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 - a llenes, then its reaction mode may be changed from the Myers - Saito c yclization to a novel C2-C6 cyclization resulting in a net intramolecu lar Diels-Alder or ene reaction. As a consequence, the thermal cycliza tion of readily accessible acyclic enyne-allenes can be utilized for t he synthesis of complex benzofulvene and benzofluorene derivatives. Ki netic results of the C2-C6 cyclization reaction indicate a two-step re action pathway with a benzofulvene biradical intermediate.