Novel Isatin-Derived Thiosemicarbazones: Synthesis, Characterization, and Assessment of Antioxidant Activity

A series of new of isatin-derived thiosemicarbazones (1–8) were synthesized using different isatin derivatives and N-(4-ethylphenyl)hydrazinecarbothioamide. The intermediated thiosemicarbazide was obtained by the reaction of 1-ethyl-4-isothiocyanatobenzene with hydrazine monohydrate. The identities and purities of all newly compounds were confirmed through standard spectroscopic techniques, including proton and carbon nuclear magnetic resonance (¹H and ¹³C NMR), Fourier-transform infrared (FT-IR) spectroscopy, and elemental analysis. In this study, the antioxidant potential of the newly synthesized compounds was evaluated using the 1,1-diphenyl-2-picrylhydrazyl (DPPH•) free radical scavenging assay. Compound 7 showed higher activity than the standard Trolox and the other compounds. The remaining compounds exhibited lower activity than the standard. The IC50 values of the compounds were determined to range from 15.88 ± 1.21 to 145.34 ± 2.68 μM. The antioxidant activity of the compounds decreased in the following order: 7 > 2 > 3 > 8 > 6 > 5 > 4 > 1. Particular attention was given to the relationship between structural modifications and antioxidant activity, with a focus on how different functional groups influence the compounds' ability to neutralize free radicals.