Hydrochloric Acid → 2-[4-(3-Cyclohexyl-ureido)-butyrylamino]-3-(4-hydroxy-phenyl)-propionic acid

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To a solution of 632 (0.45 g, 2.0 mmol) and 1-ethyl-3-(3-(dimethylamino)-propyl) carbodiimide (0.5 g, 2.2 mmol) in 15 mL of DMF, 0.53 g (2.3 mmol) of tyrosine methyl ester and 2.4 mmol of diisopropylethylamine were added. The mixture was heated at 60° C. for 6 h. Then, 50 mL of 0.1 N NaOH were added and the mixture was left at room temperature overnight. The reaction mixture was then acidified with concentrated HCl and extracted twice with a 2:1 mixture of chloroform:methanol. The organic phases were pooled, dried and evaporated. The residue was purified by chromatography on a silica column (5×4 cm). Elution with a 75:25:1 mixture of ethyl acetate:methanol:acetic acid yielded 140 mg (yield: 18%) of the target product as a brown oily liquid. LC-MS-ES negative mode: 390.3 (100%, [M−H]−), 290.9 (10%, (M−C6H10N]−), 264.9 (5%, [M−C7H12NO]−); positive mode: 392.5 (40%, [M+H]+), 264.95 (100%, [M−C7H10NO]+).

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