Synthesis, characterization, in silico molecular docking, and antibacterial activities of some new nitrogen-heterocyclic analogues based on a p-phenolic unit

Nitrogen-containing heterocycles have shown pharmacological properties against various diseases. Herein, in our study, flavoHB enzyme is a highly promising well-validated target for identification of antibacterial inhibitors using in silico and in vitro techniques. To identify a new class of antimicrobial agents, N-(4-hydroxyphenyl)-3-oxobutanamide was utilized as a precursor in the synthesis of several nitrogen-based heterocycles (pyridine, pyrimidine, and pyrazole) attached to p-phenolic substrates 2–8. Treatment of 3-oxobutanimide 1 with malononitrile and/or ethyl cyanoacetate in ethanolic piperidine afforded the pyridinone analogues 2a,b. On the other hand, treatment of 1 with arylidene cyanothioacetamide furnished the pyridinthione derivative 3. The reaction of starting material 1 with salicylaldehyde and/or dimethyl formamide dimethyl acetal (DMF-DMA) yielded the pyridinones 4 and 5, respectively. Reaction of 1 with terephthalaldehyde and urea or thiourea gave bis structures 6a,b. The reaction of compound 1 with ethyl isothiocyanate and hydrazine hydrate afforded pyrimidine and pyrazole derivatives 7 and 8, respectively. The structures of newly prepared compounds 2–8 were elucidated using elemental data and spectral analyses such as IR, (1)H NMR, (13)C NMR, and MS. In addition, an in-house nitrogen-containing heterocycle analogues library 2–8 was examined and screened in vitro for their antibacterial effects against Gram-negative bacteria, Escherichia coli and Gram-positive bacteria, Staphylococcus haemolyticus, Kocuria kristinae, Enterococcus casseliflavus, and Bacillus cereus. Compounds 6a and 6b have also shown the highest antibacterial activity against all types of bacteria strains tested except Kocuria kristinae. Further, the molecular docking study of the newly prepared compounds with the target enzyme flavohemoglobin (flavoHB) was undertaken to explore their potential inhibitory activities. The results of the docking study indicated that compounds 6a and 6b have exerted the highest docking scores against the active site of flavoHB. As a result, the in vitro and molecular docking study findings suggested that the compounds 6a and 6b (with pyrimidine moiety, amide linkage, and phenolic substrate) might be potent bacterial flavohemoglobin (flavoHB) inhibitors and they could set a promising starting point for future design of antibacterial agents.