C1([NH2+]C2CCCCC2)CCCCC1.[CH2:14]([O:21][C:22]([NH:24][C@@H:25]([C@H:29]([O:31][C:32]([CH3:35])([CH3:34])[CH3:33])[CH3:30])[C:26]([O-])=[O:27])=[O:23])[C:15]1[CH:20]=[CH:19][CH:18]=[CH:17][CH:16]=1.ClC(OCC(C)C)=O.CN1CCOCC1>C1COCC1>[CH2:14]([O:21][C:22](=[O:23])[NH:24][C@@H:25]([C@H:29]([O:31][C:32]([CH3:35])([CH3:34])[CH3:33])[CH3:30])[CH2:26][OH:27])[C:15]1[CH:16]=[CH:17][CH:18]=[CH:19][CH:20]=1A solution of (2S,3R)-2-(((benzyloxy)carbonyl)amino)-3-(tert-butoxy)butanoic acid dicyclohexylammonium salt (500 mg, 1.0 mmol) in 10 ml of THF and isobutyl chloroformate (167 mg, 1.2 mmol, 1.2 equiv) at −25° C. was added N-methylmorpholine (124 mg, 1.2 mmol, 1.2 equiv), the mixture was stirred at same temperature for 10 min and filtered. The filtrate was cooled to −20° C. and to it was added NaBH4, followed by 2 ml of water immediately afterwards. The reaction mixture was stirred at same temperature for 5 min. then gradually warmed to room temperature for 25 min, poured into water (10 ml) and extracted with ethyl acetate (2×20 ml). The combined organic phases were washed with water, brine and dried over Na2SO4. The solvent was removed to yield benzyl((2R)-(3R)-3-(tert-butoxy)-1-hydroxybutan-2-yl)carbamate as a clear oil. No further purification was required for next step. 1H NMR (400 MHz, CDCl3) δ 7.37-7.16 (m, 5H), 5.25 (d, J=8.0 Hz, 1H), 5.02 (s, 1H), 4.04 (ddd, J=12.0, 8.9, 2.8 Hz, 1H), 3.92-3.75 (m, 1H), 3.59 (dddd, J=32.3, 14.6, 8.2, 4.3 Hz, 2H), 1.10 (s, 9H), 1.09-1.06 (m, 3H).
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