Chiral N‐Heterocyclic Carbene‐Catalyzed Formal [4+2] Cycloaddition of Ketenes with Enones: Highly Enantioselective Synthesis of trans‐ and cis‐δ‐Lactones

A chiral N-heterocyclic carbene was found to be an efficient catalyst for the [4+2] cycloaddition of disubstituted ketenes to enones to give δ-lactones with α-quaternary-β-tertiary stereocenters (see scheme). Both the trans- and cis-isomers of the δ-lactones could be obtained in good yields with high diastereo- and enantioselectivities by in situ thermodynamically controlled epimerization and kinetically controlled protonation, respectively.