A formal [3 + 3] cycloaddition reaction. Improved reactivity using alpha,beta-unsaturated iminium salts and evidence for reversibility of 6pi-electron electrocyclic ring closure of 1-oxatrienes.

A detailed account regarding a formal [3 + 3] cycloaddition method using 4-hydroxy-2-pyrones and 1,3-diketones is described here. This formal cycloaddition reaction or annulation reaction is synthetically useful for constructing 2H-pyranyl heterocycles. The usage of alpha,beta-unsaturated iminium salts is significant in controlling competing reaction pathways to give exclusively 2H-pyrans.