Electronic absorption spectra and acidity of new condensed systems of uracil-derivatives of 1,3-dithiolo[4,5-d]pyrimidine

The first and second ionization constants of certain halouracils and a series of derivatives of a new condensed heterocyclic system — (4H,6H)-1,3-dithiolo[4,5-d]pyrimidine-5,7-dione — were evaluated spectroscopically. Substituents at the 2-position of this system were discovered to have considerable influence on the NH-acidity, which increases in the series of 2-O=, 2-S=, 2-Se=, 2-[(C₂H₅)₂N=]⁺. The stability of the spectrum at pH 14 for the 2-S= and 2-Se= derivatives indicates their unusual stability toward hydrolysis, compared with the 2-O= derivative. The mono- and diionization considerably influence the absorption of the dithiolthione and dithiolselenone system, causing bathochromic shifts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 416–421, March, 1992.