Calixarene agents

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

C36 263 caldum 74 calibration runs 164 calixarenes 133 cancer chemotherapy agents 9... 

Based on the theory, the separation of enantiomers requires a chiral additive to the CE separation buffer, while diastereomers can also be separated without the chiral selector. The majority of chiral CE separations are based on simple or chemically modified cyclodextrins. However, also other additives such as chiral crown ethers, linear oligo- and polysaccharides, macrocyclic antibiotics, chiral calixarenes, chiral ion-pairing agents, and chiral surfactants can be used. Eew non-chiral separation examples for the separation of diastereomers can be found. 

The calixarenes have become extremely popular and versatile cation complexing agents and supramolecular scaffolds. 

Therefore, another strategy was developed, based on the induced-fit concept, which uses flexible receptors in order to optimize the interactions between the donor atoms and the metal ion. In fact, the coordination environment is built upon complexation thanks to the flexibility introduced into the complexation agent, which is now termed predisposed ligand . These receptors are either large macrocycles able to wrap around the guest or small macrocycles fitted with pendant arms. The latter approach has proved to be very successful, particularly with calixarene (Asfari et al., 2001) and cyclen (1,4,7,10-tetraaza-dodecane) (Lukes et al., 2001) derivatives. 

Both lower rim and upper rim substitutions have been used to modify the coordination strength of the calixarenes and the properties of the resulting edifices. We shall only give a few examples here, with an emphasis on the efforts aimed at designing efficient luminescent probes and extraction agents. 

The gadolinium complex with Cxi displays a reasonable relaxivity and it has been assessed as a potential contrast agent for magnetic resonance imaging. Its stability constant is however too low to make this complex a good candidate for in vivo applications. Until now, no calixarene derivative has proved efficient in this field, contrary to the pendant-arm substituted coronands. 

An alternative and often facile route to appropriately functionalised ICPs, that avoids the synthetic problems outlined in (ii) above, is the use of sulfonated species containing the desired molecular recognition/receptor site as the dopant anion for the conducting polymer chains. For example, calixarene-containing polypyrroles [34] and polyanilines [35] for selective metal ion detection have recently been prepared via the use of sulfonated calixarenes as dopant anions. We have similarly found that the incorporation of metal complexing agents such as sulfonated 8-hydroxyquinoline as dopants in polypyrroles provides a simple route to metal ion-selective ICPs [36]. 

Ramakumar, J., Nayak, S.K., and Maiti, B., Transport of uranyl ion across a bulk liquid membrane using calixarene and synergistic agents as carriers. J. Membr. Sci., 2002, 196 203-210. 

Research continues into other agents, apart from surfactants (which are discussed in Chapter 6), which can enhance the solubility of dmgs. The calixarenes are another type of host, existing in a cup-shape in a rigid conformation. The 4-sulfonic calix[n]arenes can form host-guest type interactions with drugs such as nifedipine, a poorly water soluble agent, seen in Fig. 5.9. 

Still another calixarene derivative of interest and utility is the trimethylsilyl ether. The hexa-trimethylsilyl ealix[6]arenes and octa-trimethylsilyl calix[8]arenes can be prepared 23) by using standard trimethylsilylating agents such as hexamethyldisilazene and chlorotrimethylsilane. The tetra-trimethylsilyl ethers of the calix[4]arenes, however, do not form under these conditions and require the use of the very reactive N,0-i (trimethylsilyl (acetamide 109). 

---