L-Tryptophyl-L-seryl-L-tyrosyl-D-phenylalanyl-L-leucine methyl ester, 1.53 g., is dissolved in 35 ml. of dimethylformamide, the solution cooled in ice and treated with 620 mg. of Nα -t-butoxycarbonyl-O-benzyl-L-serine, 311 mg. of 1-hydroxybenztriazole and 475 mg. of dicyclohexylcarbodiimide (10% excess). The reaction is stirred overnight to room temperature and twenty-four hours additional at room temperature. The mixture is filtered and the filtrate evaporated under reduced pressure. The residue is dissolved in 500 ml. of ethyl acetate and washed with 0.1N hydrochloric acid, saturated salt solution, 5% sodium bicarbonate solution, saturated salt solution and water. The ethyl acetate solution is dried over magnesium sulfate and evaporated. The residual solid is dried under reduced pressure; 2.29 g.; m.p. 200°-202° C.; [α]D23 -19.5° (c 1.03, methanol); ultraviolet in methanol λmax 289.5 E11 52; λmax 280 E11 68.
