Chiral poly crown ethers

Racemic modifications may be resolved. There are very few examples of this approach having been employed successfully. The racemic cylic ether (RS)-36, which contains two CH2OCH2CO2H arms attached to the 3 and 3 positions on the axially chiral binaphthyl units, has been resolved (48-50, 93, 94) to optical purity in both its enantiomers by liquid-liquid chromatography using a chiral stationary phase of either (R)- or (S)-valine adsorbed on diatomaceous eaitii. Very recently, the optical resolution of crown ethers (/ S)-37 and (/ 5)-38, incorporating the elements of planar chirality in the form of a rron -doubly bridged ethylene unit, has been achieved (95) by HPLC on (+)-poly(triphenyl-methyl methacrylate). 

Very recently, the synthesis of a series of chiral poly(9,9 -spirobiflu-orene)crown ethers [e.g., (SS)-139, (SSSS)-140, (SSSSSS)-141, and (S55SSS5S)-142] with 26-, 52-, 78-, and 104-membered rings have been described (162). The last three are rare examples of compounds with C4, 5, and Cg synunetries, respectively. They were isolated chromatographically from a reaction mixture consisting of (S)-2,2 -bisbromomethyl-9,9 -spirobifluorene, ethylene glycol, KOr-Bu, and Csl in tolune. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

Cryptands are macro-bi- or -poly-cycles able to encapsulate an ion by providing it higher protection because of their cagelike structures, as in (147) and (148). For these ligands the correspondence between cavity size and complex stability is more pronounced than for simple crown ethers. Recent approaches to improve the metal-ion selectivity of cryptands, as for example by replacement of ethylene units between each donor atoms with propylene units, or by insertion of several substituents into the macrocycles, have been reviewed.245 A new, interesting family of cryptands is constituted by borocryptands (149), which are useful receptors for chiral substrates, where enantiomeric differentiation can be achieved by using NMR spectroscopy.246... 

Treatment of a number of covalent polymers substituted with molecular recognition capability with suitable guests leads to chiral induction. The same crown ether-amino acid complementary pair described for the rosettes above was employed in the form of crown-ether pendant cis-transoidai poly(phenylacetylene). When the achiral polymer is treated with amino acids (in the form of their hydroperchlorate salts in acetonitdle) a large induced CD signal is observed in the backbone of the polymer. The polymer is sensitive to small enantiomeric excesses in the amino acid, as little as 0.005% enantiomeric excess of alanine can be detected. In a similar vein, c/5-transoidal poly(carboxyphenylacetylene) shows induced circular dichroism when treated with nonracemic chiral amines In addition, the system displays chiral memory, in that treatment of the complex with achiral amino alcohols results in retention of the chiral polymer backbone. 

Poly[(R,R)-dlbenzo-19-crown-6] Poly[(S,S)-dlbenzo-19-crown-6] Polymeric chiral crown ethers hostguest complexation discussed. [180]... 

This review presents the syntheses of different analogs of sucrose. Transformations of this di-saccharide at the terminal positions (Cl, or C6, or C6 ) provide modified sucroses such as amines, uronic acids, thiols, or phosphorus derivatives. Preparation of so-called higher sucroses i.e. derivatives in which the terminal positions are elongated by a poly-hydroxylated carbon chain is also reviewed. Special attention is directed to the synthesis and properties of macrocyclic derivatives crown- and aza-crown ethers with sucrose scaffold. Such macrocyclic receptors exhibit interesting complexing properties towards chiral ammonium salts. 

The rather exciting and stimulating "phase transfer" synthetic procedure based on the use of polymer supported active functional moieties, such as onium groups, crown ethers, cryptands and poly-glymes, becomes even more naive when the active sites are bound to chiral matrices, whose prevalent chirality can be either intrinsi-... 

In 1971, Davankov et al. achieved the first baseline separation of enantiomers using a small molecule-based CSP consisting of L-proline [1], Since then, a wide range of chiral small compounds, which include amino acids, cyclodextrins, macrocyclic glycopeptides, cinchona alkaloids, crown ethers, jt-basic or rt-acidic aromatic compounds, etc., have been used as CSPs [2—6], On the other hand, the polymer-based CSPs are further divided into two categories, i.e., synthetic and natural chiral polymers [7, 8]. Typical examples of the synthetic polymers are molecularly imprinted polymer gels, poly(meth)acrylamides, polymethacrylates, polymaleimides, and polyamides, and those of the natural polymers include polysaccharide derivatives and proteins. 

Figure 6.5 (a) Schematic representation of the macromolecular helicity induction on poly(1 -co-5 ) upon complexation with L-alanine. The achiral fullerene and crown ether pendants represented by yellow and blue rings for clarity arrange in a helical array along the one-handed helical polymer backbone induced by noncovalent chiral interactions with L-alanine. (b) CD spectra of poly(1o.i-co-5o,) with L- and o-Ala HClO ([Ala-HCl04]/[poly(1o.,-co-5o,)] = 2) in dichloromethane-acetonitrile (8/2, v/v) at 25... 

The incorporation of two nonidentical chiral residues, each supporting C2 symmetry, into a mactocyclic poly ether affords a chiral crown compound with C2 symmetry provided its structure is constitutionally symmetrical. Thus, base-promoted reaction of the half-crown diol prepared from (5)-birraphthol with the half-crown ditosylate d-72 synthesized tom diacetone-manrritol affords (144) the 20-crown-6 derivative (S)-d-113 with C2 symmetry. When d-72 is condensed in like fashion with (/ 5)-binaphthol, then the diastereoisomeric 20-crown-6 derivative (/ )-d-114 can be separated chromatogiaphically tom (S)-d-113. In this matmer, (/ 5)-binaphthol is resolved by the carbohydrate unit during the synthesis. 

While the above studies are representative of the area, it should be noted that a considerable number of other [2]-pseudorotaxanes have been investigated. These include a system exhibiting enantioselective self-assembly during its formation as well as systems incorporating a new structural motif involving a l,2-bis(pyridini-um)ethane axle and a 24-crown-8 ether wheel .A number of poly-pseudoro-taxanes (and related poly-rotaxanes) as well as constitutionally asymmetric and chiral [2]-pseudorotaxanes have been reported. New, multiply-stranded and multiply-encircled systems have also been prepared. The solid state structure of... 

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