N-Heterocyclic Carbene-Promoted Annulation Reaction of Allenals with Chalcones: Synthesis of Polysubstituted Pyranyl Aldehydes

A general N-heterocyclic carbene (NHC)-promoted annulation reaction of allenals with chalcones affording polysubstituted pyranyl aldehydes has been developed. Three mechanistic possible pathways have been proposed. The NHC attacks the center carbon of allenals or acts as a base for deprotonation and then undergoes a 1,4-addition with chalcones, followed by sequential reactions giving the final products.