Bisurea calix[4]arenes
1 and
2 possessing
L-amino acid moieties at the lower rim were synthesized byreaction of the methyl esters of glycine,
L-alanine, or
L-isoleucine with the appropriate isocyanate (
12 or
13), obtained with a safe and efficient Curtius rearrangement from the corresponding carboxylic acidderivatives. The conformational properties of the ligands
1 and
2 were investigated by means of a combinedNMR and molecular modeling study which evidences that they are deeply influenced by strongintramolecular H-bonds between the urea NH groups and the vicinal phenolic oxygen atoms or the oppositeurea C=O group. Complexation studies performed by ESI-MS and NMR spectroscopy in acetone solutionshow that the binding ability of these bisurea hosts decreases by increasing the side chain size of theamino acid. Host
2b has a remarkable binding ability for the
N-acetyl-
D-phenylalaninate anion with aninteresting enantioselectivity (
![](/isubscribe/journals/joceah/72/i09/eqn/jo062410xe10001.gif)
/
![](/isubscribe/journals/joceah/72/i09/eqn/jo062410xe10002.gif)
= 4.14), which is explained on the basis of a three-pointinteraction mode of binding.