**1. Introduction**

The development of physiologically highly potent fused pyrimidines is a challenging task for synthetic organic chemists [1]. It is well known that pyrimidines either in isolated or in fused state are associated with a number of biological activities [2–6]. Moreover the pyrimidine nucleus containing thiouriedo linkage (-NH-C(S)-NH-) is pharmaceutically important as the development of medicine mainly arose from the heterocyclic compounds containing nitrogen and sulphur atoms [7–9]. Due to a wide range of biological activities exhibited by pyrimidine derivatives, these compounds occupy a unique place in the field of biological and medicinal chemistry. In view of such wide applications, several derivatives of fused pyrimidines were synthesized in maximum yields by using 1,3-diarylthiobarbituric acids, **1** (DTBA) which can be prepared in one pot reaction by treating 1,3-diarylthioureas with malonic acid in presence of acetyl chloride [10]. They are generally stable at room temperature and can be stored indefinitely without apparent

**Figure 1.** *Thiobarbituric acid derivatives.*

decomposition. Having an active methylene group, they can furnish several condensation products for easy cyclization to give various fused heterocyclic compounds of interest. When DTBA was reacted with ethyl orthoformate and benzaldehyde to the condensation products, namely, 5-ethoxymethylene-1,3-diaryl-2-thiobarbituric acids (**2**) and 5-phenyl-methylene-1,3-diaryl-2-thiobarbituric acids (**3**), respectively were formed (**Figure 1**) [11]. As **2** and **3** possess three electrophilic centres, they could undergo cyclocondensation with various binucleophiles to give various fused heterocycles of pyrimidine derivatives [12].
