Exploring the scope of Bergman Cyclization mediated cascade reaction of alkenyl enediynes: synthesis of [5]helicene and amino acid appended [4]helicenes

Bergman Cyclization mediated cascade radical process is employed for the synthesis of [5]helicenes as well as diastereoselective mixtures of chiral amino acid appended [4]helicenes. Maximum selectivity (2.5:1) was found in case of l-leucine attached cyclic enediyne. Mechanistic considerations for this tandem radical reaction revealed that the major pathway probably involved quenching of the biradical, formed after aryl radical addition to pendant aromatic ring, by the solvent followed by aromatization. Intramolecular H-abstraction by the sp2-hybridized radical in the central ring or a 1,5-H shift followed by self-quenching could be the other possible ways to lead to the helicenes.