New guanidinium-based carboxylate receptors derived from 5-amino-pyrrole-2-carboxylate: synthesis and first binding studies

The syntheses of two new guanidinium-based carboxylate receptors 2a,b derived from 5-amino pyrrole-2-carboxylate 4 are described. These receptors bind N-acetyl alanine carboxylate and O-acetyl lactate efficiently in aqueous DMSO as could be shown by NMR studies. However, compared to previously reported guanidiniocarbonyl pyrrole receptors 1, the reversal in the direction of the amide group in 2a,b changes both the substrate selectivity (amides are now preferred over esters) and their relative binding affinities. Both effects can be explained based on the calculated complex structure.