The carboxylic acid of example 2 (0.075 g, 0.24 mmol) and TBTU (1.2 equiv., 0.29 mmol, 0.093 g) were dissolved in DMF (0.5 mL) and DIEA (5 equiv., 1.2 mmol, 0.21 mL) was added followed by tyrosine methyl ester hydrochloride (1.2 equiv., 0.29 mmol, 0.036 g). The mixture was stirred 30 min at room temperature. The reaction mixture was added drop-wise to 1 N NaOH (10 mL) and the mixture stirred until complete hydrolysis of the methyl ester (as determined by HPLC analysis). The pH of the solution was then adjusted to 5-6 by drop-wise addition of 1 N HCl. The gray precipitate that formed was collected by filtration, washed with water and dried (125 mg). The solid was further purified by reversed-phase HPLC using 0.1% TFA—0.1% TFA in acetonitrile gradients to give after lyophilisation, the TFA salt of the title compound as a white amorphous solid (35 mg).
