To a solution of 0.42 g (1 mmol) of L-phenylalanyl-L-leucine ethyl ester trifluoroacetate, 0.14 mL (1 mmol) of triethylamine and 0.26 g (1 mmol) of N-(tert-butoxycarbonyl)-3-chloro-L-alanine in 8 mL of chloroform, 0.21 g (1 mmol) of dicyclohexylcarbodiimide was added at 0° C. The resultant reaction mixture was stirred overnight at room temperature. The precipitate which formed was removed by filtration and the filtrate was washed successively with 0.5 N hydrochloric acid, water, 3% aqueous sodium bicarbonate and water, then dried over anhydrous sodium sulfate. Removal of the solvent afforded a white solid, which was mixed with 4 mL of a 1:1 mixture of ethyl acetate and n-hexane. Insoluble materials were removed by filtration and the filtrate was evaporated. The residue was recrystallized from a mixture of ethyl acetate and n-hexane to give 0.367 g (66.5%) of the desired product. The product, which has the structural formula ##STR75## melted at about 110° C. Its identity was further confirmed by IR and NMR analyses and by thin layer chromatography (Rf =0.34 in 5:1 chloroform/ethyl acetate).
