To a solution of 10.0 mmol of Fmoc Asp-OtBu (42) in 50 mL of dry tetrahydrofuran, kept at −20° C. under nitrogen, is added 1.77 mL (12.7 mmol) of triethyl amine, followed by the slow addition of 1.57 mL (12.0 mmol) of isobutylchloroformate. The mixture is stirred for 30 minutes, and then poured slowly over an ice-cold solution of 3.77 g (99.6 m mol) of sodium borohydride in 10 mL of water, keeping the temperature below 5° C. The reaction is stirred at 0° C. for 15 minutes, and then quenched with 1N hydrochloric acid solution. The reaction mixture is diluted with 100 mL of ethyl acetate, and the layers separated. The organic layer was washed with 2×25 mL of 1N hydrochloric acid solution, 2×25 mL of water, dried over magnesium sulfate and concentrated, and purified by silica gel column chromatography. Purified compound is then dissolved in 30 mL of tetrahydrofuran, and 12 mmol of 60% sodium hydride dispersion in mineral oil is added, followed by 0.2 mmol of tetrabutylammonium iodide and 12 mmol of benzyl bromide, and the mixture is stirred overnight, quenched with 50 mL of saturated aqueous sodium bicarbonate, and extracted with 100 mL of ethyl acetate. The compound is then purified by silica gel column chromatography.
