The N-hydroxysuccinimide ester of Z-Phe-OH (19.15 g, 48.3 mmol) was dissolved in 50 mL of dioxane and the resulting solution was filtered into a solution consisting of H-Gly-Leu-OH (10.0 g, 53.1 mmol), triethylamine (10.1 mL, 72.4 mmol), and water (50 mL). This reaction mixture was stirred overnight at about 23°, and then concentrated approximately 60% by evaporation. The resulting concentrated solution was diluted with 1.0N hydrochloric acid until acidic. Product Z-Phe-Gly-Leu-OH was extracted into ethyl acetate, and the resulting extract was washed with 0.2N hydrochloric acid, followed by saturated aqueous sodium chloride adjusted to 0.10N hydrochloric acid. After washing, the ethyl acetate solution was dried over anhydrous sodium sulfate and filtered. The ethyl acetate was then evaporated to yield 22.5 g of a white, foamy product, which was subsequently crystallized from ethyl acetate to yield 16.7 g of Z-Phe-Gly-Leu-OH (m.p. 147°-148°).
