**2. Pyrazolo-pyrimidines**

The methods reported for the synthesis of pyrazolo-pyrimidines involved a number of steps and yields were poor [13–18]. A convenient route for the synthesis of pyrazolopyrimidines was described [11, 19]. The condensation products, **2** and **3** have been utilized as three carbon fragments for the synthesis of 4-oxo-pyrazolo[3,4-*d*]pyriimidines, **4** and **6**, respectively. When **2** was treated with hydrazine hydrate in the presence of ethanol and acetic acid, the corresponding pyrazolo[3,4-*d*]pyrimidin-6-thione (**4**) was yielded in 65–82% overall yields (**Figure 2**). Similarly, when **2b** was treated with phenyl hydrazine in ethanol and acetic acid, 2-phenyl-5,7-bis(2′-methylphenyl)-4 oxo-pyrazolo[3,4-*d*]pyrimidin-6-thione (**5b**) was produced in 55% yield. However, when **3** was refluxed with hydrazine hydrate in ethanol and acetic acid, the corresponding (3*H*)-3-phenyl-5,7-diaryl-4-oxo-pyrazolo[3,4-*d*]pyrimidin-6-thiones (**6**) were obtained in 60–75% overall yields (**Figure 2**). It was observed that **6** could be oxidized to give the respective 3-oxo-pyrazolo-[3,4-*d*]pyrimidin-6-thiones (**7**) [11, 12, 19].

**Figure 2.** *Pyrazolo-pyrimidines.*

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

It is well known that the fused heterocycles possessing the pyrazolo[3,4-*d*]pyrimidine nucleus serve as a class of compounds which exhibited a remarkable variety of biological activities [20–23]. The pyrazolo[3,4-*d*]pyrimidine compounds have been found to show similar characteristics to purines, since they are isomeric structural purine analogues, which have resulted in several potent antagonists in biological systems [24, 25].
