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 re
arrangement from the corresponding c
arboxylic acidderivatives. The conformational properties of the ligands
1 and
2 were investigated by means of a combinedNMR and molecul
ar modeling study which evidences that they
are deeply influenced by strongintramolecul
ar 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 rem
arkable binding ability for the
N-acetyl-
D-phenylalaninate anion with aninteresting enantioselectivity (
![](/isubscribe/journals/joceah/72/i09/eqn/jo062410<font color=)
xe10001.gif">/
![](/isubscribe/journals/joceah/72/i09/eqn/jo062410<font color=)
xe10002.gif"> = 4.14), which is explained on the basis of a three-pointinteraction mode of binding.