Oxidative dimers produced from protocatechuic and gallic esters in the DPPH radical scavenging reaction.
DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging reactions of protocatechuic and gallic acids, and their methyl esters, have been investigated by NMR. In acetone, methyl protocatechuate was gradually converted to a Diels-Alder adduct of two molecules of the intermediate quinone in the reaction with DPPH radical, whereas methyl gallate rapidly gave a symmetrical dimer via a putative quinone precursor. Both dimers are rather unstable and their structures have been deduced by in situ NMR measurements of the reaction mixtures. Gallic acid also gave a corresponding symmetrical dimer in the same reaction as methyl gallate, although protocatechuquinone produced from protocatechuic acid did not yield a Diels-Alder adduct, unlike its methyl ester. Interestingly, these dimer formations were not observed in methanol solution.
Kawabata
Jun
J
Laboratory of Food Biochemistry, Division of Applied Bioscience, Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan. junk@chem.agr.hokudai.ac.jp
Okamoto
Yasuko
Y
Kodama
Asuka
A
Makimoto
Terumasa
T
Kasai
Takanori
T
D000096:Acetone; D016227:Benzoquinones / Q000737:chemistry; D001713:Biphenyl Compounds; D019281:Dimerization; D016166:Free Radical Scavengers / Q000737:chemistry*; D005707:Gallic Acid / Q000737:chemistry*; D062385:Hydroxybenzoates / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D010084:Oxidation-Reduction; D010851:Picrates / Q000737:chemistry*
D016227:Benzoquinones; D001713:Biphenyl Compounds; D016166:Free Radical Scavengers; D062385:Hydroxybenzoates; D010851:Picrates; D000096:Acetone; C009091:protocatechuic acid; C004532:quinone; D005707:Gallic Acid; C004931:1,1-diphenyl-2-picrylhydrazyl