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Cut-And-Sew Route Makes Fused Rings

Rhodium-catalyzed method overturns typical selectivity for C–C bond activation

by Carmen Drahl
July 2, 2012 | A version of this story appeared in Volume 90, Issue 27

Dong’s reaction cleaves a usually less-reactive bond (red) in a cyclobutenone (left).
A scheme showing the new synthetic pathway for alkaloids.
Dong’s reaction cleaves a usually less-reactive bond (red) in a cyclobutenone (left).

With help from a chelating ligand, chemists have found an efficient new route to fused-ring systems similar to those in alkaloid drugs such as morphine or codeine (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201202771). The work also presents a new twist on carbon-carbon bond activation. Chemists typically forge alkaloid core structures with lengthy multistep syntheses. Now, assistant professor Guangbin Dong and postdoctoral researcher Tao Xu have stitched the three-ring cores together in one rhodium-catalyzed step. The pair, from the University of Texas, Austin, began with benzocyclobutenones. They had to avoid cleaving their substrate’s most reactive C–C bond, and break a less reactive C–C bond instead. They learned that 1,1-bis(diphenylphosphino)butane ligands helped achieve that selectivity. They think an olefin in the substrate also guides the reaction, which requires zinc and rhodium in tough cases. “While the fundamental metal-catalyzed approach to multiring systems using four-membered ring strain is not without precedent, the authors have been creative and insightful in their chosen, and nontrivial, application,” says organometallic chemist Lanny S. Liebeskind of Emory University. He looks forward to reading about gentler reaction conditions and a wider array of substrates in follow-up work.


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