The DPPH radical-scavenging abilities of the naturally occurring phenolic acid, 2,3,4-trihydroxybenzoic acid, and its methyl ester were evaluated. Both compounds in acetonitrile scavenged as many as four radicals compared to three or fewer radical consumption in acetone or ethanol. Only the ester showed relatively high ability in methanol. Oxidation with o-chloranil in acetonitrile resulted in methyl 2,3,4-trihydroxybenzoate giving a novel benzocoumarin-type dimer, its chemical structure being confirmed by spectroscopic evidence. The formation of this dimer might partly account for the higher radical-scavenging efficiency of the ester in acetonitrile or methanol.Key words: 2,3,4-trihydroxybenzoic acid; phenolic acid; oxidative coupling; DPPH radical; o-chloranil Hydroxybenzoic acids and esters are widely distributed in plants and have attracted considerable attention for their antioxidative activities. [1][2][3][4] The radical-scavenging reaction of catechol-and pyrogallol-type hydroxybenzoic acids typically comprises two distinctive phases, initially fast and subsequently slow. 5,6) The former phase is recognized as the conversion of an o-diphenol to the corresponding o-quinone, whereas complex reactions including oligomerization of the resulting quinone would occur in the latter slow phase. The difference in total radical-scavenging efficiency of each hydroxybenzoic acid inherently depends on this latter phase, although little has been found for this complex stage. So far, a protocatechuic (3,4-dihydroxybenzoic) ester (1) gave its quinone dimer, 6) quinone acetal 7) and adducts, [8][9][10][11] and a gallic (3,4,5-trihydroxybenzoic) ester (2) and its analogs also produced quinone dimers 6,12,13) and adducts.12) In contrast to protocatechuic and gallic acids, there has been no report on the oxidation reaction of another naturally occurring phenolic acid, 2,3,4-trihydroxybenzoic acid (3).14-16) We have investigated the DPPH (2,2-diphenyl-1-picrylhydrazyl) radicalscavenging abilities and the oxidant-mediated reaction of 2,3,4-trihydroxybenzoic acid (3) and its ester (4) (Fig. 1), and identified a novel oxidative dimer in the reaction mixture with o-chloranil (3,4,5,6-tetrachloro-obenzoquinone) in acetonitrile. Materials and MethodsReagents. 2,3,4-Trihydroxybenzoic acid (3) (Tokyo Kasei Kogyo Co.), 2,2-diphenyl-1-picrylhydrazyl (DP-PH) radical (Wako Pure Chem. Ind.) and o-chloranil (Aldrich Chem. Co.) were purchased from each supplier. Methyl 2,3,4-trihydroxybenzoate (4) was prepared from 3 by heating with methanol containing 10% hydrogen chloride. All solvents used were of technical grade.Colorimetric radical-scavenging tests. DPPH radical scavenging tests were conducted as described in the previous paper. y To whom correspondence should be addressed. Fax: +81-11-706-2496; E-mail: junk@chem.agr.hokudai.ac.jpBiosci. Biotechnol. Biochem., 71 (7), [1731][1732][1733][1734] 2007 tion, UV 260 or 400 nm. Under these conditions, 3 and 4 were eluted at t R ¼ 23:4 and 37.5 min, respectively.Isolation of benzocoumarin dim...