CC[O:3][C:4]([C:6]1([CH3:17])[CH2:9][CH2:8][N:7]1[C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])=[O:5].[OH-].[Na+]>CCO>[C:13]([O:12][C:10]([N:7]1[CH2:8][CH2:9][C:6]1([CH3:17])[C:4]([OH:5])=[O:3])=[O:11])([CH3:16])([CH3:14])[CH3:15]To a solution of 2-methyl-azetidine-1,2-dicarboxylic acid 1-tert-butyl ester 2-ethyl ester (63 g, 1 eq.) in 260 mL of EtOH was added 260 mL of an aqueous solution of NaOH 2N (2 eq.). The reaction was stirred at 20° C. for 15 h. The solvent was removed under reduced pressure and the crude was partitioned between water and EtOAc. The organic layer was discarded and the aqueous layer was acidified by addition of a solution of citric acid 10% in water until pH=5 and thoroughly extracted with EtOAc. The organic layers were combined, dried over MgSO4, filtered and concentrated under reduced pressure to afford 2-methyl-azetidine-1,2-dicarboxylic acid 1-tert-butyl ester (50 g, yield=89%).
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