Electrocyclic ring closure template

(2E,4Z,6E)-Octa-2,4,6-triene closes disrotatory to cis-5,6-dimethylcyclohexa-1,3-diene; (2E,4E)-hexa-2,4-diene closes conrotatory to trans-3,4-dimethylcyclobutene.

Mechanism step by step

Thermal 6π closures are disrotatory, thermal 4π closures conrotatory (Woodward–Hoffmann)

1. 6π, thermal – Disrotatory: the HOMO ψ3 has the same phase at both termini, so both methyls turn the same way out: cis
2. 4π, thermal – Conrotatory: the HOMO ψ2 has opposite phases at the termini, so the methyls end up trans

Key points

• Woodward–Hoffmann table: 4n π electrons close conrotatory by heat, disrotatory by light; 4n + 2 π electrons close disrotatory by heat, conrotatory by light.
• FMO: look at the HOMO termini; light promotes an electron, so the new HOMO (one level up) has the opposite symmetry.
• Stereospecific: (E,Z,E)-octatriene gives only cis-5,6-dimethylcyclohexadiene by heat; (E,E)-hexadiene gives trans-3,4-dimethylcyclobutene.
• The 4π cyclobutene–butadiene equilibrium lies on the open side (ring strain): the conrotatory ring opening is the one usually seen.

Drawn with 44 atoms, 8 arrows, 4 shapes – every one editable in MolDraw.

Sources

• Woodward & Hoffmann, Angew. Chem. Int. Ed. Engl. 1969, 8, 781 Review
• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook
• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 35 Textbook