Calixarene structures, modified phenolics

Calixarene-A Versatile Host Calixarenes are macrocyclic host molecules made from phenol units finked through methylene bridges. The great freedom to structurally modify calixarenes allows us to create various types of host structures. 

Calixarenes have been extensively studied as solvent extractants for inorganic ions, stationary phases for capillary gas chromatography (GC), selectivity modifiers in CE, and as components of HPLC. The calixarene structure is quite versatile in that the lower rim can be modified to adjust solubility and the phenol groups on the upper rim are suitable for many chemical modihcations that may introduce specific selectivity. Here, we describe the application of modified calixarenes to ion chromatographic separations. 

Calixarenes were developed later than crown ethers and cyclodextrins but have stillbeen extensively researched. Macrocycles of calix[n]arenes are constructed by linking a number of phenol residues via methylene moieties (Fig. 2.16). Like crown ethers, the name calixarene reflects the structures of these molecules, since a calix is a chalice. Calixarenes with various cavity sizes have been designed, each of which has conformation isomers, and their phenolic hydroxyl groups are often modified. These structural characteristics allow us to create calixarene derivatives with various structural modifications. 

The structures and abbreviations used for designating the calix[4]arenes are shown in (5). These calix[4]arenes, have both a wide (upper) and a narrow (lower) rim that can be chemically modified to produce complexants that are selective for particular metal ions. In the simple calixarene framework the wide rim has hydrocarbon functionalities, and the narrow rim phenolic groups. Calixarenes are conformationally mobile, and the extreme structures for the calix[4]arenes have been termed the cone, partial cone, 1,3-alternate, and 1,2-alternate conformations (6). Because of the conical geometry of the calix[4]arene structure, the cavity size of the wide rim is larger than that of the narrow rim. 

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