Chiral Relay Auxiliaries

Abstract

Chiral auxiliaries and templates are effective tools for the asymmetric synthesis of homochiral molecules.1 Most chiral auxiliaries are small heterocyclic compounds which rely on sterically demanding functional groups to control the conformation of their ring systems. Under ideal circumstances, the conformation of an auxiliary should be constrained to ensure that its prochiral centre reacts with a reagent via diastereoisomeric transition states which are sufficiently different in energy to ensure that only a single diastereoisomer is formed as product. In order to maximise the diastereoselectivity observed for an auxiliary, it would appear reasonable that the Stereocontrolling functional group adopts a position in space as close as possible to the newly forming stereogenic centre. Structural considerations dictate that realisation of this ideal is not always attainable and numerous examples of chiral auxiliaries that rely on relatively remote stereogenic centres to control diastereoselectivity are known. Alkylation of the enolates of Seebach’s imidazolidinone (1),2 or Schöllkopf’s bis-lactim ether auxiliary (2),3 for example, are controlled via 1,3- and 1,4- asymmetric induction respectively (scheme 1).KeywordsStereogenic CentreChiral AuxiliaryAsymmetric InductionAuxiliary DesignAmide UnitThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.