Electrocyclic reaction

Interconversion of a conjugated pi system and a ring by formation or cleavage of one sigma bond at the pi-system termini, accompanied by pi-bond redistribution. Terminal rotation connects starting geometry with product stereochemistry.

Formation of a ring by concerted conversion of terminal pi-system overlap into a new sigma bond. Thermal four-pi closure is conrotatory, subject to the appropriate electronic-state assumptions.

Ring closure of a seven-carbon conjugated anion with eight pi electrons; thermal orbital-symmetry selection favors disrotatory motion.

For ordinary closed-shell systems, a thermal $4n$-electron electrocyclic process is conrotatory and a thermal $4n+2$-electron process is disrotatory. The corresponding simple photochemical rules are reversed. These rules relate terminal orbital phases and occupation to allowed concerted bond formation or cleavage.

Terminal rotation in opposite senses under a common viewing convention during an electrocyclic reaction.

Terminal rotation in the same rotational sense under a common viewing convention during an electrocyclic reaction. It is distinguished from disrotatory motion; looking from opposite ends without changing conventions can reverse an apparent clockwise label.