C([NH:18][C@H:19]([C:32]([OH:34])=O)[CH2:20][C:21]1[CH:26]=[CH:25][C:24]([O:27][C:28]([CH3:31])([CH3:30])[CH3:29])=[CH:23][CH:22]=1)(OCC1C2C(=CC=CC=2)C2C1=CC=CC=2)=O.[NH4+].C(OC(OC(OC(C)(C)C)=O)=O)(C)(C)C.[N:51]1C=CC=CC=1>O1CCOCC1.C(OCC)(=O)C.ClCCl.C(NCC)C>[C:28]([O:27][C:24]1[CH:25]=[CH:26][C:21]([CH2:20][C@@H:19]([C:32]([NH2:51])=[O:34])[NH2:18])=[CH:22][CH:23]=1)([CH3:31])([CH3:30])[CH3:29]N-Fmoc-O-tert-butyl-tyrosine (3.27 g, 7.12 mmol) was dissolved in dioxane (15 mL). Ammonium hydrogenocarbonate (732 mg, 9.26 mmol), di-tert-butyl-dicarbonate (2.02 g, 9.26 mmol) and pyridine (0.4 mL) were added and the mixture was stirred under nitrogen atmosphere at room temperature for 12 h. It was then diluted with ethyl acetate and the organic phase was washed with brine (2×), 5% sulfuric acid (1×) and brine again. It was dried over magnesium sulfate and concentrated. The crude thus obtained (3.17 g) was dissolved in dichloromethane (50 mL) and diethylamine (10 mL). The mixture was stirred at room temperature for 12 h, concentrated under reduced pressure and purified by flash chromatography with DCM/MeOH/NH4OH (95/5/1 then 90/10/1) to give 1.37 g of the desired compound (yield=81%).
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