stirred at 0° C. for 45 min and then water was added. Organic layer was separated, and aqueous layer was extracted with dichloromethane. Combined organic solution was dried over sodium sulfate, and concentrated in vacuo. The residue was purified by flash column chromatography (ethyl acetate/n-hexane=15/85) to give the desired product (4 h, 470 mg, 74%) as a light yellow solid.
