Six Stereocenters Forged In One Step

Chemists starting out with a pair of reactants and a new copper chiral diamine catalyst have stitched together cyclohexanes with six contiguous stereocenters in a tandem [2+2+2] cycloaddition reaction (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201107495). A team led by Hanmin Huang of the Lanzhou Institute of Chemical Physics, in China, accomplished the feat by coupling ketoesters with nitroalkenes. The cyclohexanes assemble in a domino fashion: A ketoester first forms a copper enolate intermediate, which adds one nitroalkene molecule and then another, followed by a ring-closing operation to yield a single diastereomer. Contiguous multiple stereocenters are a highly desirable structural motif found in natural and synthetic bioactive compounds, the researchers note. Two other groups have reported cyclohexanes with six contiguous stereocenters, Huang says, but either as a by-product or in low yield and without providing the absolute structures of the compounds. A compound with eight contiguous stereocenters is also known, he adds, although two steps were needed to make it. “Our method could be claimed to be a record for the synthesis of the most contiguous stereocenters in a molecule made in just one step,” Huang tells C&EN.