Calixarene - A Versatile Host

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 conformational isomers of a cahxarene with four phenol residues are shown in Fig. 2.17. The isomers vary in terms of the orientations of their phenol groups (a) has a cone structure with all of the phenols pointing to the same direction (b) has a partial cone structure with one phenol pointing in a different direction to the others (c) has a 1,3-alternate structure with neighboring phenols pointing in opposite directions. These isomeric hosts have different selectivities for metal ion inclusion in the upper cavity and the lower cavity. Of course, changing the number of phenol residues alters the guest size appropriate for effective inclusion. 

The calix[8]arene depicted in Fig. 2.18 can bind fullerenes (see Chap. 3) the fullerene soccer ball is trapped in the calix. Fullerenes are usually prepared as mixture of Ceo, C70, 75, and so on, and separating them is not always easy. The calix[8]arene has a cavity with an inner diameter of 1 nm, which is therefore suitable for Ceo, since it has a diameter of 0.7nm. When the calixarene is added to a toluene solution of a mixture of fullerenes, a 1 1 complex of the calixarene and Ceo selectively precipitates. Isolation of the precipitates followed by dispersion of them in chloroform results in the precipitation of dissociated Ceo. Repeating these processes results in Ceo with high purity. 

Spectral Shift upon Change of Hydrophobic Environment 

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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, when in their cone-conformation (54), represent versatile host systems for metalated container molecules and many examples have been reported in the literature (55-61). Reinaud and coworkers have carried out extensive work concerned with calix[6]arenes that are functionalized at the small rim by nitrogen arms (62), aiming to reproduce the hydro-phobic binding site of mononuclear zinc and copper metalloen-zymes. A recent example is the calix[6]arene ligand L1 (Fig. 3), in which a tris(2-methylpyridyl)amine unit covalently caps the calixarene small rim (63). The ligand forms copper complexes of... 

A Versatile Class of Macrocyclic Compounds, J. Vicens, V. Bohmer, Eds., Kluwer, Dordrecht, 1991, p. 65 G. D. Andreetti, F. Ugozzoli, Inclusion Properties and Host-Guest Interactions of Calixarenes in the Solid State, ibid., p. 87. 

In summary, our approach of using cyclopeptides with natural amino acids and 3-aminobenzoic acid subunits for the development of macrocydic receptors has afforded remarkably efficient hosts. The cation affinity of 4b, for example, exceeds that of many calixarene derivatives. Even more interesting is the high anion affinity of 5 in aqueous solution. By introdudng additional functional groups such as car-boxylates to the periphery of the cavity, we recently also obtained cydopeptides that interact with neutral substrates, for example, carbohydrates [25]. Our peptides therefore represent a versatile dass of artificial receptor that should prove useful in supramolecular and bioorganic chemistry. 

Calixarenes and their derivatives have been of growing interest to the scientific community due to their widespread applications as unique and versatile hosts [34-42]. This class of compounds possesses a cavity with distinct hydrophobic and hydrophihc sites represented by upper and lower rims. By suitable modifications of these sites, one may obtain several different calixarenes for a wide variety of applications. It has been demonstrated that a variety of guests with different size, shape, properties, and characteristics can interact with calixarenes and their derivatives [35,36]. 

A -Alkyl Ammonium Resorcinarene Salts A Versatile Family of Calixarene-Related Host Molecules... 

The calixarenes [1, 2] are a versatile group of host molecules which have indeed been used to probe various aspects of enzyme functions [3]. Nonetheless, it is not to be expected that a single molecular structure could provide a basis for the mimicry of all enzymic behaviour, and thus new molecules with controllable architecture are valuable. This work describes such a group of molecules, closely related to the calixarenes obtained through the syntheses which combine aspects of cyclo-phane and host/guest chemistry. It defines the group of a//-homocalixarenes . 

The most readily available calixarenes are compounds derived from p-t-butylphenol, 1-3. In order to introduce functional groups at the upper rim of the calix, the f-butyl group has to be removed, and this can be performed in 70-90% yield by reacting the p-f-butylcalixarenes with an excess of anhydrous aluminium trichloride in toluene in the presence of phenol at room temperature (Scheme 7.10). The unsubstituted p-//calixarenes thus obtained are versatile intermediates which give access to hosts having a large variety of properties. 

All these examples illustrate the great versatility of calixarenes as supports for a variety of other functions able to participate in host-guest complexing and cation and anion sensing. 

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