SynthesisandHighAntioxidantActivityofC-Alkyl Calix[4]resorcinarene and C-Alkyl Calix[4]pyrogallolarene Derivatives

Abstract

In the present work, a successful synthesis and high antioxidant activity of C-alkylcalix[4]resorcinarene and C-alkylcalix[4]pyrogallolarene derivatives were reported. The C-alkylcalix[4]resorcinarenes were prepared from a cyclization reaction of resorcinol and either pentanaldehyde or octanaldehyde under acidic conditions. Meanwhile, the Calkylcalix[4]pyrogallolarenes were prepared from a cyclization reaction of pyrogallol with pentanaldehyde or octanaldehyde. Four synthesized products, i.e., nBu-CR, nHep-CR, nBu-CP, and nHep-CP, were successfully prepared in 92.4-96.4% yield. Their chemical structure was elucidated by Fourier transform infrared (FTIR), liquid chromatography-mass spectrometry (LC-MS), and proton nuclear magnetic resonance (1H-NMR) analysis. The antioxidant activity assay of these compounds was evaluated through an in vitro assay employing 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals. The DPPH assay showed that the half-maximal inhibitory concentration (IC50) values of nBu-CR, nHep-CR, nBu-CP, and nHep-CP compounds were 25.1, 22.9, 11.5, and 21.9 µg mL-1, respectively. The IC50 value of the synthesized compounds was 2.0-4.3 times lower than the positive standard of butylated hydroxytoluene (BHT, IC50 = 49.9 µg mL-1), which was remarkable. This finding demonstrates that both Calkylcalix[4]resorcinarenes and C-alkylcalix[4]pyrogallolarenes are better antioxidant agents than BHT. The nHep-CR compound was found as the best antioxidant agent among the other compounds due to weaker intramolecular and intermolecular hydrogen bondings and a longer alkyl chain.