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

Abstract

Abstract 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.