[C:1]([O:5][C:6]([NH:8][C:9]([CH3:14])([CH3:13])[C:10]([OH:12])=O)=[O:7])([CH3:4])([CH3:3])[CH3:2].Cl.C(N=C=NCCCN(C)C)C.Cl.[CH3:28][NH:29][O:30][CH3:31].C(N(CC)CC)C>ClCCl.O>[C:1]([O:5][C:6](=[O:7])[NH:8][C:9]([C:10](=[O:12])[N:29]([O:30][CH3:31])[CH3:28])([CH3:14])[CH3:13])([CH3:2])([CH3:3])[CH3:4]A mixture of afford 2-tert-butoxycarbonylamino-2-methyl-propionic acid (compound K; 60.0 g) and 1-ethyl-3(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC.HCl; 86.4 g) in dichloromethane (900 mL) was stirred at ambient temperature, then N,O-dimethyl hydroxylamine hydrochloride (35.3 g) and triethylamine (150 mL) were added. The resulting mixture was stirred at ambient temperature for 3 days. Water was added and most of volatiles were removed in vacuo. The residue was partitioned between DCM and aqueous NaHCO3. The organic layer was washed with 3M aqueous HCl, subsequently with brine before it was dried over Na2SO4, filtered, and concentrated in vacuo. The residue was purified by silica gel chromatography to give [1-(methoxy-methyl-carbamoyl)-1-methyl-ethyl]-carbamic acid tert-butyl ester (compound L; 28.2 g) as a white solid sufficiently pure for the next step.
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