**8. Pyrido-pyrimidines**

Very few substituted pyrido[2,3-*d*]pyrimidines were synthesized from 6-aminouracils [49, 50]. The synthesis of substituted 3-cyano-6,8-diaryl-2,5-dioxopyrido[2,3-*d*]pyrimidin-7-thiones (**20**) from the corresponding condensation product (**2**) derived from 1,3-diarythiobarbituric acids were also reported [12]. Thus, the reaction of **2** with cyanoacetamide in presence of sodium isopropoxide and isopropanol afforded the corresponding pyrido[2,3-*d*]pyrimidines (**20**) in 62–74% overall yields (**Figure 10**).

**Figure 10.** *Pyrido-pyrimidines.*
