Diels–Alder reaction
The retro-Diels–Alder reaction proper is the microscopic reverse of the Diels–Alder reaction: a concerted (but not necessarily synchronous), pericyclic, single-step process. Evidence for the retro-Diels–Alder reaction was provided by the observation of "endo"-"exo" isomerization of Diels–Alder adducts. It was postulated that at high temperatures, isomerization of kinetic "endo" adducts to more thermodynamically stable "exo" products occurred via an rDA/DA sequence. However, such isomerization may take place via a completely intramolecular, [3,3]-sigmatropic (Cope) process. Evidence for the latter was provided by the reaction below—none of the "head-to-head" isomer was obtained, suggesting a fully intramolecular isomerization process.