Synthesis, Structural Characterization, and Antioxidant Potential of N⁴-Substituted Thiosemicarbazones Derived from Hydroxybenzaldehydes Containing a Morpholine Fragment

In the context of the increasing resistance to antibiotics and antitumor therapy, the development of new compounds with these biological effects remains important. Thiosemicarbazones are organic compounds that exhibit a broad spectrum of biological activities, including antibacterial, antioxidant, antifungal, and anticancer properties. According to the literature, thiosemicarbazones containing free hydroxyl (OH) groups in the phenyl fragment demonstrate good antioxidant activity. Therefore, the synthesis of thiosemicarbazones based on 2-hydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, and 2,3,4-trihydroxybenzaldehyde was proposed in this study.

For this purpose, N-[2-(morpholin-4-yl)ethyl]hydrazinecarbothioamide 1 equivalent was mixed with 1 equivalent of the corresponding aldehyde. The reaction mixture was refluxed in ethanol for 4 h, then cooled, and the precipitated solid was filtered and recrystallized from ethanol. The structures of the obtained compounds were confirmed by NMR and FTIR spectroscopy.

Subsequently, the antioxidant activity of the synthesized compounds was evaluated using the ABTS assay. The obtained data suggest that the synthesized compounds are effective antioxidants. The IC₅₀ values were 19.83 (± 0.46) μM for the 2-hydroxybenzaldehyde -based thiosemicarbazone, 3.51 (±0.03) μM for the 2,4-dihydroxybenzaldehyde-based thiosemicarbazone, and 3.18 (±0.02) μM for the 2,3,4-trihydroxybenzaldehyde-based thiosemicarbazone. All synthesized compounds exhibited stronger antioxidant activity than the reference compound Trolox 33.3 (± 0.3) μM, highlighting their enhanced antioxidant potential.

In conclusion, three thiosemicarbazones based on 2-hydroxybenzaldehyde, 2,4-dihydroxybenzaldehyde, and 2,3,4-trihydroxybenzaldehyde containing a morpholine fragment at position N⁴ were synthesized in this study. All compounds exhibited good antioxidant activity. It was observed that antioxidant activity increased with the number of hydroxyl (OH) groups present in the compound.