The etc, ett, and ttt isomers of l,2-di(prop-l'-enyl)cyclopropane isomerize at 165°at comparable rates to give, respectively, c/s,dj,dj-di(prop-l'-enyl)cyclopropane, írans-6,7-dimethylcyclohepta-l,4-diene, and cis-6,7dimethylcyclohepta-1,4-diene. These highly stereoselective and stereospecific reactions are interpreted in terms of a common mechanism involving a one-center epimerization as the rate-limiting step, followed by rapid Cope rearrangement of the trans,cis,trans-and m.ds.fra/w-dipropenylcyclopropanes.Among the archetypal thermal isomerizations, the l cA-divinylcyclopropane (1) to 1,4-cycloheptadiene (2) reaction occupies a prominent interstice between l 2 the electrocyclic conversion of 1,3,5-hexatriene to 1,3cyclohexadiene and the degenerate [3,3] sigmatropic rearrangement of 1,5-hexadiene. Since the initial demonstration of this process by Vogel in I960,2-5 and the first studies exploiting this reaction to demonstrate fluxional isomerism by Doering and Roth,6•7 numerous examples have been investigated.