Nucleophilic Cycloaromatization of Ynamide-Terminated Enediynes

Introduction of a nitrogen atom at one of the acetylenic termini of 10-, 11-, 12-, and 13-membered benzannulated cyclic enediynes results in a complete suppression of the conventional radical Bergman reaction in favor of a polar cycloaromatization. The latter reaction is catalyzed by acids and proceeds via initial protonation of an ynamide fragment. The resulting ketenimmonium cation then cyclizes to produce naphthyl cation, which rapidly reacts with nucleophiles or undergoes Friedel Crafts addition to aromatic compounds. In alcohols, addition of the nucleophilic solvent across the activated triple bond competes with the cyclization reaction. The ratio of cyclized to solvolysis products decreases with the increase in ring size.