Bicyclic guanidinium-carboxylate

Figure 7. A sketch of the proposed solution ensemble of bicyclic guanidinium-carboxylate associated species. The configurations can also be considered as snapshots taken from an individual host-guest pair in certain time intervals, because all configurations should be rapidly interconverting.

In 1992, de Mendoza and coworkers reported the synthesis of the bifunction-alized chiral bicyclic guanidinium 34 for the purpose of enantioselectively binding zwitterionic aromatic amino acids (i.e., tryptophan and phenylalanine) [55]. By including binding elements for the carboxylate and ammonium of the amino acids which were noncomplementary, they hoped to avoid the potential... 

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]. 

---