[4+2] Diels-Alder: aromatic 6π transition state (thermally allowed, Woodward-Hoffmann)

Diels-Alder reaction (Diels & Alder 1928) — concerted [4+2] cycloaddition between a conjugated diene (4π) and a dienophile (2π) — proceeds through a 6π aromatic transition state under the Woodward-Hoffmann rules (1965). TS π-count = 6 =…