Receptors Based on the Amidic Bond

In the last two decades, chiral receptors containing amidic functions were designed almost exclusively for binding protected amino acids [49-57], oligopeptides [54,58], and lactic [59], tartaric [60,61] or camphoric acid derivatives [62]. Usually, chiral building blocks such as spirobifluorene [49, 60], binaphthalene [51,57],or amino acid chains containing macrocycles [52-56,58] were employed. An interesting receptor was synthesized via connection of the calix[4]arene moiety with an aza-crown derivative [61]. 

Although the works concerning amide-based receptors for anions are rather rare, several very interesting publications dealing with this topic can be found. One review on peptide and glycocalixarenes has recently appeared [63]. 

The design of receptors 32-34 was based on the attachment of chiral amino acid chains to benzo-18-crown-6 ether [64]. These molecules were able, upon the addition of cesium salts of amino acids, to form sandwich complexes and distinguish between enantiomers of the substrate added. The enantioselective 

The discrimination between enantiomers ofZ-phenylalaninate was observed only using receptors 32 and 33 for the complexation. For 34, no chiral recognition of enantiomers was detected. The same transport ratios and slow transport of Z-phenylalaninate by this receptor can be explained by formation of an external complex (Z-phenylalaninate is not bound between the amino acid chains of the receptor 34 and no sandwich complex is formed). 

Calix[4]arene is a very popular building block for the design of molecules with well-defined mutual positions of substituents. The attachment of amidic residues to this molecule resulted in receptors 35 [65] and 36 [66]. Upon the H NMR titration of 35 by tetrabutylammonium salts of amino acids in acetone-d6y moderate chiral recognition was observed N-Ac-L-Ala K=4,900 M-1, AT-Ac-D-Ala K=5,700 M 1, AT-Ac-L-Phe iC=7,900 M-1, AT-Ac-D-Phe K= 10,500 M 1. The receptors 36a-c formed stable complexes with AT-tosyl-L-phenylalaninate in deuteriochloroform (36a K= 1,626 M 1, 36b 1C=4,836 M 1, 36c K=6y924 M-1). Unfortunately, the measurement using AT-tosyl-D-phenylalaninate has not been reported. 

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