[NH2:1][C@H:2]([CH2:6][CH2:7][C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[C:3]([OH:5])=[O:4].O.Cl[C:17]([O:19][CH2:20][C:21]1[CH:26]=[CH:25][CH:24]=[CH:23][CH:22]=1)=[O:18].Cl>C1COCC1.CCOC(C)=O>[CH2:20]([O:19][C:17]([NH:1][C@H:2]([CH2:6][CH2:7][C:8]([O:10][C:11]([CH3:14])([CH3:13])[CH3:12])=[O:9])[C:3]([OH:5])=[O:4])=[O:18])[C:21]1[CH:26]=[CH:25][CH:24]=[CH:23][CH:22]=1
(R)-2-amino-5-(tert-butoxy)-5-oxopentanoic acid (1.00 g, 4.92 mmol) was suspended in THF (18.45 ml)/Water (6.15 ml) and cooled to 0° C. using an ice bath. To the cold slurry was slowly added benzyl chloroformate (0.773 mL, 5.41 mmol) and the reaction mixture was allowed to stir for 2.5 h. The slurry was diluted with 50 mL of EtOAc before being acidified to pH 3 using an aqueous 1N HCl solution. The crude product was extracted three times with EtOAc. The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated. The crude product was obtained as a clear oil and was subjected to silica gel chromatography using 100% Hexanes to 100% ethyl acetate as eluant. The product, (R)-2-(((benzyloxy)carbonyl)amino)-5-(tert-butoxy)-5-oxopentanoic acid, was obtained as a colorless oil (1.41 g, 71%). 1H NMR (400 MHz, chloroform-d) δ 7.44-7.29 (m, 5H), 5.53 (d, J=7.5 Hz, 1H), 5.13 (s, 2H), 4.41 (d, J=5.1 Hz, 1H), 2.56-2.30 (m, 2H), 2.21 (dd, J=13.5, 6.5 Hz, 1H), 2.06-1.88 (m, 2H), 1.45 (s, 9H).
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