Urea- and Thiourea-Based Receptors

The bicyclic receptors 40 and 41 were also prepared for enantioselective recognition of chiral carboxylates [69, 70]. The substitution of two benzene rings by pyridines in the receptor 41 enriched this molecule with two additional sites capable of hydrogen bond formation. 

The ability of receptor 41 to bind enantioselectively to the chiral substrates has not yet been examined. The preliminary NMR studies performed with this receptor demonstrated the high affinity of 41 to N-Ac-asparagine (K=8,700 M 1) and AT-Ac-glutamine (K=55,000 M-1). 

Thiourea functions were used to attach chiral saccharide units to the molecule of calix[4]arene [72]. The complexation properties of these molecules toward chiral anions have not yet been examined. However, in the preliminary complexation studies (XH NMR titrations in DMSO-d6) the affinity toward acetate and AT-Ac-L-alaninate was observed. 

The synthesis of receptor 43 was based on calix[5]arene [73]. The NMR studies in tetrachloroethane/methanol 2 1 showed the ability of this receptor to bind achiral zwitterionic c-aminohexanoic acid. 

We have prepared a very simple binaphthalene-based receptor 44 capable of enantioselective recognition of lactate and mandelate [74]. This extremely 

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Abstract This review article provides a broad overview to the area of anion coordination by synthetic organic receptors and includes examples of different functional groups used to bind anions. The first section examines neutral anion receptors containing amide-, sulfonamide-, urea- and thiourea-based receptors. Then aromatics such as pyrrole, car-bazole and indole are discussed before concluding the discussion of neutral systems with examples of hydroxy OH donors. A brief overview of charged systems is also provided. 

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