Cavitands calixarenes

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... 

A further category of cavitands are the calixarenes (Gutsche, Dhawan, No Muthukrishnan, 1981 Gutsche Levine, 1982). Structure (255) illustrates an example of this type which is readily prepared by treatment of 4-f-butylphenol with formaldehyde and base. The compound may exist in other conformations besides the saucer-shaped one illustrated by (255). Similarly, f-butyl-calix[4]arene (256 R = CH2COOH) has an enforced hydrophilic cavity in the shape of a cone the alkali and ammonium salts of this host are soluble in water (Arduini, Pochini, Reverberi Ungaro, 1984). 

Cavitands, prepared by the Twente group, differ from the wide-rim CMPO calixarene not only by the basic scaffold, but also by the distance to the rim and in the amido function (tertiary amide versus secondary amide). In comparison to CMPO calixarenes, compounds Cvl and Cv2 did not lead to improvements in terms of extracting ability or selectivity.17169... 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

In order to find compounds able to perform an efficient lanthanide/actinide separation, the Twente group prepared two cavitands (Cvl and Cv2) functionalized with CMPO groups and two cavitands (Cv3 and Cv4) with carbamoylmethylphosphonate (CMP) groups192193 (see Section 4.7). Distribution ratios displayed by CMPO cavitands are much lower than those found for the calixarene counterpart. This important decrease of extracting ability of cavitand is probably due to the presence of a carbon atom between the benzene unit and the nitrogen atom, causing N-protonation below pH 2. Furthermore, the Am/Eu selectivity is less than that of CMPO-calix[4]... 

In certain cases, cosan derivatives were used to improve the distribution ratios. In particular, it was interesting to know if linking a calixarene and cosan could lead to a synergistic effect in comparison to a mixture of these compounds. For the first time, very sophisticated synthesis led to mixed compounds with cosan linked to the wide rim or narrow rim of calixarenes or cavitands, Cosl and Cos2, precursors of new extractants.18... 

Much interest has centred on the branch of cyclophanes known as calixarenes. They are polyphenol systems that can act as hosts in the formation of inclusion compounds, where a small guest molecule resides completely in a cavity within a single host they are cone-shaped cavitands . Several accounts have appeared of their history. The discovery by Baeyer of a formaldehyde/phenol resin led to Bakelite and to the work of A. Zincke and E. Ziegler, who gave to the first oligomer a tetrameric structure of a calix[4]arene. Later syntheses by Gutsche (1978) led to calixarenes with 4, 6 or 8 phenol residues.107-109... 

Calixarenes and cyclodextrins (CDs) are the simplest combination of organic compounds able to form molecular cavities. The relative ease of functionalization makes them versatile enzymatic binding site mimics [24]. These cavitands can af-... 

Calixarenes, cavitands, carcerands, and cyclophanes as nanoscale molecular containers 02BCJ393. 

Only few investigations have been published on the gas-phase ion chemistry of host-guest complexes of calixarenes. With the advent of ESI mass spectrometry, especially when combined with ion-trap and FT-ICR mass spectrometry, this field has started to be developed. Binding selectivities of alkali metal ions to cahxarene-based crown ethers and open-chain ethers have been studied , the inclusion of neutral guests into the protonated resorcarene-based cavitand hosts by gas-phase ion-molecule reactions with amines have been studied and the formation of capsules from various calixarene tetraether derivatives and alkylammonium ions as ionic guests (notably enabling their detection by mass spectrometry) have been described recently . ... 

Two calixarenes have also been connected in various ways to porphyrins. 109 may be taken as an example, in which the porphyrin plane separates two chambers confined by the calix[4]arenes . The two self-folding cavitands (compare 94) in the C-shaped isomer of 110 (formed in mixture with the S-shaped isomer) can simultaneously include two different guests . ... 

Figure 5.8 Capsule formation by self-complementary calixarenes (left) and cavitands (right)...

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