this invention relates to a two - step synthesis of high purity dialkyl 2 - vinyl - cyclopropane - 1 , 1 - dicarboxylate in which a mixture of trans - and cis - 1 , 4 - dihalobutene - 2 is first isomerized to essentially trans - 1 , 4 - dihalobutene - 2 followed immediately by cyclocondensing the resultant trans - 1 , 4 - dihalobutene - 2 with a malonic ester in the presence of an alcoholic solution of a metallic alkoxide .