Synthesis of 5-(4-Formylphenyl)barbituric Acid to Access Enolizable Chromophoric Barbituric Acids

Barbituric acids mono-substituted at the 5-position show keto–enol tautomerism. In the keto form, conjugation to an aryl substituent is interrupted due to the sp³-hybridized carbon atom at the 5-position of barbituric acid. The enol form generates a conjugated π-system to the aryl substituent and acts as an electron-donating group. If the aryl substituent is electron-deficient, a push-pull system is generated that shows typical UV/Vis absorption. These types of compounds are difficult to access synthetically due to their intrinsic convertibility. The synthesis of barbituric acids with a 4-formylphenyl functionality at the 5-position is reported. This compound, 5-(4-formylphenyl)barbituric acid, could be used to introduce extended π-systems with electron-withdrawing groups in great variety by simple condensation reactions. We demonstrate this by a Horner–Wadsworth–Emmons reaction that forms the enolizable dye (E)-5-(4-(4-nitrostyryl)phenyl)barbituric acid.