Pteridines. Part XLI. Synthesis and properties of 6,7,8‐trimethyl‐4‐thiolumazine

The first representative of the 8‐substituted 4‐thiolumazine series has been synthesized. In a sequence of reactions, 4,6‐dichloropyrimidin‐2‐(1H)‐one (1) is first converted into 4‐chloro‐6‐(methylamino)pyrimidin‐2(1H)‐one (6), then the Cl‐atom displaced by the thioxo group (→7) followed by a coupling reaction with 4‐chlorophenyldiazonium chloride to introduce the necessary N‐function into the 5‐position (→ 9; Scheme 1). Reduction of the p‐chlorophenylazo group leads to the 6‐(methlyamino)‐4‐thiouracil‐5‐amine (10) which on condensation with diacetyl gives 6,7,8‐trimethyl‐4‐thiolumazine (8). The physical properties of 8 are compared with the 2‐thio analog and 6,7,8‐trimethyllumazine indicating that 8 possesses the highest acidity and the longest UV absorption.