[CH3:1][O:2][C:3](=[O:9])[CH2:4][C:5]([CH2:7][Cl:8])=[O:6].[CH:10](=O)[CH2:11][CH3:12].C(O)(=O)C>ClCCl.N1CCCCC1>[CH3:1][O:2][C:3](=[O:9])[C:4](=[CH:10][CH2:11][CH3:12])[C:5]([CH2:7][Cl:8])=[O:6]To a solution of 4-chloroacetoacetic acid methyl ester (700 mg : 4.65 mMol.) and propionaldehyde (405 mg : 6.97 mMol.) and acetic acid (28 mg : 0.47 mMol.) in dichloromethane (3 ml) is added a solution of piperidine (24 mg : 0.28 mMol.) in dichloromethane (0.5 ml), and the mixture is stirred at -27° C. for 120 minutes. The reaction mixture is washed with dilute hydrochloric acid and water and concentrated under reduced pressure at lower than 15° C. to give 4-chloro-2-propylideneacetoacetic acid methyl ester (818 mg). Yield: 92%.
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