A @[email protected] is pseudopericyclic if the primary changes in bonding occur within a cyclic array of atoms at one (or more) of which nonbonding and bonding atomic orbitals interchange roles. A formal example is the enol \(\rightarrow \) enol prototropy of pentane-2,4-dione (acetylacetone). Because the \(\pi \)- and \(\sigma \)-atomic orbitals that interchange roles are orthogonal, such a reaction does not proceed through a fully @[email protected] and is thus not a @[email protected] and therefore not governed by the rules that express @[email protected] restrictions applicable to pericyclic reactions.