Synthesis of modified calixarenes for imaging applications
Description
The recent increase of interest in metals used in medical imaging procedures, led to the investigation of calixarenes as possible ligands for imaging applications. Calixarenes are cyclic oligomers produced from the base catalyzed condensation reaction between p-tert-butyl phenol and formaldehyde. Two methods for synthetically modifing calixarenes so that they were transformed from a hexaanionic ligand to a trianionic ligand were examined. The first method consisted of bridging adjacent hydroxy groups of p-tert-butylcalix (6) arene with a compound with only one anionic binding site. Of the various bridging groups tried, dichloroethyl phosphate showed the most promise. However, only two phosphate groups could be incorporated into the lower rim. The conformation of this compound has been determined by $\sp1$H and $\sp{31}$P NMR. The second method that was tried was the selective functionalization of the lower rim. This was accomplished by first selectively methylating p-tert-butylcalix (6) arene in the 1, 3, 5 positions. The remaining hydroxy groups were functionalized with alcohol, and carboxylate groups. The acid dissociation constants for these two calix (6) arenes were measured by potentiometric titrations. Attempts to measure the stability constants of these calix (6) arenes and various metals used in imaging procedures such as gallium, indium, gadolinium, yttrium and iron all failed. The failures were due to the precipitation of the metal hydroxides prior to complexation by the calix (6) arenes