The reaction of thione 3 or its 2-methylthio derivative 4 with hydrazonoyl halides 5a-l, in the presence of triethylamine, yielded the corresponding triazolo[4,3-alpha]pyrimidin-5(1 H)-ones 8a-l. The structure of compounds 8a-l was further confirmed by the reaction of 3 with the appropriate active chloromethylenes 11a-c followed by coupling of the products with benzenediazonium chloride to afford the azo-coupling products 6b, f, and j, which were converted in situ to 8b, f, and j. 2-Hydrazinyl-pyrido[3',2':4,5]thieno[3,2-d]pyrimidin-4(3 H)-one (13) was prepared and condensed with different aldehydes 14a-f to give the corresponding hydrazone derivatives 15a-f. Oxidative cyclization of the hydrazones 15a-f give the corresponding triazolo[4,3-alpha] pyrimidin-5(1 H)-one derivatives 16a-f. The antimicrobial activity of the products was evaluated and the results revealed that compounds 8f and 15f showed strong activity against gram-positive bacteria while compound 15d showed the highest activity against gram-negative bacteria. Moreover, compounds 15b, 8d, 8e, 8c, 8l, and 8j exhibited significant antifungal activity. In addition, the antitumoral activity of the synthesized products against different cancer cell lines was determined and the results revealed that compound 12c was the most active against MCF-7, HepG-2, HCT-116, and HeLa with IC50 values of 0.51, 0.72, 0.95, and 0.95, respectively, as compared with doxorubicin as positive control.