β-Siloxy unsaturated nitriles: stereodivergent cyclizations to cis- and trans-decalins

Nitrile and enolate anions exhibit divergent intramolecular cyclization stereoselectivities. Enolates cyclize to cis-decalones, whereas nitrile anions are predisposed to pyramidalize, cyclizing instead through a less-congested conformation to trans-decalins. Conjugation of nitrile anions with adjacent sp2 centers prevents pyramidalization, and redirects cyclization through a planar-delocalized anion to cis-decalins. Collectively these cyclizations allow conversion of a single β-siloxynitrile to either cis- or trans-decalins that are ideally suited for elaboration into terpenoid natural products.