[NH:1]([C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[C@H:2]([C:5]([OH:7])=[O:6])[CH2:3][SH:4].[OH-].[Na+].[CH3:17][CH:18]([SH:20])[CH3:19].II.Cl>O.C1COCC1.CCO>[C:11]([O:10][C:8]([NH:1][C@@H:2]([CH2:3][S:4][S:20][CH:18]([CH3:19])[CH3:17])[C:5]([OH:7])=[O:6])=[O:9])([CH3:14])([CH3:13])[CH3:12]Boc-L-Cys-OH (221 mg, 1.0 mmol) was dissolved in H2O/THF (50% v/v, 20 mL). To the solution was added 10 N sodium hydroxide aq. (1.2 mL, 12 mmol) and 2-propane thiol (929 uL, 10 mmol) at 0° C. Then, a solution of iodine in 95% EtOH was added dropwise until the color of the reaction system changed from colorless to brown. The mixture was stirred for 1 d with warming to room temperature. After removing THF under reduced pressure, reaction mixture was neutralized by 1 N HCl aq. until pH 2-3. The solution was extracted with EtOAc. The extract was washed with saturated aq. NaS2O4 and brine, and dried over MgSO4. Concentration under reduced pressure followed by flash chromatography over silica gel in EtOAc/Hexanes/AcOH (75:25:1) gave Boc-L-Csp-OH (145.7 mg, 49% yield): 1H NMR (360 MHz, CDCl3) δ 5.34 (s, 1H), 4.59 (br, s, 1H), 3.24-3.14 (m, 2H), 3.09-3.00 (m, 1H), 1.47 (s, 9H), 1.32-1.30 (dd, 6H); LRMS (ESI) m/z calcd for C11H22NO4S2 (M+H)+ 296.1. found 296.1.
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