Cl.[CH:2]12[NH:11][CH:6]([CH2:7][C:8](=[O:10])[CH2:9]1)[CH2:5][O:4][CH2:3]2.[C:12](Cl)([O:14][CH2:15][C:16]1[CH:21]=[CH:20][CH:19]=[CH:18][CH:17]=1)=[O:13]>ClCCl>[O:10]=[C:8]1[CH2:9][CH:2]2[N:11]([C:12]([O:14][CH2:15][C:16]3[CH:21]=[CH:20][CH:19]=[CH:18][CH:17]=3)=[O:13])[CH:6]([CH2:5][O:4][CH2:3]2)[CH2:7]13-oxa-9-azabicyclo[3.3.1]nonan-7-one hydrochloride (CAS 1126795-00-3) (10 g) was dissolved in 200 ml of dichloromethane which was cooled to 0° C. Then, CbzCl (14.3 g, 84.7 mmol) was added to the mixture followed by the dropwise addition of a solution of TEA (14.6 ml, 112.8 mmol) in 20 ml of dichloromethane. The mixture was allowed to warm up to room temperature. After stirring 2 h, it was quenched with 20 ml of a saturated aqueous solution of NaHCO3. The two layers were separated and the aqueous layer was extracted with dichloromethane (3×150 ml). The combined organic layers were washed with water, dried over Na2SO4 and the solvent was evaporated under reduced pressure. The crude product was purified by chromatography on silica gel, yielding 10.1 g of benzyl 7-oxo-3-oxa-9-azabicyclo[3.3.1]nonane-9-carboxylate 100b (64%).
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