Fused pyrimidine derivativesFused pyrimidine derivatives R 0515 Cationic and Mesoionic 1,3-Thiazolo[3,2-c]quinazoline Derivatives. -The synthesis of otherwise not available cationic derivatives of the thiazolo[3,2-c]quinazoline ring system [cf. (V), (XII)] and related mesoionic compounds (IX) by intramolecular ring-closure reactions of appropriate S-and N-alkylated derivatives of quinazolinethiones (II) is described. -(WASFY, A.