Reaction of Barbituric, 2-Thiobarbituric Acids and Their Derivatives with 2-Carboxybenzaldehyde and Opianic Acid: Synthesis and Tautomerism of 5-(3'-Oxo-1',3'-dihydroisobenzofuran-1'-yl)barbituric Acids and Their 2-Thio Analogs

The reaction of barbituric, N-alkylbarbituric acids, and their 2-thio analogs with carboxybenzaldehyde and 2-carboxy-3,4-dimethoxybenzaldehyde leads to the formation of the corresponding 5-(3'-oxo-1',3'-dihydroisobenzofuran-1'-yl)barbituric and 2-thiobarbituric acids, the structures of which were studied by ¹H and ¹³C NMR spectroscopy and mass spectrometry. In DMSO the derivatives of barbituric acid exist in the form of mixtures of the ketone and enol tautomers, while their 2-thio analogs exist in the enol form. In chloroform the tautomeric equilibrium is displaced fully toward the ketone form.