N‐Heterocyclic Carbene (NHC)‐Catalyzed/Lewis Acid Mediated Conjugate Umpolung of Alkynyl Aldehydes for the Synthesis of Butenolides: A Formal [3+2] Annulation

Abstract

Reverse to success! A new formal [3+2] annulation reaction combining alkynyl aldehydes and β,γ-unsaturated α-ketoesters has been disclosed by using a NHC-catalyzed/Lewis acid mediated strategy. This cooperative catalysis strategy first allows the "allenolate" intermediate as a nucleophilic synthon at the β-position to react with activated electrophilic reagents by an addition reaction as the key C-C bond-forming step.