Substitution of the propargylic carbon for nitrogen in cyclic 10-membered ring enediynes results in at least 2 orders of magnitude enhancement of reactivity. The mechanism of cycloaromatization of aza-enediynes, however, switches from radical (Bergman) to polar, which proceeds by initial protonation of a reactive yneamine moiety. Ketenimmonium cation produced on this step undergoes intramolecular nucleophilic cyclization. Reaction is strongly catalyzed by acids and exhibits substantial kinetic isotope effect.