Transition-metal-catalyzed ring-closing metathesis is a useful tool for organic synthesis. The stereochemistry of the resulting olefin is important for the properties of the product, and the E– and Z– isomers can be difficult to separate. Thus, finding stereoselective approaches to ring-closing metathesis is important.

Robert H. Grubbs, California Institute of Technology (Caltech), Pasadena, USA, and colleagues have developed a ruthenium-based metathesis catalyst for the stereoretentive synthesis of E-macrocycles (pictured). The team prepared ruthenium complexes bearing N-heterocyclic carbene (NHC) and dithiolate ligands. Thes catalysts were then used to convert diene starting materials bearing two E-olefins into the corresponding 12- to 18-membered E-macrocycles.

The reactions proceed in moderate to good yields and with high selectivities (up to 99 % E). According to the researchers, the selectivity is due to a sterically favored arrangement in the metallacyclobutane intermediate.

• Using stereoretention for the synthesis of E-macrocycles with ruthenium-based olefin metathesis catalysts,
  Tonia S. Ahmed, T. Patrick Montgomery, Robert H. Grubbs,
  Chem. Sci. 2018.
  https://doi.org/10.1039/c8sc00435h