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.