Radical cyclization of mesitylsulfonylhydrazones provides a useful method for the formation of unsubstituted five-and six-membered cyclic compounds. Thus, diethyl 3-methyl-3-cyclopentene-1,1-dicarboxylate (2) was formed by reaction of diethyl 5-bromo-1-mesitylsulfonylhydrazono-5-hexene-3,3-dicarboxylate (1b) with tributyltin hydride and 2,2'-azobisisobutyronitrile in benzene.