Here we provide a report on the synthesis, structural recognition and tautomeric behavior of a new series of coumarin-based hydrazide-hydrazones (4-methylcoumarin-7-yl-oxymethyl arylaldehyde acetohydrazones) and their 1,3,4- oxadiazoline and oxadiazole derivatives. The final products were tested for antibacterial activity on both Gram-negative and Grampositive bacteria. Both types of hydrogen, the hydrazone N-H and methylene protons can take part in tautomeric isomerization equilibrium. The synthesis of 1,3,4-oxadiazoles and oxadiazoline bearing coumarin ring was conducted for inferring the isomerization-tautomerization of their hydrazone precursors According to the NMR investigation, We found that, the amide-iminol tautomerization is more probable than keto-enol one. It was further explained via converting the hydrazones to the corresponding 1,3,4-oxadiazoles and oxadiazolines based on the modified procedures. All the synthesized oxadiazoles and oxadiazolines have antibacterial effect on Gram-negative bacteria with respect to Gram-positive bacteria. The active ingredient of the synthesized derivative 8 has the best effect on both Gram-negative and Gram-positive bacteria. Manuscript profile