Oxidation of methyl 3-(4-methoxyphenyl)propynoate in the system CF ₃COOH-CH₂Cl₂-PbO₂ at 0-2°C within 2 h affords in a preparative yield of 55% a product of oxidative dimerization, dimethyl 2,3-bis(4-methoxyphenylcarbonyl)but-2-ene-1,4- dicarboxylate (II) whose double bond is in E-configuration according to X-ray diffraction data.