Sodium borohydride → Benzyl ((2S,3S)-4-chloro-3-hydroxy-1-phenylbutan-2-yl)carbamate

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Reaction Steps

To a solution of N-benzyloxycarbonyl-L-phenylalanine chloromethyl ketone (75 g, 0.2 mol) in a mixture of 800 mL of methanol and 800 mL of tetrahydrofuran was added sodium borohydride (13.17 g, 0.348 mol, 1.54 equiv.) over 100 min. The solution was stirred at room temperature for 2 hours and then concentrated in vacuo. The residue was dissolved in 1000 mL of ethyl acetate and washed with 1N KHSO4, saturated aqueous NaHCO3, saturated aqueous NaCl, dried over anhydrous MgSO4, filtered and concentrated in vacuo to give an oil. The crude product was dissolved in 1000 mL of hexanes at 60° C. and allowed to cool to room temperature where upon crystals formed that were isolated by filtration and washed with copious amounts of hexanes. This solid was then recrystallized from hot ethyl acetate and hexanes to provide 32.3 g 43% of N-benzyloxycarbonyl-3(S)-amino-1-chloro-4-phenyl-2(S)-butanol, mp 150-151° C., FAB MS: MLi+=340. Part B: 3(S)-[N-(benzyloxycarbonyl)amino]-1,2(S)-epoxy-4-phenylbutane A solution of potassium hydroxide (6.52 g. 0.116 mol, 1.2 equiv.) in 970 mL of absolute ethanol was treated with N-benzyloxycarbonyl-3(S)-amino-1-chloro-4-phenyl-2(S)-butanol (32.3 g, 0.097 mol). This solution was stirred at room temperature for 15 minutes and then concentrated in vacuo to give a white solid. The solid was dissovled in dichloromethane and washed with water, dried over anhyd MgSO4, filetered and concentrated in vacuo to give a white solid. The solid was crystallized from hexanes and ethyl acetate to give 22.3 g, 77% of 3(S)-[N-(benzyloxycarbonyl) amino]-1,2(S)-epoxy-4-phenylbutane, mp 102-103° C., FAB MS: MH+=298. Part C: N-[3(S)-benzyloxycarbonylamino-2(R)-hydroxy-4-phenyl]N-isobutylamine A solution of N-benzylcarbonyl-3(S)-amino-1,2-(S)-epoxy-4-phenyl butane (50.0 g, 0.168 mol) and isobutylamine (246 g, 3.24 mol, 20 equivalents) in 650 mL of isopropyl alcohol was heated to reflux for 1.25 hours. The solution was cooled to room temperature, concentrated in vacuo and then poured into 1 L of stirring hexane whereupon the product crystallized from solution. The product was isolated by filtration and air dried to give 57.56 g, 92% of N[3(S)-benzyloxycarbonylamino-2(R)-hydroxy-4-phenyl]-N-isobutylamine, mp 108.0-109.5° C., MH+ m/z=371. Part D: phenylmethyl [2(R)-hydroxy-3-[N-(2-methylpropyl)-N-(4-methoxyphenylsulfonyl)amino]-1S-(phenylmethyl)propyl]carbamate The amine from Part C (936.5 mg, 2.53 mmol) and triethylamine (2.88.5 mg, 2.85 mmol) was dissolved in 20 mL of dichloromethane and treated with 4-methoxybenzenesulfonyl chloride (461 mg, 2.61 mmol). The solution was stirred at room temperature for 16 hours and then concentrated in vacuo. The residue was dissolved in ethyl acetate and this solution was washed with 1N KHSO4, saturated aqueous NaHCO3, brine, dried over anhyd MgSO4, filtered, and concentrated to give a clear oil 1.234 g. The oil was crystallized from a mixture of ether and hexanes, 729.3 mg, 56.5% mp 95-99° C., FAB MS: MH+=511. Part E: 3S-amino-1-[N-(2-methylpropyl)-N-(4-methoxyphenylsulfonyl)amino]-4-phenyl-2R-butanol A solution of phenylmethyl [2(R)-hydroxy-3-[N-(2-methylpropyl)-N-(4-methoxyphenylsulfonyl)amino]1-S-(phenylmethyl) propyl carbamate (671.1 mg, 1.31 mmol) from Part D in 10 mL of methanol was hydrogenated over 50 mg of 10% palladium on carbon at 40 psig at room temperature for 15 hours. The catalyst was removed by filtration through diatomaceous earth and the filtrate concentrated to give a white foam, 474.5 mg, 96%, FAB MS: MH+=377.

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