Hydrochloric Acid → (S)-4-Benzyloxy-2-{2-[benzyloxycarbonyl-(2-tert-butoxycarbonyl-ethyl)-amino]-ethylamino}-butyric acid methyl ester

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Reaction Steps

3.71 g (11.55 mmol) of 3-[benzyloxycarbonyl-(2-oxo-ethyl)-amino]-propionic acid tert-butyl ester and 3.00 g (11.55 mmol) of (S)-2-amino-4-benzyloxy-butyric acid methyl ester; hydrochloride (intermediate 7) were dissolved in 1,2-dichloroethane/ethanol 1:1 80 ml) and treated with 1.93 ml (13.86 mmol) of triethylamine, 3.03 ml of acetic acid and 3.03 ml (24.25 mmol, 8 M in pyridine) of pyridine-borane complex (cooling with a water bath to room temperature). The reaction was stirred at room temperature over 1¼ h, then partitioned between aq. sodium hydrogencarbonate solution and diethyl ether (3×). The organic phases were washed with sat. aq. sodium hydrogencarbonate solution, 10% aq. sodium chloride solution, dried over Na2SO4 evaporated and purified by flash silica gel column (dichloromethane/n-heptane 1:1 to dichloromethane, then dichloromethane/ethyl acetate 4:1 to ethyl acetate) to yield 4.81 g (79%) of the title compound as colorless oil. MS: 529.2 (MH+).

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