N-Heterocyclic Carbene (NHC)-Catalyzed Stereoselective Synthesis of Six-Membered Heterocycles and Carbocycles

Abstract

N-Heterocyclic carbene (NHC) based catalysis is employed for the generation of α,β-unsaturated acylazoliums followed by their interception with various nucleophiles, the formal [3+3] annulation reactions for the construction of six-membered rings. In this context, we have demonstrated the NHC-catalyzed routes for the enantioselective synthesis of thiazinones, 5,6-dihydroindolizines, tetra-substituted tetralines and tetrahydro-indolizines, where these reactions proceed via the selective Michael addition to the catalytically generated α,β-unsaturated acylazoliums. Moreover, a related strategy has been developed for the synthesis of quinazoline-2,4-diones. In addition, oxidative NHC catalysis has been utilized for the rapid access to functionalized pyrrolo-oxazinones. In another phase of the work, the synthesis of 2‑aryl naphthoquinones by the cross-dehydrogenative coupling involving an NHC-catalyzed endo-Stetter reaction has been realized.