Mg-promoted reduction of ethyl cinnamate (la) in the presence of carbon dioxide gave a mixture of beta-carboxylated compound 2a and alpha,beta-dicarboxylated compound 3a. Similar reductive carboxylation of 1 a followed by acidic decarboxylation of one of the two geminal carboxyl groups of the generated 3a and esterification afforded selective formation of diethyl phenylsuccinate (2a) in good yield.