High π‑Facial and <i>exo</i>-Selectivity for the Intramolecular Diels–Alder Cycloaddition of Dodeca-3,9,11-trien-5-one Precursors to 2-<i>epi</i>-Symbioimine and Related Compounds

An unconstrained exocyclic stereogenic center and a removable trimethylsilyl group are combined to induce high π-facial selectivity and near-exclusive <i>exo</i>-selectivity in the intramolecular Diels–Alder cycloaddition of dodeca-3,9,11-trien-5-ones. This strategy provides direct access to polysubstituted <i>trans</i>-1-decalones related to the symbioimines in good yield and acceptable diastereoselectivity