All-Carbon (4+2) Annulations Catalysed by N-Heterocyclic Carbenes

Less than five years ago we reported the NHC-catalysed (4+2) annulation of dienol ethers and unsaturated acyl fluorides. From a mechanistic perspective, this reaction likely involves a vinylogous Michael addition followed by an aldol/beta-lactonisation cascade. In this account, the discovery of this reaction and ensuing studies into its mechanism and utility in multistep synthesis will be examined. The subsequent development of chiral catalysts designed for this reaction and the achievement of a first-generation and later second-generation approach to an enantioselective variant of this reaction will be discussed. Finally, related redox isomerisation cascades leading to benzaldehydes will be introduced, as will reactions in the field of NHC catalysis that exploit similar reaction cascades. 1 Introduction 2 Reaction Design and Discovery 3 Mechanistic Studies and beta-Lactone Interception 4 Enantioselective Cyclohexenyl beta-Lactone Synthesis 5 Enantioselective Cyclohexadiene Synthesis 6 Redox Isomerisation 7 Related NHC Catalysis 8 Conclusions