Electrocyclic Ring Closure or Opening
Electrocyclic rearrangements are a type of pericyclic mechanism which involve ring formation by converting a double alkene double pi-bond into a pair of single sigma-bonds.

Pericyclic reactions exhibit great stereochemical control over the product isomer distribution.

For more information look in the Chemogenesis webbook sections on rearrangement reactions and pericyclic reactions.