Chirality in pyridoxal

Pyridoxal and its iminium derivatives form cyclic compounds via an intramolecular nucleophilic attack. This results in a configurational chirality on C4' which can be related to out-of-plane orientations around the C4C4' bond in the open forms. 1H- and 13C-NMR structural studies of the cyclic compounds were performed in solution and in the solid state (13C NMR only). It was found that the presence of an additional chiral centre in the iminium moiety gives rise to an excess of one of the diastereomers for the cyclic aminal. This indicates a preference for one of the axially chiral C4-C4' conformers prior to ring closure. The present results may be of relevance with respect to the function of the coenzyme pyridoxal phosphate, which forms iminium compounds with l or d amino acids. Axial chirality in these systems has been proposed by us as a mechanism for the stereospecificity of enzymatic amino acid transformations.