Conversion of 1,3-Dimethyl-5-(Arylazo)-6-Amino-Uracils to 1,3-Dimethyl-5-(Arylazo)-Barbituric Acids: Spectroscopic Characterization, Photophysical Property and Determination of pKa of the Products

The study of inter‐conversion between molecules, especially biologically and pharmaceutically important molecules, is extremely important. This study reports the inter‐conversion between two azo‐derivtives: azo‐6‐aminouracils to azo‐barbituric acids. We successfully converted the 1,3‐dimethyl‐5‐(arylazo)‐6‐aminouracils (Uazo‐1 to Uazo‐4) to 1,3‐dimethyl‐5‐(arylazo)‐barbituric acids (BAazo‐1 to BAazo‐4) (where aryl⇒C6H5‐(1); p‐MeC6H4‐(2), p‐ClC6H4‐(3), and p‐NO2C6H4‐(4)) following an acid‐hydrolysis path. The products were characterized using spectroscopic tools like UV‐vis, IR, and NMR spectroscopy. UV‐vis spectra of the as‐prepared dyes reveal that in contrast to the azo‐6‐aminouracils they are hardly responsive towards solvatochromism. IR spectra exhibit three characteristic >CO frequencies for as‐prepared azobarbituric acids instead of two for mother azo‐6‐aminouracils. 1H NMR spectra which reflect the existence of solution species evidence the absence of >CNH group (characteristic imido‐H at the 6‐position of hydrazone species of azo‐6‐aminouracils) and consequence presence of >CO group at the same position in as‐prepared azobarbituric acids. They exhibit structural emissions in the range of 400–440 nm upon excitation at 360 nm. The determined acid dissociation constant (pKa) values of BAazos increase according to the following sequence: BAazo‐2>1>3>4. The study of inter‐conversion between biologically and pharmaceutically significant molecules is extremely important. The conversion from azo‐6‐aminouracils to azo‐barbituric acids, of two pyrimidine bases, has thus been projected. In doing so, solvatochromism, photophysics and pKa values of the products: azo‐barbituric acids have been explored. In contrast to azo‐6‐aminouracils, the values of pKa of azo‐barbituric acids fall in the basic region.