(S)-2-((((9H-Fluoren-9-yl)methoxy)carbonyl)amino)-3-(1H-indol-3-yl)propanoic acid (Fmoc-Trp-OH) (1.26 g, 2.96 mmol) and N-hydroxysuccinimide (341 mg, 2.96 mmol) in methylene chloride (5.9 mL) were mixed with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (WSCI.HCl) (568 mg, 2.96 mmol) at 0° C., and the reaction solution was stirred at room temperature for 16 hours. N-Ethyl-N-isopropyl propane 2-amine (DIPEA, 517 μL, 2.96 mmol) and (S)-2-amino-3-(tert-butyldisulfanyl)propanoic acid (H-D-Cys(StBu)-0H) (Compound SP641) (620 mg, 2.96 mmol) were then mixed at 0° C., and the reaction solution was stirred at room temperature for 5 hours. After washing with 1 N hydrochloric acid, the organic layer was extracted with ethyl acetate, and the resulting organic layer was washed with a saturated aqueous sodium chloride solution and then dried over sodium sulfate. Following concentration under reduced pressure, the resulting residue was crude purified by reverse-phase silica gel column chromatography (0.1% aqueous formic acid solution/0.1% formic acid-acetonitrile solution) to afford (S)-2-((S)-2-((((9H-fluoren-9-yl)methoxy)carbonyl)amino)-3-(1H-indol-3-yl)propanamido)-3-(tert-butyldisulfanyl)propanoic acid (Fmoc-Trp-D-Cys(StBu)-OH) (Compound SP642).
