Synthesis and binding properties of an inherently chiral calixarene
Calixarenes are macrocyclic compounds composed of phenol and formaldehyde units and have been utilized extensively as supramolecular host molecules. Formation of chiral calixarenes for recognition of chiral guest species has been accomplished by two general methods. The first method involves the covalent attachment of a chiral auxiliary to the calixarene framework while the second method involves desymmetrization of the substitution pattern (inherent chirality) of the phenol rings. Of the two methods, the former is much more common and only limited examples of inherently chiral calixarenes exist to date. Due to their smaller size and number of phenol rings, inherently chiral calixarenes have been explored more and few examples of inherently chiral calixarenes have been reported. To date, only one example of an inherently chiral calixarene has been reported. We have recently synthesized a racemic mixture of an inherently chiral calixarene-crown-6 compound with an (A,B,H) substitution pattern by monobridging a disubstituted calixarene with penta(ethylene glycol) di-p-tosylate in a diagonaltransannular-distal motif. X-ray crystallography confirms that the molecule adopts a cone conformation and provided the relevant dimensions of compound. When the racemic mixture was treated with 2 and 5 equivalents of Pirkle’s chiral reagent, two diastereomeric complexes were formed as evidenced by shifts in certain 'H NMR resonances for the host. Unfortunately attempts to optically resolve the racemic mixture into separate enantiomers by either chromatographic methods or via attachment of a chiral auxiliary have been unsuccessful. Efforts in this area are ongoing as well as chiral recognition studies. The alkali metal binding properties of the chiral calixcrown were probed by electrospray ionization mass spectrometry. The results clearly demonstrate that the compound forms a 1:1 complex with both sodium and cesium ions, but has a large preference for cesium. The binding properties with the same metal ions were also studied by liquid-liquid extraction experiments followed by UV analysis of the picrate salts extracted into chloroform. Those experiments contradicted the ESI/MS experiments in that no apparent selectivity was observed for Na+, K+, Rb+ or Cs+ ions which all showed percent extraction ranging from 5-11%. In addition, the ammonium ion affinity of the calixcrown was briefly examined via extraction experiments using ammonium picrate and the picrate salt of a chiral amine and both of these species formed 1:1 complexes.
Salazar, R. (2005). <i>Synthesis and binding properties of an inherently chiral calixarene</i> (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.