REMOTE STEREOCHEMICAL CONTROL OF BOTH REACTING CENTERS IN KETYL-OLEFIN RADICAL CYCLIZATIONS - INVOLVEMENT OF A SAMARIUM TRIDENTATE LIGATE

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 reac tion include the fact that remote diastereoselection is achieved in a radical process and that high levels of stereochemical induction are o bserved at both new stereocenters created in the transformation. The s ource of the asymmetric induction is postulated to be a highly ordered , tricyclic transition structure made possible by three-point chelatio n 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 inductio n in a radical reaction. Because of this chelation, the sense of relat ive stereoselectivity is unusual for a SmI2-mediated cyclization, prov iding consistently high ratios of cis/trans isomers. A double-diastere odifferentiating experiment provides additional support for this mecha nistic hypothesis. The preparation of enantiomerically enriched cyclop entanediols and -lactols can be achieved through this novel asymmetric cyclization protocol.