To a solution of di-tert-butyl (4S)-4-(3-hydroxypropyl)-N-trityl-L-glutamate (438 mg, 0.78 mmol) and triethylamine (327 μL, 2.35 mmol) in dichloromethane (7.0 mL) was added 2,4,6-trichlorobenzenesulfonyl chloride (328.6 mg, 1.17 mmol) at 0° C. The reaction mixture was stirred at 0° C. for 1 h and then water was added. The 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=10/90) to give the desired product (415 mg, 66%) as a white solid.
