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| - Remote Stereochemical Control of Both Reacting Centers inKetyl-Olefin Radical Cyclizations: Involvement of a SamariumTridentate Ligate
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| - High diastereoselection in a samarium(II) iodide-promotedketyl-olefin cyclization reaction has been achievedusing tartramide-derived keto allylic acetals as chiral auxiliaries.The unique features of the reaction include thefact that remote diastereoselection is achieved in a radical processand that high levels of stereochemical inductionare observed at both new stereocenters created in the transformation.The source of the asymmetric induction ispostulated to be a highly ordered, tricyclic transition structure madepossible by three-point chelation between theketyl intermediate and the samarium counterion. As such, thistransformation also demonstrates the first exampleof the use of a chelating metal to affect high levels of remoteasymmetric induction in a radical reaction. Becauseof this chelation, the sense of relative stereoselectivity is unusualfor a SmI2-mediated cyclization, providingconsistentlyhigh ratios of cis/trans isomers. Adouble-diastereodifferentiating experiment provides additional supportfor thismechanistic hypothesis. The preparation of enantiomericallyenriched cyclopentanediols and -lactols can be achievedthrough this novel asymmetric cyclization protocol.
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