**3. Isoxazolo-pyrimidines**

Most of the methods for the preparation of the isoxazolo[5,4-*d*]pyrimidine system are initiated from the construction of an isoxazole ring first and followed by pyrimidine ring closure [26, 27]. DTBAs were also used as precursors for the synthesis of various isoxazolo[5,4-*d*]pyrimidines [28]. When **2** and **3** were treated with hydroxylamine hydrochloride in dehydrated alcohol and acetic acid gave 5,7-diaryl-4-oxo-isoxazolo[5,4-*d*]pyrimidin-6-thiones **(8)** in 65–85% overall yields and 3,5,7-triaryl-2,3-dihydro-4-oxo-isoxazolo[5,4-*d*]pyrimidin-6-thones (**9**) in 60–70% overall yields, respectively (**Figure 3**) [28].

It was reported that the isoxazolo-pyrimidine derivatives exhibited antifungal, antibacterial and many other important biological properties [29–31]. The biological activities in terms of the antifungal and antibacterial properties of the compounds (**8** and **9**) were determined by the standard disc diffusion method. The bacteria (*Escherichia coli* and *Bacillus subtilis*) and fungi (*Penicillium* species, *Aspergillus* sp., *Fusarium* sp. and *Trichoderma* sp) were grown in nutrient agar and potato dextrose agar (PDA) plates, respectively. Compound **9c** was found to have the highest activity against coli form organisms whereas other compounds were moderated and have less activity. The studied compounds were found to be inactive against some species of fungi, except **8b** was found to have antibiotic property in *penicillium* species.
