Diisopropylethylamine (0.90 ml.), then diphenylphosphinyl chloride (1.23 g.), were added to a solution of N-(tert-butyloxycarbonyl)-O-(tert-butyl)-N-methyl-L-tyrosine (1.76 g.) in tetrahydrofuran (15 ml.) maintained at -20° C., and the mixture was stirred for ten minutes. A solution of N-(3-phenylpropyl)-D-methioninamide hydrochloride (1.58 g.) and diisopropylethylamine (0.90 ml.) in tetrahydrofuran (10 ml.) was then added. The mixture was stirred for one half hour at -20°, one hour at 0° C. and overnight at room temperature, then filtered and stripped of solvent. A solution of the residue in ethyl acetate (100 ml.) was washed with water, aqueous citric acid, water again, aqueous sodium bicarbonate and water again, dried and stripped of solvent. Purification of the residue by high pressure liquid chromatography on silica gel (350 g.) using hexane-ethyl acetate (2:1) as the eluant afforded in fractions 6-7 (k'=8-14) [N-(tert-butyloxycarbonyl)-O-(tert-butyl)-N-methyl-L-tyrosyl]-N-(3-phenylpropyl)-D-methioninamide as a clear gum (2.1 g.).
