In the present work three series of Mannich bases of 1,3,4-oxadiazole-2(3H)-thione (121170) have been successfully synthesized with good to excellent yield and comprehensively characterized by FTIR, 1H-NMR, 13C-NMR, HRMS and elemental analysis. Furthermore, their in-Silico and in-vitro evaluation showed them as excellent urease inhibitors which were even better than standard. All the synthesized compounds were also evaluated for their antioxidant properties as well as antiviral activities against the four poultry viruses (IBA, H9N2 , NDV, and IBDV). The synthetic strategy for the synthesis of these Mannich base derivatives are as under. Synthesis and Biological evaluation of pyrazinyl based derivatives (119-135) The strategy for the synthesis of targeted compounds is elaborated in Scheme 29. Ethyl pyrazine-2-carboxylate (119) was prepared by the esterification of pyrazine-2-carboxylic acid (118), which was further converted into pyrazine-2-carbohydrazide (120) by refluxing with 80% hydrazine hydrate in ethanol. The synthesized hydrazide underwent cyclization to yield 5-(pyrazine-2-yl)-1,3,4-oxadiazole-2-thione (121) upon reacting with CS2 in alkaline medium. Which was then further converted to respective Mannich bases 3-[{(substituted phenyl)amino}methyl]-5-(pyrazine-2-yl)-1,3,4-oxadiazole-2-thione or 5-(pyrazin-2-yl)-3[{(substitutedpyridin-4-yl)amino}methyl]-1,3,4-oxadiazole-2-thione (122-135).