To a stirred solution of N-(2-amino-2-methyl-1-oxopropyl)-L-tyrosine ethyl ester monohydrochloride (1.33 g, 3.70 mmol) in dimethylformamide (50 mL) and triethylamine (0.52 mL, 3.70 mmol) is added (S)-2-acetylthio-3-methylbutanoic acid (0.65 g, 3.70 mmol). To this mixture is added 1-hydroxy-7-azabenzotriazole (0.55 g, 4.07 mmol) and 1,3-dicyclohexylcarbodiimide (0.84 g, 4.07 mmol). The mixture is stirred overnight and then water (50 mL) is added to the mixture. The solution is extracted with ethyl acetate (3×50 mL). The combined organic layers are washed with a saturated solution of sodium bicarbonate (20 mL), brine, dried over magnesium sulfate and the solvent is removed to give a yellow foam. The foam is purified by flash chromatography (SiO2 :hexane/ethyl acetate, 6:4) to give as a white solid N-[2-(2(S)-acetylthio-3-methyl-1-oxobutylamino)-2-methyl-1-oxopropyl]-L-tyrosine ethyl ester, m.p. 137°-138° C., the compound of formula II wherein R1 is 4-hydroxyphenyl, R2 is ethyl, R3 is acetyl, R4 is isopropyl, and R6 and R7 are methyl.
