[C:1]([O:5][C:6](=[O:10])[CH:7]=[N+]=[N-])([CH3:4])([CH3:3])[CH3:2].[Sn](Cl)Cl.[CH3:14][O:15][C:16](=[O:27])[CH2:17][C:18]1[CH:23]=[CH:22][C:21]([CH:24]=[O:25])=[C:20]([Cl:26])[CH:19]=1>C(Cl)Cl.[Cl-].[Na+].O>[C:1]([O:5][C:6](=[O:10])[CH2:7][C:24]([C:21]1[CH:22]=[CH:23][C:18]([CH2:17][C:16]([O:15][CH3:14])=[O:27])=[CH:19][C:20]=1[Cl:26])=[O:25])([CH3:4])([CH3:3])[CH3:2]A flame-dried flask is charged with tert-butyl-diazoacetate (0.62 mL, 4.48 mmol) and anhydrous tin(II) chloride (168 mg, 0.88 mmol) in dry DCM (10 mL). (3-Chloro-4-formyl-phenyl)-acetic acid methyl ester 14 (500 mg, 2.35 mmol) is dissolved in DCM (5 mL) and added dropwise to the reaction mixture. After stirring for 6 h at room temperature the reaction mixture is poured into brine (50 mL) and extracted with ether (3×50 mL). The combined organic layers are dried (MgSO4), filtered, concentrated and purified on reverse phase HPLC (H2O/MeCN gradient) to afford 3-(2-Chloro-4-methoxycarbonylmethyl-phenyl)-3-oxo-propionic acid tert-butyl ester 56 as a white solid: MS calculated for C16H19Cl2NaO5 (M+Na+) 349.1, found 349.1.
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