Redox azolium-catalyzed

The Bode group documented detailed research on the asymmetric NHC-catalyzed annulation reaction of ynals and stable enols (kojic acid) to afford enantioenriched dihydropyranone products in excellent yields. Mechanistically, the key a,p-unsaturated acyl azolium intermediate was accessible via an NHC-catalyzed redox neutral reaction of ynals. The annulation occurs via... 

Unlike the NHC-catalyzed redox esterifications described above, which were first reported in 2004, chemists have long recognized the potential of azolium... 

N-heterocyclic carbene catalysis has become one of the major categories in orga-nocatalysis. Azolium salts are ready deprotonated by weak bases to generate a carbene, which then adds to an aldehyde to form an acyl anion equivalent, generally called the Breslow intermediate. The reactive acyl anion attacks an electrophile to promote the various transformations such as benzoin, Stetter, and redox reactions [107]. Recently, an interesting approach for NHC-catalyzed generation of an enol/enolate intermediate was reported. Enantio-enriched (i-amino acid derivatives (217) are formed by the reaction between the a-aryloxyaldehyde 214 and N-tosyl-imines (215) in the presence of phenyalanine-derived azoUum salt 216 as a pre-catalyst and aryl phenoxide as a base (Scheme 28.28) [108]. 

Unlike the NHC-catalyzed redox esterifications described above, which were first reported in 2004, chemists have long recognized the potential of azolium salts such as thiamine to promote esterification reactions in the presence of an external oxidant. These reactions are inspired by the biological process whereby an activated aldehyde or Breslow intermediate is oxidized by... 

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