The carboxylic acid of example 2 (0.500 g, 1.61 mmol), TBTU (0.621 g, 1.94 mmol) and 4-nitro-L-phenylalanine methyl ester hydrochloride (0.461 g, 1.77 mmol) were dissolved in DMSO (2.0 mL) and DIEA (6.44 mmol, 1.12 mL) was added. The mixture was stirred for 2 h at room temperature (HPLC: complete). The reaction mixture was added dropwise with stirring to a mixture of water (45 mL) and AcOH (0.8 mL). The precipitate that formed was collected by filtration, washed with water and dried (0.768 g, 92% yield).
