Double Calixarenes and Their Analytical Applications

The synthesis of various types of double calixarenes is reported.The complexation and extraction behaviors of these calixarenes are summarized. TABLE OF CONTENTS1) Introduction 2) Synthetic Strategies for Double Calixarenes A) Head-to-Head Double Linked Calixarenes B) Tail-to-Tail Double Linked Calixarenes C) Head-to-Tail Double Linked Calixarenes D) Miscellaneous 3) Applications of Double Calixarenes 4) Future Scope and Conclusion 5) References Vol. 21.No. 1, 2008 Double Calixarenes and Their Analytical Applicationsrecognize guest molecules, a rigid, conformationally defined calixarene is essenlial.Calix[4]arenes possess four possible basic conformations that are termed "cone", "partial cone", "1,2-alternate", and "1,3-alternate", respectively 111.However, this is not the case with calix[6)arene.Considering only the relative orientations (syn or anti) of the aromatic nuclei, there are eight possible conformations for calix[6]arenes, i.e. "cone", "partial cone", "1,2alternate", "1,3-alternate", "1,4-alternate", "1,2,3-alternate", "1,2,4alternate", and "1,3,5-alternate", and the number increases dramatically if the value of the angles that each aromatic nucleus makes with the average plane of the molecule is taken into account 111.The mobility of conformation of calix[6]arene has been reduced by bridging, including capping, of calix[6]arene.To freeze the conformation of calix[6)arene, another route is the bridging of two or capping of three or more phenolic rings.Here, the term "bridge" means that two phenolic rings are connected by a spacer, and "cap" means that the spacer links three or more phenolic rings of caiix[6]arene, or the calixarene is linked by a spacer with triple or more linkage.The bridging or capping of calix[6]arene not only represents a potential method of freezing its conformation, but also constructs an interesting kind of host molecules with a special conformation, which possess good complexation ability HI.Researchers are also interested in one of the most useful building blocks for the construction of a complex molecule with an internal cavity capable of including large molecules.This goal requires a large cavity to hold the large molecules.Large molecules having internal cavities capable of including guest molecules are of great interest to workers.So, aiming at the elaboration of larger systems based on calixarenes units, organic chemists have been attracted to the design and production of double calixarenes /3/.Double calixarenes exhibit a potential to recognize and include larger species, such as neutral organic molecules and metal ions /4 a, b/.This account deals with the synthesis, conformational behavior and complexing properties of double calixarenes.These double calixarenes can be represented as a receptor in which two calixarenes subunits are linked via different linkages [Fig 1 j.