Chiral crown ether receptors

Another model was developed, by the group of J. M. Lehn (278) of Strasbourg. They prepared a complex between a pyridinium substrate and a chiral crown ether receptor molecule having four dihydronicotinamide derivatives as the side chain (Fig. 7.1). 

A different concept of chiral recognition was used by Lehn et al. (1978) for the differentiation between pairs of enantiomeric anions. Following the terminology used for metallo-enzymes, the chiral crown ether [309] acts as an apo-receptor, complexing a metal cation and thus becoming a chiral metal receptor that may discriminate between enantiomeric anions (cascade-type complexation). Extraction experiments with racemic mandelic acid dissolved in... 

The functionality that is often necessary for a chiral crown ether to serve a particular purpose can usually be introduced by the synthetic chemist without too much difficulty. The practice here, however, can be very much more demanding on account of the promiscuous receptor properties of the compounds that have to be separated and isolated pure from reaction mixtures containing many components. 

Closely related to the synthetic work reported in the previous section is the incorporation (131) of a 2,5-anhydro-3,4Hdi-0-methyl-D-mannitol residue (Figure 15) into the 18-crown-6 derivative d-91. Other derivatives of D-mannitol that have been built into crown ether receptors include l,4 3,6-dianhydro-D-maiuiitol (132), l,3 4,6-di-0-methylene-D-marmitol (13 134), and 1,3 4,6-di-O-benzylidene-D-mannitol (134). Examples of chiral crown compounds containing these residues include dd-92, dd-93, d-94, and d-95. Although not derived from carbohydrates—but rather (135) from the terpene, (-t-)-pulegone—... 

V. CHIRAL CONJUGATED POLYMERS CONTAINING CROWN ETHER RECEPTORS [28b]... 

Chirality derived from the readily accessible a-amino acids has been incorporated into the side chains of aza and diaza macrocyclic polyethers. A number of procedures suitable for peptide synthesis have proved (178) to be unsuitable for acylating the relatively unreactive secondary amine groups of aza crown ethers. Eventually, it was discovered that mixed anhydrides of diphenylphos-phinic acid and alkoxycarbonyl-L-alanine derivatives do yield amides, which can be reduced to the corresponding amines, e.g., l-172. By contrast, the corresponding bisamides of diaza-15-crown-S derivatives could not be reduced and so an alternative approach, involving the use of chiral A-chloroacetamido alcohols derived from a-amino acids, has been employed (178) in the synthesis of chiral receptors, such as ll-173 to ll-175, based on this constitution. 

H and 13C NMR have also been widely used to study the binding of alkylammonium cations to crown ethers. The shift in substrate proton resonances can give detailed information of the preferred structure of the complex. This is particularly important in the study of chiral recognition by asymmetric receptors (79CSR85) (Section 5.21.3.2.2) and also the development of bioorganic models and catalysts (Section 5.21.5.1.1). 

Dinitroazobenzene was used as the fluorescent residue in a chiral receptor molecule that incorporated both a crown ether and calixarene, the structure of which is shown in Figure 15. Formally, this comprises a ditopic cryptand but such a simple nomenclature is clearly inadequate <2004CH1174>. 

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