One-Pot Preparation of Bridged Tricyclic and Fused Tetracyclic Scaffolds via Rhodium(III)-Catalyzed Asymmetric Borylative Cyclization

Abstract

Summary: Bridged and fused polycyclic structures are found in a huge number of natural products. Significant progress has been made on their syntheses; however, facile and practical strategies to afford polycycles with tunable substituents and functional groups remain rare. Here, we report a practical rhodium(III)-catalyzed asymmetric borylative cyclization of cyclohexadienone-containing 1,6-enynes, affording highly enantioenriched cis-bicyclic frameworks with great functional group compatibility, and further extend it to construct bridged tricyclic and fused tetracyclic skeletons in one-pot protocols. The combination of borylative cyclization with a subsequent oxidation/aldol reaction sequence goes through an epimerization of the stereocenter adjacent to the acetyl group, yielding various tricyclic compounds. In addition, borylative cyclization is combined with a Suzuki-Miyaura coupling/Michael addition sequence, leading to diverse tetracyclic compounds. Starting from simple starting materials, both one-pot transformations yield complex products bearing several consecutive stereocenters with excellent enantioselectivities. Furthermore, an unnatural steroid-like compound is also readily prepared, demonstrating the potential of this methodology.