Organocatalytic Enantioselective Intramolecular Michael Addition by In Situ Generated Aminoisobenzofulvenes: Construction of Spiro Quaternary Carbon Stereocenters

An unprecedented enantioselective organocatalytic spirocyclization strategy is presented by in situ generation of aminoisobezofulvenes. The reaction sequence involves a reductive Michael/aldol-condensation/Michael addition cascade by iminium-enamine catalysis. The key success of this spirocyclization was the formation of intermediatory nucleophilic aminoisobenzofuvenes accountable for intramolecular Michael addition. Benzospirononanes featuring an all carbon qauternary spirocenter were obtained using proline-derived amino-organocatalyst in moderate to good yields and excellent diastereo- and enantioselectivities (up to >20 : 1 dr, and 99 % ee). Post-methodological manipulation of benzospirononanes was also demonstrated.