Zinc(II) complexes of phosphonic acid analogues of aspartic acid and asparagine

An analogue of aspartic acid obtained by the replacement of the α-carboxylic group by a phosphonic acid group underwent chemical transformation upon the action of zinc(II) ions under neutral and slightly alkaline conditions yielding, most probably, a cyclic phosphonamidate composed of two ligand molecules. In contrast, an analogue obtained by the replacement of the β-carboxylic group by a phosphonic acid moiety did not undergo such a transformation. In order to determine which structural features are responsible for the observed cyclization a series of aminophosphonic acids was synthesized and their behaviour in zinc(II) solutions studied by means of NMR spectroscopy. Cyclization of 1-aminoalkylphosphonic acids upon the action of zinc(II) ions in neutral and slightly alkaline media seems to be a general property of these acids. The crystal structures of two of the studied compounds were also determined.