Contracting rings with light

Ring-contracting reactions let chemists create, for example, a five-membered ring from a six-membered ring, giving them a tool for creating complex molecular architectures. Now, chemists at the University of Texas, San Antonio, have developed a ring-contracting reaction with a bonus: It creates a side chain containing a stereocenter on the new five-membered ring (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b07128). The reaction is a photoinduced carboboration, which uses light to convert cis-cyclohexenes to more reactive trans-cyclohexenes. These then react with organoboranes via a unique mechanism to produce substituted five-membered rings. Subsequent reactions can convert the resulting boranes to alcohols (example shown), amines, or alkenes. “The reaction offers a new shortcut to five-membered carbocycles and heterocycles from the six-membered ring precursors that are abundant among natural products or that can be easily prepared by the Diels-Alder reaction,” says Oleg V. Larionov, who spearheaded the research effort. “The reaction can also produce molecules with contiguous quaternary stereocenters that are particularly difficult to access synthetically.” The reaction’s discovery corrects work from the 1970s, when chemists first looked at the reaction of trans-cyclohexene and organoboranes and assumed incorrectly that the transformation produced six-membered rings.