N-Heterocyclic Carbene-Catalyzed Michael-Michael-Lactonization Cascade for the Enantioselective Synthesis of Tricyclic δ-Lactones.

Enantioselective synthesis of tricyclic δ-lactones with three contiguous stereocenters has been demonstrated by the N-heterocyclic carbene (NHC)-catalyzed functionalization of benzylic C(sp3)-H bonds. The NHC-catalyzed reaction of enals with dinitrotoluene derivatives under oxidative conditions proceeds via the chiral α,β-unsaturated acylazoliums and produces the δ-lactones in good yields and excellent diastereoselectivity and enantioselectivity. This mild and atom-economic cascade reaction takes place in a Michael/Michael/lactonization sequence and tolerates a broad range of functional groups.