**14. Conclusion**

Many fused pyrimidine derivatives are belonged to the most important heterocyclic systems, and there are numerous synthetic preparative methods for these compounds. However, in some cases, difficult access to key intermediates, or to their precursors, was a serious limitation for the above syntheses. Various 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-oxopyrimido[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, etc.

*Construction of Biologically Active Five- and Six-Membered Fused Ring Pyrimidine Derivatives… DOI: http://dx.doi.org/10.5772/intechopen.108842*
