[CH3:1][O:2][C:3](=[O:7])[CH2:4][CH:5]=[CH2:6].[Cl:8][C:9]([Cl:14])(Cl)[C:10](Cl)=[O:11]>COCCOC.C(OCC)C.[Cu].[Zn]>[CH3:1][O:2][C:3](=[O:7])[CH2:4][CH:5]1[CH2:6][C:10](=[O:11])[C:9]1([Cl:14])[Cl:8]To a mixture of but-3-enoic acid methyl ester (1.00 g, 10 mmol) and zinc-copper couple (1.97 g) in 1,2-dimethoxyethane (4.89 ml) and diethyl ether (37 ml), trichloroacetyl chloride (2.98 ml, 26.7 mmol) is added at room temperature under nitrogen. The mixture is stirred at room temperature for 3 days. The mixture is filtrated and washed with diethyl ether. The filtrate is concentrated under reduced pressure, and the obtained residue is purified by silica gel column chromatography to give (2,2-dichloro-3-oxo-cyclobutyl)-acetic acid methyl ester (2.93 g, quant.); TLC (hexane/AcOEt, 3:1) Rf 0.35, 1H NMR (400 MHz, chloroform-d) δ ppm 2.68-2.74 (m, 1H), 2.94-3.00 (m, 1H), 3.06-3.13 (m, 1H), 3.33-3.41 (m, 1H), 3.51-3.57 (m, 1H), 3.75 (s, 3H).
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