Synthesis and structure of chiral macrocycles containing 2,2'-bipyridine subunits

A series of macrocyclic receptors has been prepared containing bipyridine groups linked to two amino acids. Variations in both the amino acid and the linking spacer have been made. The structure of the resulting macrocycles has been investigated using ¹H NMR spectroscopy and X-ray crystallography. The use of L-valine leads to an open conformation for the macrocycle in which the 2-propyl substituents are directed perpendicular to the plane of the ring leaving the bipyridine and amide groups accessible for binding to a metal or complementary substrate. Proline-based macrocycles take up a twisted arrangement with the linking chain stretched across the face of the bipyridine which takes up a trans conformation. The metal ion binding properties of these derivatives have been investigated and shown to occur only to the valine macrocyles which have the two pyridine rings preorganized for complexation. These macrocycles have also been shown to bind to phenolic hydroxyl groups by using hydrogen-bond donors and acceptors from the amide groups in the linking chain.