N -Heterocyclic Carbene-Catalyzed Generation of Homoenolates:  γ-Butyrolactones by Direct Annulations of Enals and Aldehydes

N-Heterocyclic carbenes, prepared in situ from diarylimidazolium salts, serve as highly effective catalysts for the generation of reactive homoenolates from alpha,beta-unsaturated aldehydes. The catalyst-bound homoenolate reacts with electrophilic aldehydes leading, via the key intermediacy of an activated carboxylate, to gamma-butyrolactones in good yields and stereoselectivities. Importantly, this process demonstrates an unprecedented reaction mode for the generation of nucleophilic carbanions with a multifunctional organocatalyst under exceptionally mild and convenient reaction conditions.