A Surprising Switch from the Myers–Saito Cyclization to a Novel Biradical Cyclization in Enyne–Allenes: Formal Diels–Alder and Ene Reactions with High Synthetic Potential

If there is an aryl substituent on the acetylene terminus of enyne-allenes, then its reaction mode may be changed from the Myers-Saito cyclization to a novel C2–C6 cyclization resulting in a net intramolecular Diels-Alder or ene reaction. As a consequence, the thermal cyclization of readily accessible acyclic enyne-allenes can be utilized for the synthesis of complex benzofulvene and benzofluorene derivatives. Kinetic results of the C2–C6 cyclization reaction indicate a two-step reaction pathway with a benzofulvene biradical intermediate.