Stetter reactions umpolung addition

In addition to the stabilized carbanions, electron-rich aromatic compounds, for example indole derivatives have emerged as new Michael donors [25], In these reactions, aromatic sp2-C-H transformation is involved. These reactions are described in detail in Section 111.1.3.1.3. A highly enantioselective intramolecular Stetter reaction, in which umpolung reactivity of a formyl group was accomplished using a chiral triazolium salt, has also been reported by Rovis [26]. 

The use of species 118 for the umpolung of a carbonyl group, the Stetter reaction, was demonstrated for the intramolecular asymmetric conjugate addition of a formyl group to the enoate moiety of 119 to give the cyclization product 120 in high ee and yield (Scheme 15) [64]. 

Besides the Michael addition of heteroatomic nucleophiles initiating cyclocondensations, acceptor substituted unsaturated systems can also be reacted with carbon nucleophiles stemming from aldehydes in the sense of an umpolung, generally referred to as the Stetter reaction [244-246]. This process is organocatalytic and furnishes in turn 1,4-dicarbonyl compounds, intermediates that are well suited for Paal-Knorr cyclocondensations giving rise to furans or pyrroles. Among numerous heterocycles furans and pyrroles have always been the most prominent ones since they constitute important classes of natural products [247-249], of synthetic... 

In this chapter, asymmetric organocatalyzed umpolung conjugate additions are considered. Particular emphasis on deconjugative Michael additions with a,a-dicyanoalkenes, the intramolecular Rauhut-Currier reaction, and the Stetter reaction is placed. These are very efficient transformations in the synthetic chemist s arsenal, but they are also challenging to control. 

NHC-catalyzed reactions are unique in organic synthesis, and very useful for the construction of carbon-carbon bonds. Great success has been made for the NHC-catalyzed benzoin condensation, Stetter reactions, and a -d Umpolung reactions in the past decades. NHC catalysis has also hnd application in many other reactions, such as umpolung of Michael acceptors, Morita-Baylis-Hilman reaction, Michael additions, redox reaction, and reactions of ketenes. With the rapid development of NHC catalysis, more reactions will surely be found, and the wide applications in organic synthesis could be expected. 

Triazolium salt-based NHCs have been used to promote asymmetric intra- and inter-molecular " Stetter reactions of cyclohexadienones and simple acrylates, respectively, resulting in umpolung addition of aldehydic carbon. 

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