Calixarene core

The chiral cavitands 3.109 have been developed by combining the amino acid residue L-alanine with macrocyclic cavitands (calixarenes - Section 3.14).51 These ammonium ion receptors are able to complex a range of amino acids and their methylester hydrochloride salts, all of which contain an -NH3+ functionality capable of interaction with the carboxylate residues of the host. In general amino acids are bound only very weakly in aqueous solution, while association constants with the chiral methyl esters range from 620 M-1 for L-tryptophan methylester to 110 M-1 for L-alanine methylester. The methylester of glycine is not bound at all. Receptors related to 3.109 with variable four peptide loops arrayed around a central calixarene core have been used to bind to the surfaces of proteins. The... 

The synthesis of eight arm star PIB was recently described by Jacob et al. [37], where eight PIB arms emanate from a calixarene core (multifunctional initiators 20 (tert-hydroxy derivative) and 21 (terf-methoxy derivative)). The synthetic strategy is shown in Scheme 3. 

Dodecaferrocene 64 (Figure 8.3) was prepared for the study of multiple redox centers. The calixarene core was constructed by the acid-catalyzed condensation of ferroce-necarboxaldehyde with resorcinol. Beer et alJ58 have also constructed poly Na+ salt 65 by the connection of crown ethers to bipyridine ligands, which were subsequently organized by the complexation to a Ru(n) metal core. 

A series of calixarene-core-type initiators (MI-40, MI-47, and MI-51) were prepared and employed for the radical polymerizations of MMA, styrene, and tBA. Ruthenium catalysts357 were first employed, and copper catalysts are equally effective.421,422 PMMA obtained with the dichloroacetate version of MI-40 had narrow MWDs (MJMn = 1.1—1.2) and controlled numbers of arms, which were ascertained by the scission of arms from the cores after the polymerizations.357... 

The star polymers obtained with the bromoester-type calixarene-based initiators were analyzed by SEC equipped with a multiangle laser light scattering (MALLS) detector. The arm numbers were well controlled (close to the initiator s functionality), although the octafunctional initiator MI-51 induced star—star coupling in the styrene-polymerization at conversions higher than 20%.421 A similar series of tetra-, hexa-, and octafunctionalized initiators with calixarene cores were synthesized for sulfonyl chloride versions (MI-41, MI-48, and MI-52) and employed for copper-catalyzed MMA polymerizations.343... 

Scheme 56. Preparation of calixarene-core multifunctional initiators and the structure of calix [4] arene modified with acetyl chloride groups [346]...

Calixarenes [11] turned out to be especially useful for practical applications. Starting from readily available para-ferf-butyl-calix[4]-, -[6]- and -[8]arenes many derivatives have been prepared, and their properties have been determined [12]. Methods for nearly all desired functionalizations of the calixarene core are available by now, so that future work in this area will be directed mainly towards applications. A current area of interest is the use of substituted [ln]phanes as artificial receptors and sensor molecules [12 a]. 

The lower rim O-alkylation of a calixarene core with pyridine-based appendages would provide universal ligands for both hard and soft metal ions. Since minor changes in the regioselectivity of derivatization and conformation of modified calixarenes would influence their complexation, a variety of such pyridine conjugates varying in nature of linkers, site of attachment in pyridine, have been investigated. 

Inhibition of protein-protein interactions offers a potential target for medicinal chemists. Hamilton et al. have illustrated this approach through the synthesis of a range of calixarene derivatives that bind to and inhibit chy-motrypsin. The calixarene core in these molecules is appended with anionic peptide domains, which are spatially matched to positively charged domains on the chymotrypsin surface, resulting in strong electrostatic attraction. This approach has recently been extended to self-assembled systems that utilize a quadruplex DNA core. °... 

Appended to the calixarene core is a bicyclic guani-dinium group, which has the ability to hydrogen bond to and electrostatically stabilize the tetrahedral intermediate developed during the methanolysis of the carbonate functional group. This is shown in a schematic form below. This electrostatic/hydrogen bonding stabilization of the transition state leads to a 150-fold rate enhancement for the reaction relative to methanolysis in the absence of this artificial enzyme. 

In polymer synthesis, calixarenes - cyclic phenol-formaldehyde oligomers - are rarely studied and reported. There are two main applications of calixarenes in polymer synthesis use of metal calrxarene complexes as catalysts for polymerization and star-shaped polymers with a calixarene core. 

---