Elsevier

Tetrahedron

Volume 61, Issue 34, 22 August 2005, Pages 8101-8108
Tetrahedron

Effects of electron-withdrawing substituents on DPPH radical scavenging reactions of protocatechuic acid and its analogues in alcoholic solvents

https://doi.org/10.1016/j.tet.2005.06.040Get rights and content

Abstract

The DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity of protocatechuic acid (3,4-dihydroxybenzoic acid) and its related catechols was examined. Compounds possessing strong electron-withdrawing substituents showed high activity. NMR analysis of the reaction mixtures of catechols and DPPH radical in methanol showed the formation of methanol adducts. The results suggest that high radical scavenging activity of catechols in alcohol is due to a nucleophilic addition of an alcohol molecule on o-quinones, which leads to a regeneration of a catechol structure. Furthermore, the radical scavenging activity in alcohols would largely depend on the electron-withdrawing/donating substituents, since they affect the susceptibility toward nucleophilic attacks on o-quinone.

Introduction

Polyphenols such as hydroxybenzoic and hydroxycinnamic acids derivatives are known to exhibit potent antioxidant activities. In particular, catechol-type o-diphenols such as protocatechuic acid (3,4-dihydroxybenzoic acid, 1) and caffeic acid show high antiradical activity,1, 2, 3, 4 since they would be readily converted to the corresponding o-quinones and further complex products.1, 5 In recent years, kinetic studies on reactions of phenolic antioxidants with radicals, including substituents effects,6, 7, 8 have been extensively investigated.6, 7, 8, 9, 10 However, to understand the whole antioxidant mechanisms of catechols and explain why they show high radical scavenging activity, it is necessary to study the reaction events after the formation of o-quinones. Previously, we reported the solvent dependency of radical scavenging activity of protocatechuic acid and its esters.11 In non-alcoholic acetone or acetonitrile, protocatechuic acid and its esters consumed two radicals and were converted to their quinones. In contrast, protocatechuic esters rapidly scavenged more than four radicals with a concomitant conversion to the corresponding quinones, 3-hemiacetals,12 and their alcohol adducts at C-2 in methanol or ethanol. We found that regeneration of catechol structures by a nucleophilic addition of a solvent alcohol molecule on o-quinones is a key reaction for the higher radical scavenging activity of protocatechuic esters in alcoholic solvents than in non-alcoholic solvents.11, 13 Interestingly, 1 showed significantly low activity in alcohol compared to its methyl ester (2), and no alcohol adduct was formed. In addition, the radical scavenging activity of 3′,4′-dihydroxyacetophenone (3), 3,4-dihydroxybenzaldehyde (4) and 3,4-dihydroxybenzonitrile (5), which bear electron-withdrawing groups (–COMe, –CHO, –CN) at C-1 of the catechol ring, was comparable to that of 2.14 It seems that the radical scavenging activity is greatly affected by the C-1 substituents on the catechol ring. Therefore, in the present study, we extended our investigation on radical scavenging reactions to other C-1 substituted catechols. The objective of this study is to determine whether the radical scavenging mechanisms of protocatechuic ester analogues, possessing electron-withdrawing/donating groups at C-1, are similar or different to that of 2, and to examine the effects of the electronic properties of the substituents on the radical scavenging reactions beyond the formation of o-quinones. In this study, the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity of protocatechuic acid and its related compounds, which bear electron-withdrawing/donating substituents on the catechol ring (Fig. 1), was compared in acetonitrile and methanol. In addition, the reaction mixtures of catechols and DPPH radical were analyzed by NMR and HPLC, to determine what went on after the quinone formation with the aid of molecular orbital calculations, and the radical scavenging mechanism of protocatechuic ester analogues in alcoholic solvents is proposed.

Section snippets

Results and discussion

The DPPH radical scavenging activity of catechols (113), which possess electron-withdrawing/donating substituents at C-1 of the aromatic ring except 11, was evaluated in methanol and acetonitrile. The relative radical scavenging equivalences of compounds 113, when that of dl-α-tocopherol as standard was designated as 2.0, are listed in Table 1. In inert acetonitrile, all test compounds scavenged approximately two radicals in 30 min, and there was no significant difference in activity among

Conclusion

In conclusion, the radical scavenging mechanisms of catechols, bearing strong electron-withdrawing substituents such as –COMe, –CN at C-1, were similar to that of methyl protocatechuate. Moreover, the results suggested that the radical scavenging activity largely depends on the electron-withdrawing/donating properties of the substituents, since they affect the susceptibility toward nucleophilic attack on the o-quinones. In this study, nitrogen-centered DPPH radical was used as a model radical.

Chemicals

3,4-Dihydroxybenzaldehyde, 3′,4′-dihydroxyacetophenone, 4-fluoro-1,2-dimethoxybenzene, and 4-chlorocatechol were purchased from Tokyo Kasei Kogyo Co. 3,4-Dihydroxybenzonitrile and 4-methylcatechol were obtained from Aldrich Chemical Co. and protocatechuic acid from Sigma Chemical Co. Methyl protocatechuate (2) and methyl 3,4-dihydroxy-2-methoxybenzoate (2c) were prepared by the method described previously.11 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical and other reagents were purchased from

Acknowledgements

We are grateful to Mr. Kenji Watanabe and Dr. Eri Fukushi, of the GC–MS and NMR Laboratory of our school, for measuring mass spectra. This work was supported by research fellowship for young scientists from the Japan Society for the Promotion of Science (to S.S.).

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