C[O:2][C:3]([C@@:5]1([CH3:9])[CH2:8][CH2:7][NH:6]1)=[O:4].[OH-].[Na+].[Br:12][C:13]1[CH:21]=[CH:20][C:16]([C:17](Cl)=[O:18])=[CH:15][CH:14]=1.C(O)(=O)C>O>[Br:12][C:13]1[CH:21]=[CH:20][C:16]([C:17]([N:6]2[CH2:7][CH2:8][C@:5]2([CH3:9])[C:3]([OH:2])=[O:4])=[O:18])=[CH:15][CH:14]=1To a solution of (R)-2-methyl-azetidine-2-carboxylic acid methyl ester (1 eq.) in water were added aqueous NaOH (1M) (3 eq.) then 4-bromo-benzoyl-chloride (1 eq.) dropwise. The reaction was stirred for 4 h at 20° C. The reaction was neutralised with acetic acid. The solvent was partially removed under reduced pressure and the crude was extracted with EtOAc. The organic layer was dried over MgSO4, filtered and concentrated under reduced pressure. The crude was purified by chromatography on silica gel (elution with DCM/MeOH/AcOEt/AcOH/iPr2O: 95/5/98/2/300) to afford (R)-1-(4-bromo-benzoyl)-2-methyl-azetidine-2-carboxylic acid.
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