C(OC([N:8]1[CH2:13][CH2:12][O:11][CH2:10][C@@H:9]1[C:14]([OH:16])=[O:15])=O)(C)(C)C.S(Cl)(Cl)=O.[CH3:21]O>>[NH:8]1[CH2:13][CH2:12][O:11][CH2:10][C@@H:9]1[C:14]([O:16][CH3:21])=[O:15]To a solution of (R)-morpholine-3,4-dicarboxylic acid-4-tert-butyl ester (5.00 g, 21.6 mmol) in MeOH (54 mL) was added thionyl chloride (3.16 mL, 43.2 mmol) dropwise, and the resulting mixture was heated at reflux for 16 h. The solution was cooled to ambient temperature and concentrated in vacuo. The residue was made basic with saturated aqueous NaHCO3 and extracted with CH2Cl2 (3×). The combined organic extracts were dried over Na2SO4, filtered, and concentrated in vacuo to yield the title compound.
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