Ring-chain tautomerism of 2-aryl-6-oxohexahydropyrimidine-4-carboxylic acid sodium salts

Examples of ring-chain tautomeric equilibria in the series of saturated pyrimidine derivatives were given in the papers [1, 2], summarized in reviews [3-5], and in a monograph [6]. The condensation products of the natural amino acid L-asparagine with aromatic aldehydes have been assigned a linear [7] or cyclic 6-oxohexahydropyrimidine structure [8-10]. The possibility of simultaneous presence of linear and cyclic forms as a solution-phase tautomeric equilibrium has not been considered in the literature. 6-Oxohexahydropyrimidine-4-carboxylic acid derivatives have been used as key intermediates in the synthesis of β-amino acids [8, 11], in asymmetric cycloaddition reactions [9], and in the preparation of liposomal microencapsulated biopreparations [10]. The aim of the present work was a structural study of L-asparagine (1) condensation products with a series of aromatic aldehydes, and also establishing the dependence of tautomeric equilibrium on the electronic properties of aromatic aldehyde component. Compounds 2a-h were obtained in 65-90% yield after maintaining L-asparagine, the corresponding aromatic aldehyde, and an equivalent amount of sodium hydroxide in methanol solution for 48 h at 25°C.