A solution of 1.471 g of (S)-alpha-[[(1,1-dimethylethoxy)carbonyl]amino]-4-[2-(1,1-dimethylethoxy) -2-oxoethyl]benzenepropanoic acid methyl ester in 25 mL of methanol and 5 mL of 1N sodium hydroxide solution was stirred at room temperature for 2 hours. The mixture was acidified with a slight excess of hydrochloric acid, was diluted with 100 mL of ether, and was washed with water and saturated sodium chloride solution. The residue obtained after filtration and evaporation was chromatographed over 100 g of silica gel, eluting with 40:59:0.5 ethyl acetate-hexane-acetic acid. The product containing fractions were combined evaporated, diluted with toluene and evaporated finally under high vacuum to give 1.105 g of (S)-alpha-[[(1,1-dimethylethoxy) carbonyl]amino]-4-[2-(1,1-dimethylethoxy)-2-oxoethyl]benzene-propanoic acid [a]D +21.41 (ethanol).
