To a solution of 3-[1-[2-amino-3-(4-fluorophenyl)propionyl]-4-(2-naphthalen-2-ylethyl)-5-oxo-piperazin-2-yl]propionitrile, 33, (47.2 g, 100 mmol), N-acetyl-L-tyrosine (22.3 g, 120 mmol), 1-hydroxybenzotriazole (16.2 g, 120 mmol), and N-methylmorpholine (132 mL, 120 mmol) in DMF (150 mL) is cooled to 0° C. and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (24.9 g, 130 mmol) is slowly added. The resulting mixture is stirred for 2 hours at 0° C. then allowed to warm and stir at room temperature another 2 hours. The reaction solution is diluted with water and the resulting solid is collected by filtration, re-dissolved in EtOAc and extracted with water, dried and concentrated in vacuo to afford the crude product which is purified over silica gel.
