Spirooxindole 5-(Trifluoromethyl)benzothiazole Hybrids: Synthesis, Crystal Structure, and Antioxidant Activity

Antioxidants neutralize free radicals to reduce oxidative stress and cellular damage, preventing various chronic diseases. In this study, the antioxidant activities of novel 5-(trifluoromethyl)-3H-spiro[1,3-benzothiazole-2,3'-indol]-2'(1'H)-ones (4a-h), along with the spirocyclic 2-indolinones (4i-m and 5a-n) previously synthesized by our group, were investigated for the first time, to find a multi-modal antioxidant and to understand the impact of the trifluoromethyl group located at the 5-position of the benzothiazole on antioxidant activity. For this purpose, the 2,2-diphenyl-1-picrylhydrazyl (DPPH•) free radical scavenging, superoxide anion scavenging, ABTS^•+ radical cation scavenging, trolox equivalent antioxidant capacity, ferric ion reduction antioxidant potential (FRAP), and lipid peroxidation (TBARs) assays were carried out. The cytotoxic effects of compounds (4g, 4h, 4i, 4j, 4l, 5a, 5b, and 5h), which showed promising in vitro antioxidant activity in at least two in vitro tests, were investigated in healthy human fibroblast (HFF-1) cells. The effects of non-toxic doses of these compounds on intracellular reactive oxygen species were also evaluated. As a result, it was determined that compound 4l (R₁ = 5-F and R₂ = H) exhibited greater radical scavenging activity than the other compounds tested. The IC₅₀ of compound 4l was 74.13 µM while compound 4g (R₁ = 5-Cl-7-CH₃ and R₂ = H) was non-toxic (IC₅₀ > 200 μM). Compounds 4e (R₁ = 5-Cl and R₂ = H) and 4f (R₁ = 7-Br and R₂ = H), exhibited higher lipid peroxidation inhibitory activity than α-tocopherol in vitro; however, the difference was not statistically significant.