Enantioselective Ruthenium(II)-Catalyzed Access to Benzonorcaradienes by Coupling of Oxabenzonorbornadienes and Alkynes

Benzonorcaradienes and benzonorcarenes are attractive scaffolds that occur in natural products as well as biologically active compounds. However, there is a significant gap in the current synthetic toolbox as methods to access benzonorcaradiene enantioselectively are very scarce. We disclose an enantioselective ruthenium(II)-catalyzed synthesis of benzonorcaradiene by coupling of oxabenzonorbornadienes and alkynes. An easily accessible chiral cyclopentadienyl ligand (Cp-x) provides access to exo-benzonorcaradiene in exclusive diastereoselectivity and with enantioselectivities of up to 97.5:2.5 er. Mechanistically, the transformation proceeds by a ruthenium-catalyzed part linking the alkene and alkyne moiety. This is followed by a series of pericyclic rearrangements converging to the thermodynamically favored exo-benzonorcaradiene isomer.