To a solution of (R)-5-(4-bromo-benzyl)-7-(3,5-dichloro-phenyl)-5-methyl-6-oxo-6,7-dihydro-5H-imidazo[1,2-a]imidazole-3-carboxylic acid (2.5 g, 5.0 mmol) in DMF (5 mL) was added (S)-2-amino-propionic acid tert-butyl ester hydrochloride (0.92 g, 5.0 mmol), HOBt (0.68 g, 5.0 mmol), EDC (0.97 g, 5.0 mmol) and DMAP (0.061 g, 0.50 mmol). The reaction mixture was heated at 80° C. for 12 h. The mixture was allowed to cool to room temperature and diluted with water (100 mL). The mixture was extracted with EtOAc (3×100 mL) and the combined organic layers were washed with water, brine and dried with MgSO4. The mixture was filtered and concentrated in vacuo to give 2.5 g of (S)-2-{[(R)-5-(4-bromo-benzyl)-7-(3,5-dichloro-phenyl)-5-methyl-6-oxo-6,7-dihydro-5H-imidazo[1,2-a]imidazole-3-carbonyl]-amino}-propionic acid tert-butyl ester as a yellow solid, m/z 623.5 [M+1]+.
