Organocatalytic asymmetric cascade bicyclization: access to chiral polycyclic bisindoles from 2-indolylmethanols and propargylic alcohols

We report a chiral phosphoric acid-catalyzed asymmetric bis-cyclization of alpha-indolyl propargylic alcohols with 2-indolylmethanols. This highly efficient reaction affords various chiral bisindole-fused polycyclic compounds in excellent yields, as well as remarkable chemo-, regio-, and enantioselectivities under mild reaction conditions. The reaction proceeds via successive formation of allene and allylic cation intermediates, leading to the construction of challenging chiral hexacyclic bisindole scaffolds (HCBIs). Significantly, the synthesized chiral HCBIs were evaluated for their antitumor activities and they exhibited promising results. DFT calculations of the key reaction steps offer an in-depth understanding of the high regioselectivity and chemoselectivity of this reaction. A chiral phosphoric acid-catalyzed asymmetric bis-cyclization of alpha-indolyl propargylic alcohols with 2-indolylmethanols was realized.