NHC-Catalyzed Michael Addition for the Synthesis of Dihydropyranones

Significance

α,β-Unsaturated aldehydes react with N-heterocyclic carbenes (NHCs) to form umpoled intermediates, which can react with homo­enolates at the β-position with various electrophiles (C. Burstein, F. Glorius Angew. Chem. Int. Ed. 2004, 43, 6205). Based on this fact and on a previous work from the authors, showing that Breslow intermediates A can be readily oxidized with mild organic oxidants to acylazolium ions B, which undergo efficient O-acylation (S. de Sarkar, A. Studer Org. Lett. 2010, 12, 1992), oxidatively generated intermediates of type B from enals 1 were used as Michael acceptors to conduct 1,4-addition using 1,3-dicarbonyl nucleophiles 2. A subsequent 1,2-addition of the additional nucleophilic site of the 1,3-dicarbonyls 2 to the generated acylazolium ion releases the NHC catalyst to afford dihydropyranones 5 in good to excellent yields in most cases. The reaction is carried out under mild conditions using the readily available quinone 4 as an oxidant as well as DBU and the triazolium salt 3 (carbene precursor) as catalytic system. The authors proposed a mechanism supported by control experiments.