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

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journal contribution
posted on 15.10.2019, 13:35 by Sung Hwan Park, Shou-Guo Wang, Nicolai Cramer
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 (Cpx) 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.