To a solution of N-(2-acetylthiomethyl-4-phenylbutanoyl)-(L)-leucine (derived from the higher Rf TLC fraction) (182 mg, 0.5 mmole) in 2 mL of THF at 0° C. was added aniline (93 mg, 1.0 mmole) and 1-hydroxybenzotriazole hydrate (HOBT′H2O), 101 mg, 0.75 mole) and N-methylmorpholine (202 mg, 2.0 mmole). The mixture was stirred at 0° C. for 15 minutes, then 192 mg of 1-ethyl-3-(3-dimethyl-aminopropyl)carbodiimide hydrochloride (EDC.HCl, 1.0 mmole) was added and the mixture stirred overnight. The solution was worked up by adding 7 mL methylene chloride and the mixture extracted with 3×3 ml of 5% sodium bicarbonate, then 2×2 mL of brine. The organic layer was dried over MgSO4, filtered and the solvent removed under reduced pressure. The residue was purified by flash column chromatography on silica gel eluted with methylene chloride/ethyl acetate (90/10) to obtain 110 mg of product (yield=50%).
