Synthesis and antioxidant structure-activity relationships of hydroxycinnamic acid derivatives

Five kinds of hydroxycinnamic acid were synthesized by Knoevenagel condensation reaction, using hydroxy benzaldehyde derivatives, malonic acid, pyridine, piperazine hexahydrate.Structural identification and purity were confirmed by melting point analysis, _1H nuclear magnetic resonance (_1H NMR) , Infrared Spectroscopy (IR) , high performance liquid Chromatography (HPLC) .To evaluate its antioxidant activity, Fe³⁺ reducing power, DPPH free radicals scavenging activity were employed, and compared with hydroxy benzaldehyde derivatives, butylated hydroxytoluene (BHT) and cinnamic acid. Results showed that the products were identified as—3, 4-dihydroxycinnamic acid, 4-hydroxy-3-methoxycinnamic acid, 3-hydroxy-4-methoxycinnamic acid, 4-hydroxycinnamic acid, 3-hydroxycinnamic acid, whose purity were 92.45%, 93.14%, 99.55%, 96.54%, 93.50% respectively.3, 4-dihydroxycinnamic acid, 4-hydroxy-3-methoxycinnamic acid had strong reducing power on the Fe³⁺ and scavenging power on the DPPH radical, which were higher than that of BHT, and the IC₅₀ (half inhibitory concentration) of DPPH radical were (11.50 ± 0.02) , (16.57 ± 0.04) , (33.04 ± 0.03) μg/m L.Phenolic hydroxyl group was the antioxidant activity center of hydroxycinnamic acid derivatives, and hydroxycinnamic acid derivatives which had catechol structure or the hydroxyl's ortho-position with electron-donating group had strong antioxidant activity.4' hydroxy group on the benzene ring had strong antioxidant activity than 3' hydroxy group on the benzene ring. α, β-unsaturated acid conjugate structure connected with benzene ring could improved the antioxidant activity of phenolic compounds.