N-Heterocyclic Carbene Catalysis as a Tool for Gaining Access to the 3,4-Dihydropyran-2-one Skeleton

An overview of the progress made in the synthesis of 3,4-dihydropyran-2-ones since the development of NHC catalysis is reported. Two distinct classes of species - electrophilic alpha,beta-unsaturated acylazolium and nucleophilic azolium enol/enolate intermediates - can easily be produced from different substrates through the intermediacy of NHC catalysis. They enable the construction of the 3,4-dihydropyran-2-one skeleton, the core of natural and synthetic products with significant biological activities, through Michael additions, cycloadditions and rearrangements.