Sodium triacetoxyborohydride → (R)-4-(4-((2-(tert-butyldimethylsilyloxy)ethyl)(ethyl)amino)-1-(phenylthio)butan-2-ylamino)-3-(trifluoromethylsulfonyl)benzenesulfonamide

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To a solution of (R)-4-(4-oxo-1-(phenylthio)butan-2-ylamino)-3-(trifluoromethylsulfonyl)benzenesulfonamide (INTERMEDIATE 45, 3.98 g, 8.25 mmol) and 2-(tert-butyldimethylsilyloxy)-N-ethylethanamine (INTERMEDIATE 63, 1.68 g, 8.25 mmol) in 1,2-dichloroethane (27.5 ml) was added sodium triacetoxyborohydride (2.62 g, 12.37 mmol). The resulting mixture was stirred in the dark for 11.5 h and was then quenched with 0.5 M aqueous sodium hydroxide (50 ml). After vigorous stirring for 10 min, the opaque mixture became clear. The layers were diluted with ethyl acetate and then separated. The organic layer was washed with saturated aqueous sodium bicarbonate and saturated aqueous sodium chloride before being dried over sodium sulfate, filtered, and concentrated under reduced pressure. The resulting residue was purified by column chromatography (ISCO, 330 g SiO2, 50→100% ethyl acetate in hexanes for 20 mins then 0→20% methanol in ethyl acetate for 20 mins followed by 0→20% methanol in ethyl acetate over 20 min) to afford the title product (3.41 g, yield: 62%).

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