Carbon–carbon bond formation by intramolecular 1,4-dipolar cycloaddition: heterocyclic betaines generated in situ from amides and N-substituted amides

Abstract

Reaction of benzamides and N-substituted benzamides with appropriate o-alkenyl and -alkynyl side-chains and (chlorocarbonyl)phenyl ketene gives transient anhydro-1,3-oxazinium hydroxides which readily undergo intramolecular 1,4-dipolar cycloaddition; loss of CO₂ from these 1:1-cycloadducts, isolable with N-substituted systems and alkenyl side-chains, gives chromeno[4,3-b]pyridin-2-ones.