Stable isotope-labeling studies on the oxidative coupling of caffeic acid via o-quinone.
The formation of ortho-quinone from ortho-diphenol is a key step in its dimerization. An NMR analysis of the oxidation of 3,4-dihydroxycinnamic acid (caffeic acid) by NaIO4 revealed the formation of 3-(3',4'-dioxo-1',5'-cyclohexadienyl) propenoic acid (o-quinone) prior to the formation of furofuran-type lignan 4,8-exo-bis (3,4-dihydroxyphenyl)-3,7-dioxabicyclo[3.3.0]octane-2,6-dione. Both electrolytic and enzymatic oxidation of caffeic acid also generated o-quinone. The yields of o-quinone from caffeic acid were quantified by NMR and HPLC analyses. A stable isotope-labeling study of the formation of lignans directly proved the random radical coupling of semiquinone radicals formed from a set of caffeic acid and o-quinone.
Tazaki
H
H
Department of Bioresource Science, Obihiro University of Agriculture and Veterinary Medicine, Japan. tazaki@obihiro.ac.jp
Taguchi
D
D
Hayashida
T
T
Nabeta
K
K
D016227:Benzoquinones / Q000737:chemistry*; D002109:Caffeic Acids / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D007554:Isotopes / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010084:Oxidation-Reduction
D016227:Benzoquinones; D002109:Caffeic Acids; D007554:Isotopes; C004532:quinone; C040048:caffeic acid