Conjugate Umpolung of β,β-Disubstituted Enals by Dual Catalysis with an N-Heterocyclic Carbene and a Bronsted Acid: Facile Construction of Contiguous Quaternary Stereocenters

A sterically hindered homoenolate has been generated by the NHC-catalyzed conjugate umpolung of beta,beta-disubstituted enals and successfully employed in a facile stereoselective annulation with isatins. The strategy provides efficient access to spirocyclic oxindoles bearing two highly congested contiguous quaternary carbon centers. The use of a Bronsted acid cocatalyst was found to be crucial for guaranteeing both excellent reactivity and high stereoselectivity.