Inverse-electron-demand Diels–Alder reactions of 4-aryl-2-pyrones with electron-rich dienophiles

The Diels–Alder reaction of 4-aryl-pyrones with electron-rich dienophiles afforded substituted biaryl derivatives in most cases. At the minimal temperatures necessary for a measurable conversion of the starting pyrones, the bicyclic lactones, the primary products of the condensation, underwent cycloreversion by extruding carbon dioxide and then aromatised through further eliminations. In the case of the more active 4-aryl-6-chloro-pyrone, a formal substitution was observed instead of the expected cycloaddition with an active dienophile, while in its reaction with a Schiff-base the primary product of the cycloaddition was trapped through the formation of a new tetrahydropyridine derivative.