The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.

Selection Rules: Thermal Activation

Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

Conjugated systems with an odd number of π-electron pairs undergo a disrotatory ring closure. For example, (2E,4Z,6E)-2,4,6-octatriene, a conjugated diene containing three π-electron pairs, yields cis-5,6-dimethyl-1,3-cyclohexadiene.