Switching Selectivity in o‐Benzoquinone Cycloadditions

Whereas m‐methoxy‐o‐benzoquinone 6, like other unsaturated ketones, operates as a dienophile in Diels–Alder cycloadditions, the corresponding methyl derivative 7a behaves as an electron‐poor 4π‐system under exactly the same conditions. With the nonsymmetric cyclopentadiene 2 (X = MeO), adduct 9a is formed with excellent chemoselectivity, face‐selectivity, and regioselectivity.