Chiral N-Heterocyclic Carbene Catalyzed, Enantioselective Oxodiene Diels−Alder Reactions with Low Catalyst Loadings

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Abstract
Chiral N-heterocyclic carbene (NHC) catalyzed redox reactions of racemic α-chloroaldehydes lead to the generation of chiral enolates suitable for highly enantioselective inverse-electron-demand 1-oxodiene Diels−Alder reactions. Significantly, these reactions proceed under mild, operationally friendly reaction conditions (EtOAc, room temperature, 2−8 h) using less than 1 mol % of a chiral N-mesityl triazolium salt as the precatalyst. A broad array of densely functionalized dihydropyran-2-ones bearing either aliphatic or aromatic substiutents are formed in excellent yields and exceptional enantioselectivities from readily available reactants. Stereochemical mismatching between the racemic starting materials and the chiral catalyst is avoided by rapid epimerization of the chloroaldehydes under the reaction conditions.