Tandem Sakurai-aldol addition reactions as a route to structurally complex carbocycles

摘要

Tandem intramolecular Sakurai-aldol reactions provide a concise and highly diastereoselective route to substituted cyclohexenone derivatives. The cyclization substrates are readily obtained using olefin isomerization-Claisen rearrangement (ICR) reactions to prepare the key chiral allyl silane precursors. The Claisen reaction products are elaborated to the chiral Sakurai-aldol substrates by an efficient two-step sequence involving vinyl organometallic-aldehyde addition and oxidation of the resulting alcohol. The reaction of the resulting enones with TiCl4 elicits a highly stereoselective allyl silane conjugate addition to produce a trichlorotitanium enolate as the reaction intermediate; intermolecular trapping of the enolate with an aldehyde provides pentasubstituted cyclohexanone derivatives in which the annulation reaction establishes four stereocenters and two new C-C bonds. A fully intramolecular variant of the Sakurai-aldol reaction that creates four stereocenters, two new C-C bonds, and establishes two new carbocyclic rings is also described.