Structural Elucidation and Antimicrobial Evaluation of Novel [1,2,4]Triazolo[4,3-a]pyrimidines and Pyrido[2,3-d][1,2,4]triazolo[4,3-a]pyrimidinones

1,3-Di(thiophen-2-yl)prop-2-en-1-one (1) was utilized in the synthesis of 4,6-di(thiophen-2-yl)-3,4-dihydropyrimidine-2(1H)-thione (2) and 5,7-di(thiophen-2-yl)-2-thioxo-2,3-dihydropyrido[2,3-d]pyrimidin-4(1H)-one (4). The latter thiones were used in the synthesis of two new series of [1,2,4]triazolo[4,3-a]pyrimidines 10a–i and pyrido[2,3-d][1,2,4]triazolo[4,3-a]pyrimidinones 5a–i via reaction with the appropriate hydrazonoyl halides using triethylamine as a basic catalyst in dioxane. The mechanism of formation of the synthesized compounds was discussed, and the assigned structure was established via microanalysis, spectral data (infrared, 1H NMR, and Mass), and density functional calculations. Moreover, the newly synthesized products were evaluated for their antimicrobial activities, and the results show that some derivatives have been well with mild activities. Finally, quantum chemistry calculations confirmed the mechanism and structure of the products.