In this study, baker's yeast promoted one-pot synthesize of new 1,2,4-triazolpyrimidooxadiazole derivatives in high yields using multicomponent reactions of isatins, guanidine, alkyl bromides, aldehydes and hydrazonoyl chlorides in the presence of baker's yeast as a high performance catalyst in water at room temperature. Investigation of antioxidant ability of synthesized compounds using radical trapping of diphenyl-picrylhydrazine (DPPH) and ferric reduction power experiment is another purpose in this research. Also, the antimicrobial activity of some synthesized compounds proved by employing the disk diffusion test on Gram-positive and Gram-negative bacteria. This procedure has some benefits such as short reaction time, product with excellent yields and simple, efficient, inexpensive, and environment-friendly catalyst. Green chemistry is the use of a set of principles to reduce or eliminate the use or generation of unsafe materials in the design, fabrication and applications of chemical products. Among solvents, water is a green solvents and very suitable for performing organic reaction. The present procedure avoids the use of toxic solvent. 1,3,4-Oxadiazoles are a known class of compounds, reported to exhibit antibacterial, antifungal, analgesic and anti-inflammatory, anticonvulsant, hypoglycemic, and anticancer activities. [ABSTRACT FROM AUTHOR]