To a solution of (2S,3S)-2-{[(benzyloxy)carbonyl]amino}-3-hydroxy-4-methylpentanoic acid (368 mg, 1.31 mmol) in anhydrous tetrahydrofuran (5 ml) was added hydroxybenzotriazole (177 mg, 1.31 mmol) and 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (251 mg, 1.31 mmol). The resulting solution was stirred at room temperature under nitrogen for 30 minutes. A solution of 1-amino-3,3-diethoxypropane (183 mg, 1.25 mmol) in tetrahydrofuran (2 ml) and N,N-diisopropylethylamine (0.228 ml, 169 mg, 1.31 mmol) were added and stirred at room temperature for 15 hours. The reaction mixture was partitioned between ethyl acetate (75 ml) and water (40 ml). The organic layer was separated, and the aqueous layer was extracted with ethyl acetate (2×75 ml). The combined extracts were washed with 2% cold hydrochloric acid, saturated sodium bicarbonate solution and saturated sodium chloride, dried over magnesium sulfate, filtered and concentrated in vacuo to provide [1-(3,3-diethoxy-propylcarbamoyl)-2-hydroxy-3-methyl-butyl]-carbamic acid benzyl ester (407 mg, 79%) as a white solid.
