Transfer of Chiral Information through Molecular Assembly

Calix[4]arenes substituted with ureas on their upper, wider rims form dimers in solution held together by a seam of 16 hydrogen bonds. The resulting molecular capsules have interiors capable of accommodating small-molecule guests. By virtue of the assembly process, a head-to-tail arrangement of ureas is formed that can assume either a clockwise or counterclockwise orientation around the equator of the capsule. Amino acids on the urea functions have a dramatic effect on the dimerization behavior of the calixarenes. First, the chiral derivatives display a preference for associating with calixarenes substituted with arylureas rather than with themselves. This selectivity for heterodimerization is exclusive with amino acids that have β-branched side chains such as isoleucine and valine. Second, the point chirality of the amino acids is transferred upon assembly giving rise to a chiral capsule with only one direction to its head-to-tail arrangement of ureas. Circular dichroic spectroscopy is used to assign the absolute configuration of these capsules. The chirality of the capsule can be sensed by chiral guests where enantioselective binding is observed.