[Li+].[OH-].[CH3:3][C:4]1[CH:5]=[C:6]([CH2:11][C:12]([NH:14][C@@H:15]([CH2:20][C:21]2[C:29]3[C:24](=[CH:25][CH:26]=[CH:27][CH:28]=3)[NH:23][CH:22]=2)[C:16]([O:18]C)=[O:17])=[O:13])[CH:7]=[C:8]([CH3:10])[CH:9]=1.O>CO>[CH3:10][C:8]1[CH:7]=[C:6]([CH2:11][C:12]([NH:14][C@@H:15]([CH2:20][C:21]2[C:29]3[C:24](=[CH:25][CH:26]=[CH:27][CH:28]=3)[NH:23][CH:22]=2)[C:16]([OH:18])=[O:17])=[O:13])[CH:5]=[C:4]([CH3:3])[CH:9]=1DMAP (122.2 mg, 0.5 mmol) and DIC (N,N-diisopropylcarbodiimide) (235 μl, 1.5 mmol) were added to a solution of tryptophan methyl ester (255 mg, 1.0 mmol) and 3,5-dimethylphenylacetic acid (164 mg, 1.0 mmol) in dry DCM (11 ml). After 17 h at room temperature the solvent was removed under reduced pressure and the crude product was purified by flash chromatography (DCM containing 1% EtOH) to afford (S)-methyl 2-(2-(3,5-dimethylphenyl)acetamido)-3-(1H-indol-3-yl)propanoate. An aqueous solution of LiOH (1N, 4.5 ml, 4.5 mmol) was added to a solution of (S)-methyl 2-(2-(3,5-dimethylphenyl)acetamido)-3-(1H-indol-3-yl)propanoate (365 mg, 1 mmol) in methanol (24.5 ml) and the reaction mixture was stirred at room temperature for 4.5 h. Then water was added and methanol was removed under reduced pressure. The aqueous residue was washed with DCM (5×20 ml), treated with HCl 2N to obtain a pH 4 and extracted with DCM (3×25 ml). The organic phases were dried with anhydrous Na2SO4 and the solvent was removed under reduced pressure to afford (S)-2-(2-(3,5-dimethylphenyl)acetamido)-3-(1H-indol-3-yl)propionic acid (93% c.y.).
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