Stereoselective N‐Heterocyclic‐Carbene‐Catalyzed Spiroannulation as an Access Towards Spirobenzofuranone‐Fused Cyclohexenones.

In this work, we present an organocatalytic spiroannulation method that provides access to spirocyclic benzofuran‐2‐ones. The reaction proceeds via the generation of an azolium dienolate from an α‐bromo‐α,β‐unsaturated aldehyde using a chiral N‐heterocyclic carbene, followed by annulation with benzofuran‐2‐one‐derived alkenes. The resulting chiral spirobenzofuranone‐fused cyclohexenones were obtained in 77–99% ee and dr from 1.2:1 to 20:1. The utility of the protocol was underscored by gram‐scale reaction and follow‐up transformations. [ABSTRACT FROM AUTHOR]