Retro hetero-Diels-Alder reaction

A retro hetero-Diels-Alder reaction is the reverse of a Diels-Alder reaction involving a heteroatom-containing partner. For a six-membered ring with sequence $O-A-B=C-D-E$, cleavage of $O-A$ and $D-E$ can give $A=B-C=D$ and $E=O$. Whether the cycloreversion is concerted or proceeds through ionic intermediates depends on the substrate and conditions.