Facile Access to Quaternary Stereocenters Through Anion Binding Catalysis

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 DESCRIPTION (provided by applicant): The fact that stereogenic all-carbon quaternary centers appear in many biologically active small molecules is indisputable. Unfortunately, they are also amongst the most challenging motifs to enantioselectively prepare. As a result, the diversity of these structures that can be deployed medicinally is extraordinarily limited and quaternary stereocenters in pharmaceuticals are almost exclusively prepared by semi-synthesis or classical resolution. Existing catalytic asymmetric methods suffer from several limitations: 1) reliance upon synthetically challenging stereodefined starting materials that restrict general accessibility; 2) reliance upon expensive transition metals that necessitate rigorous purification prior to biological testing; 3) reliance upon activating functional groups that inherently restrictthe scope of accessible products. An attractive alternative strategy would be an enantioconvergent SN1 reaction in which all-carbon quaternary stereocenters would be prepared by the enantioselective coupling of simple racemic tertiary alcohol derivatives with carbon nucleophiles. This approach would address each of the key limitations of previous methods to prepare enantioenriched quaternary stereocenters and thus would dramatically enhance the synthetic accessibility of the motif. Hydrogen bond donor organocatalysts are known to both abstract anions from neutral organic molecules to generate cationic intermediates and can effectively control the stereochemical outcome of the subsequent nucleophilic trapping. This anion abstraction reaction manifold has been tremendously effective in generating heteroatom-stabilized carbocations and exploiting them to produce highly enantioenriched products. However, extension of this catalytic manifold to enable the production of enantioenriched quaternary stereocenters is an unsolved problem. The goal of this proposal is to design an anion binding organocatalyst that will catalyze the first general and synthetically useful enantioconvergent SN1 reaction capable of preparing quaternary stereocenters. The research plan outlines an approach to develop such a catalyst system guided by hypothesis-driven experimentation, supramolecular chemistry and computational modeling. To complement the reaction development, a detailed thermodynamic and kinetic investigation anion abstraction from neutral alcohol derivatives will be undertaken. This study will enable these readily accessible and bench stable substrates to engage broadly in anion-abstraction catalysis as carbocation precursors. Overall, the research described herein will enable facile access to diverse quaternary stereocenters without reliance upon complex starting materials, metal catalysts or proximal activating groups and will have tremendous relevance to both medicinal chemistry research and further catalysis development.