To a solution of di-tert-butyl (4S)-4-(3-hydroxypropyl)-N-trityl-L-glutamate (217.5 mg, 0.39 mmol) and triethylamine (160 μL, 1.17 mmol) in dichloromethane (5.0 mL) was added naphthalene-2-sulfonyl chloride (155.4 mg, 0.58 mmol) at 0° C. The reaction mixture was stirred at 0° C. for 3 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=12/88) to give the desired product (289 mg, 82%) as a white solid (m.p.=119.3° C.).
