Synthesis and biological evaluation of new 1,3,4-thiadiazole derivatives as potent antimicrobial agents

A series of 1,3,4-thiadiazole derivatives were designed and synthesized using N-(4-nitrophenyl)acetohydrazonoyl bromide and 1-[3,5-dimethyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl]ethan-1-one as starting materials. The treatment of 1-[3,5-dimethyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl]ethan-1-one with methyl hydrazinecarbodithioate or hydrazinecarbothioamide afforded 2-[1-[5-methyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl]ethylidene]hydrazine derivatives. The targeted 1,3,4-thiadiazolyl derivatives were prepared by the reaction of 2-[1-[5-methyl-1-(4-nitrophenyl)-1H-pyrazol-4-yl]ethylidene]hydrazine derivatives with hydrazonoyl chloride derivatives. The reaction of N-(4-nitrophenyl)acetohydrazonoyl bromide with 2-[(methylthio)carbonthioyl]hydrazones in absolute ethanol in the presence of triethylamine afforded the corresponding 1,3,4-thiadiazole derivatives. The newly synthesized compounds were fully characterized by 1H NMR, 13C NMR, IR, MS, and elemental analysis. Moreover, the antimicrobial activity of the synthesized 1,3,4-thiadiazole derivatives were tested against E. coli, B. mycoides, and C. albicans. Four compounds outperformed the other produced compounds in terms of antimicrobial activity.

The chemical structures of the 1,3,4-thiadiazole compounds 17a-17h were established by their spectral data ( 1 H NMR, 13 C NMR, IR, MS) and elemental analysis. For example, the IR spectrum of 17a showed characteristic band at 1713 cm −1 could be attributed to C=O stretching frequency. Its 1 H NMR spectrum showed triplet signal at δ = 1.39 ppm corresponding to methyl group of OCH 2 CH 3 , two singlet signals at 2.31 and 2.52 ppm corresponding to the CH 3 groups, quartet signal at 4.33 ppm corresponding Scheme 2

Scheme 4
to methylene group of OCH 2 CH 3 and two doublet signals at 8.33 and 8.42 ppm of the protons of 4-NO 2 C 6 H 4 . Its 13 C NMR spectrum showed 12 signals for asymmetric carbon atoms. Its mass spectrum showed the molecular ion peak at m/z = 349.
Likewise, the treatment of the starting material 2 with methyl carbodithioate derivatives 18-21 under the same conditions afforded the corresponding 1,3,4-thiadiazole derivatives 22-25, respectively (Scheme 5). The chemical structures of 22-25 were confirmed by the spectral data ( 1 H NMR, 13 C NMR, IR, MS) and elemental analysis. For example, the 1 H NMR spectrum of 25 showed multiplet at δ = 1.97-2.01 ppm corresponding to CH 2 group, singlet at 2.50 ppm refer to the CH 3 group, two triplets at 2.86 and 3.01 ppm corresponding to two CH 2 groups, multiplet at 7.17-7.32 ppm due to three aromatic protons, doublet at 8.26 ppm due to one aromatic proton and pair of doublets at 8.34 and 8.47 ppm corresponding to protons of 4-NO 2 C 6 H 4 . The 13 C NMR spectrum showed 17 signals for asymmetric carbon atoms. Its mass spectrum showed the molecular ion peak at m/z = 379. Table 1 and Fig. 2 illustrate the inhibition zones induced by the tested synthesized 1,3,4-thiadiazole derivatives against the tested micro-organisms. All compounds were shown to be capable of reducing the growth of the tested microbial strains.

Antimicrobial activity
Antimicrobial susceptibility studies demonstrated that compound 14 outperformed the other produced compounds in terms of antimicrobial activity, with inhibition zones of 21 mm against E. coli, 23 mm against B. mycoides, and 22 mm against C. albicans (Table 1 and Fig. 2). Furthermore, compounds 13a, 17c, and 17h have inhibition zone diameters of 17-18 mm against E. coli and B. mycoides, respectively, whereas compounds 13a, 13c, and 17g have an inhibition zone diameter of 17 mm against C. albicans.
Compound 17b, on the other hand, displayed the least antimicrobial activity (inhibition zone of 14 mm) against all tested microbes (Table 1 & Fig. 2). Furthermore, the results showed that the Gram-positive bacterium example (B. mycoides) was more vulnerable to the majority of the examined produced chemicals than Gram-negative bacteria (E. coli). Streptomycin (10 mcg) as a positive control had lesser antimicrobial efficacy than compound 14, but findings are comparable to other synthetic compounds evaluated (Table 1).

S y n t h e s i s o f 5 -m e t h y l -2 -a r y l i d e n e h y d r azono-3-(4-nitrophenyl)-2,3-dihydro-1,3,4-thiadiazole derivatives 17 and 22-25
Method A A mixture of 1.3 g N-(4-nitrophenyl)acetohydrazonoyl bromide (2, 5 mmol) and the appropriate 2-((methylthio)carbonthioyl)hydrazones 16 and 18-21 (5 mmol) was dissolved in 50 cm 3 absolute ethanol. To the resulting solution, 2 cm 3 triethylamine was added and reaction mixture was stirred for 6 h at room temperature. The resulting solid product that precipitated was collected, washed with ethanol, and crystallized from a suitable solvent to give the corresponding 1,3,4-thiadiazole derivatives 17 and 22-25.