Chiral calixarenes

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. 

Probably the first examples of chiral calixarene derivatives were esters with camphorsulfonic acid24 derived from t-butyl-calixfSjarene.25 Later ethers obtained with (S)-2-methylbutyl bromide were synthesized and studied with respect to their CD spectra and their chiral recognition ability.26... 

Calixarene esters are easily available by alkylation with ethyl bromoacetate and are often used as starting materials for the introduction of chiral groups at the narrow rim. Their aminolysis by chiral amines led to chiral calixarene derivatives in high yields. Water soluble calix[4]arene amino acid derivatives 9a,b obtained in this way, were successfully used as a pseudostationary phase... 

The binding of N,N V-trimethyl- 1-phenylethylammonium iodide by 55 was studied in chloroform, where 1 1 complexes having association constants in the range (Afa =) 10 - 15 M 1 were found. The signals of the racemic salt were split into two patterns in the presence of the chiral calixarene analogues. 

If the phosphorotropic rearrangement is combined with selective electrophilic substitution of free phenolic units (e.g. bromination) and hydrolysis of ester or phosphate groups a variety of inherently chiral calixarenes becomes available,130 as demonstrated by the examples in Figure 15. 

The area covered is very widespread and the role of the calixarene molecules reach from a simple platform or skeleton on which to assemble chiral centers to an inherent part of the chiral structure. Biologically active molecules or derivatives are involved as well as artificial ligands and their metal complexes. Chiral calixarenes have been used as stationary phases in analytical separations or as host molecules in sensors. Basic properties of calixarenes, such as their conformational stabilities, have been studied with chiral derivatives as well as more... 

For earlier reviews on inherently chiral calixarenes and related compounds see (a) Bohmer, V. Kraft, D. Tabatabai, M. J. Incl. Phenom. Mol. Recogn. 1994,19,17-39. (b) Otsuka, H. Shinkai, S. Supramolecular Science 1996,3,189-205. 

Siegel, J. Biali,S.E. Mislow, K. J. Am. Chem. Soc. 1987,109,3397. In the present case, however, the directionality of the hydrogen bonds is the only stereogenic element and consequently all inherently chiral calixarenes would be cycloenantiomers. The expression cycloenantiomers was also used for instance for rotaxanes composed of wheel and an axle, which both are achiral by themselves but have a structural directionality Yamamoto, C. Okamoto, Y. Schmidt, T. Jager, R. Vogtle, V. J. Am. Chem. Soc. 1997,119, 10547-10548. 

Synthesis of Dissymmetric and Asymmetric Calixarenes Chiral Calixarenes... 

The majority of chiral calixarenes are made either by affixing a chiral moiety to the upper or lower rim of an achiral calixarene or by establishing a dissymmetric... 

The most obvious way to generate a chiral calixarene is by attachment of a chiral moiety. This was achieved as far back as 1979 by esterification with camphorsul-fonyl chloride,a chiral moiety which has again more recently been incorporated into a calix[4]arene. Several other examples have also been published describing the attachment of the chiral moiety at the lower rim via ether linkage. 

A more subtle way to generate a chiral calixarene is to create dissymmetry or asymmetry within the molecule itself One of the means for doing this with a calix[4]arene is to establish a substitution pattern of AABC or ABCD on the upper or lower rim, producing a compound that is inherently chiral in all possible conformations. Tabulations of the cone, partial cone, 1,2-alternate, and... 

We synthesized chiral calixarene 9 by the reaction of (leH) and (S )-l-bromo-2-methylbutane [16]. In an aqueous system / -(5)-2-methylbutyloxybenzenesulfonate 10, a noncyclic analogue, did not give a perceptible CD spectrum at 220-300 nm. Probably, the asymmetric carbon in 10 is too far from the chromophoric benzene ring to affect it intramolecularly. In contrast, 9 gave a clear CD spectrum in this... 

The search for inherently chiral calixarenes has been steadily pursued since the very beginning of calixarene... 

Grady T, Harris SJ, Smyth MR et al (1996) Determination of the enantiomeric composition of chiral amines based on the quenching of the fluorescence of a chiral calixarene. Anal Chem 68 3775-3782... 

The variety of types of chiral calixarenes is exceptionally broad. We will discuss here the main strategies for obtaining chiral calixarenes involving classic approach via attachment of auxiliary/ies with stereogenic centers. We will also focus ot... 

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