**Abstract**

Several derivatives of fused pyrimidines were synthesized in maximum yields by using the respective condensation products, namely, 5-ethoxymethylene-1, 3-diaryl-2-thiobarbituric acids and 5-phenyl-methylene-1,3-diaryl-2-thiobarbituric acids, which can be obtained from 1,3-diarylthiobarbituric acids (DTBA). These condensation products possessing three electrophilic centres could undergo cyclocondensation with various binucleophiles to give various fused heterocycles of pyrimidine derivatives, such as pyrazolo[3,4-*d*]pyrimidine-6-thiones, 5,7-diaryl-4-oxo-isoxazolo[5,4-*d*]pyrimidine-6-thiones, 5-oxo-pyrimido[4,5-*d*]pyrimidine-7-thiones, 2-thioxo-pyrano[2,3-d]pyrimidine-4-ones, pyrido[2,3-*d*]pyrimidines, quinazoline-4-oxo-2-thiones.

**Keywords:** diarylthiobarbituric acids, pyrazolopyrimidines, isoxazolopyrimidines, pyrimidopyrimidines, pyranopyrimidines, quinazolines
