N-Carbobenzoxy-L-alanyl-N-cyclopentylglycine (4 g) and L-phenylalanine tert-butyl ester (2.4 g) are dissolved in 20 ml of dimethylformamide and under ice-cooling and stirring, a solution of 1.9 g of diethyl cyanophosphate in 5 ml of dimethylformamide is added, followed by addition of 1.6 ml of triethylamine. The mixture is stirred at room temperature for 3 hours. Then, 200 ml of ice water is added and the mixture is extracted twice with 50 ml portions of ethyl acetate. The extract is washed with 1N hydrochloric acid and water and dried. The solvent is then distilled off under reduced pressure to give 6.7 g of N-carbobenzoxy-L-alanyl-N-cyclopentylglycyl-L-phenylalanine tert-butyl ester as colorless oil.
