Enantioselective recognition carboxylic acids

The dependence of chiral recognition on the formation of the diastereomeric complex imposes constraints on the proximity of the metal binding sites, usually either an hydroxy or an amine a to a carboxylic acid, in the analyte. Principal advantages of this technique include the ability to assign configuration in the absence of standards, enantioresolve nonaromatic analytes, use aqueous mobile phases, acquire a stationary phase with the opposite enantioselectivity, and predict the likelihood of successful chiral resolution for a given analyte based on a well-understood chiral recognition mechanism. 

Despite the relatively large number of enantioselective receptors of cations and neutral species, reports on the effective chiral recognition of anions are still rare. However, there are a number of reports in the literature describing the recognition of carboxylic acids, and amino acids in particular, performed in aqueous solution at a pH where the carboxylic group has to be at least partly ionized. These reports are also included in this review. 

Ligand 62 [83] was prepared for the enantioselective recognition of amino acids. Chiral carboxylates are bound by cooperative binding by electrostatic... 

Another interesting development is the use of bicyclic guanidinium receptors as heteroditopic systems for the enantioselective recognition of amino acids. In this case the guanidinium fragment was combined via a flexible linker with a crown ether moiety in order to promote the simultaneous binding of the carboxylate anion and the ammonium cation belonging to a zwitterionic amino acid [45]. 

The design of 11 was aimed at the enantioselective recognition of zwitterionic aromatic amino acids under neutral conditions.The features of the receptor include a chiral structure, an aromatic substituent for n-stacking. and binding sites for a carboxylate group and an ammonium... 

Boronic acid-derived fluorescent chemosensors are unique in that the inter-molecular interaction is a covalent bond, and not hydrogen bonding as is the case for most conventional fluorescent molecular sensors used for the selective reeognition of hydroxyl carboxylic acids. This chapter summarizes the development of the boronic acid-based chiral fluorescent chemosensors over recent years and the enantioselective fluorescent reeognition of chiral a-hydroxyl carboxylic acids analytes in aqueous solutions. The fundamental scaffolds of these chiral sensors include a fluorophore, an arylboronie aeid binding site, and linker between the two units. The systems usually consist of a bis-boronic acid unit, which is required for enantioselective recognition of the chiral a-hydroxyl carboxylic acid analytes. However, mono-boronic acid fluorescent chemosensors have also been developed. All three components of the chiral boronic acid sensors play an important role in determining the... 

The same author has reported chiral recognition of a-amino acids by native, anionic, and cationic a- and (3-cyclodextrins [17]. Both carboxylates and amines (monosubstituted as well as hexa- and heptasubstituted) were included in this study. The best results obtained were those from a combination of (S)- and (P)-AcTrp complexed by per-NH -[3-cyclodextrin with K=2,310 and 1,420 (1/mol). In the detailed study of chiral recognition of substituted phenyl-acetic acid derivatives by aminated cyclodextrins, these were found to be again only modest with respect to the enantioselection attained [18]. 

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