To a solution of [(1-amino(1-cyclopentyl)carbonyl]-L-tyrosine methyl ester hydrochloride (1.36 g, 3.97 mmol) in dichloromethane (50 mL) is added triethylamine (0.55 mL, 3.97 mmol). The mixture is stirred for 5 minutes and then (S)-2-acetylthio-3-methylbutanoic acid (0.70 g, 3.97 mmol), 1,3-dicyclohexylcarbodiimide (0.82 g, 3.97 mmol) and 1-hydroxy-7-azabenzotriazole (0.54 g, 3.97 mmol) are added. The mixture is stirred for 16 hours and then the solid precipitate is filtered off. The organic phase is washed with a saturated solution of sodium bicarbonate (30 mL), brine, dried over MgSO4, filtered and concentrated under reduced pressure to give a white foam. The foam is purified by flash chromatography on silica gel, eluting with hexane-ethyl acetate (1:1). The pure fractions are concentrated and the resulting foam is crystallized from ethyl acetate-hexane to afford N-[[1-[(2(S)-acetylmercapto-3-methyl-1-oxobutyl)amino]-1-cyclopentyl]carbonyl]-L-tyrosine methyl ester as a white solid, m.p. 157°-158° C., the compound of formula III wherein R1 is 4-hydroxyphenyl, R2 is methyl, R3 is acetyl and R4 is isopropyl.
