Pteridines. Part LXXXVII. Synthesis and Properties of 8‐Substituted 2‐Thiolumazines

Various 8‐substituted 2,8‐dihydro‐2‐thioxopteridin‐4(3H)‐ones (14–21) and 2‐(methylthio)pteridin‐4(8H)‐ones (27–32) have been synthesized by condensation of the appropriate 5‐amino‐6‐(substituted amino)‐1,2‐dihydro‐2‐thioxopyrimidin‐4(3H)‐ones (22–34) and 5‐amino‐6‐(substituted amino)‐2‐(methylthio)pyrimidin‐4(3H)‐ones (25, 26), respectively, with glyoxal, biacetyl, and benzil. The presence of a quinonoid cross‐conjugated π‐electron system makes this type of compounds susceptible to nucleophilic additions in position 7, which leads to intramolecular (43, 45) and intermolecular (44) covalent adducts. The newly synthesized compounds have been characterized by elemental analyses, pKa determinations, 1H‐NMR and UV spectra. UV‐Spectral changes in dependence of the pH are associated with the most appropriate molecular species including the monocations, neutral forms, covalent adducts, mono‐ and dianions.