Catalytic Asymmetric Formation of δ‐Lactones by [4+2] Cycloaddition of Zwitterionic Dienolates Generated from α,β‐Unsaturated Acid Chlorides

Smooth elaboration: Versatile δ-lactone building blocks are provided by tertiary-amine-catalyzed asymmetric [4+2] cycloadditions of α,β-unsaturated acid chlorides and the electron-poor aldehyde chloral (see scheme). Silyl-substituted acid chlorides (R1=R3Si) can be used for the diastereoselective synthesis of β-hydroxy-δ-lactones possessing quaternary stereocenters.