Remote Stereochemical Control of Both Reacting Centers in Ketyl-Olefin Radical Cyclizations: Involvement of a Samarium Tridentate Ligate

Abstract

High diastereoselection in a samarium(II) iodide-promoted ketyl-olefin cyclization reaction has been achieved using tartramide-derived keto allylic acetals as chiral auxiliaries. The unique features of the reaction include the fact that remote diastereoselection is achieved in a radical process and that high levels of stereochemical induction are observed at both new stereocenters created in the transformation. The source of the asymmetric induction is postulated to be a highly ordered, tricyclic transition structure made possible by three-point chelation between the ketyl intermediate and the samarium counterion. As such, this transformation also demonstrates the first example of the use of a chelating metal to affect high levels of remote asymmetric induction in a radical reaction. Because of this chelation, the sense of relative stereoselectivity is unusual for a SmI2-mediated cyclization, providing consistently high ratios of cis/trans isomers. A double-diastereodifferentiating experiment provides additional support for this mechanistic hypothesis. The preparation of enantiomerically enriched cyclopentanediols and -lactols can be achieved through this novel asymmetric cyclization protocol.