RESUMO
We report asymmetric bioinspired total syntheses of the fungal metabolites emerionesâ A-C via stereoselective oxidations of two bicyclo[4.2.0]octadiene diastereomers. The central bicyclic scaffolds are prepared in an 8π/6π electrocyclization cascade of a stereodefined pentaene, which contains the fully assembled side chains of the emeriones. The anti-aldol side chain is made using a Paterson-aldol addition, and the epoxide of the dioxabicyclo[3.1.0]hexane side chain via ring-closure onto an oxidized acetal. Our work has enabled the structural revision of emerioneâ C, and resulted in the synthesis of a "missing" family member, which we call emerioneâ D. DFT calculations identified two methyl groups that govern torquoselectivity in the 8π/6π cascade.
Assuntos
Estereoisomerismo , Ciclização , OxirreduçãoRESUMO
A new redox-active N-heterocyclic carbene (NHC) architecture is obtained using N-methylated pyrazinediimine iron complexes as precursors. The new species exhibit strong π-accepting/σ-donating properties and are able to ligate two metal centres simultaneously. The redox activity was demonstrated by the reversible chemical oxidation of a heterobimetallic Fe0 /RhI example, which affords an isolable ligand-based radical cation. The reversible redox process was then applied in the catalytic hydrosilylation of 4,4'-difluorobenzophenone, where the reaction rate could be reversibly controlled as a function of the catalyst oxidation state. The new NHC exhibits high electrophilicity and nucleophilicity, which was demonstrated in the reversible activation of alcohols and amines. The electronic structure of the resulting complexes was investigated through various spectroscopic and computational methods.