896792-06-6Relevant academic research and scientific papers
DPPH (=2,2-diphenyl-1-picrylhydrazyl=2,2-diphenyl-1-(2,4,6-trinitrophenyl) hydrazyl) radical-scavenging reaction of protocatechuic acid esters (=3,4-dihydroxybenzoates) in alcohols: Formation of bis-alcohol adduct
Saito, Shizuka,Gao, Hong,Kawabata, Jun
, p. 821 - 831 (2007/10/03)
Protocatechuic acid esters (=3,4-dihydroxybenzoates) scavenge ca. 5 equiv. of radical in alcoholic solvents, whereas they consume only 2 equiv. of radical in nonalcoholic solvents. While the high radical-scavenging activity of protocatechuic acid esters in alcoholic solvents as compared to that in nonalcoholic solvents is due to a nucleophilic addition of an alcohol molecule at C(2) of an intermediate o-quinone structure, thus regenerating a catechol (=benzene-l,2-diol) structure, it is still unclear why protocatechuic acid esters scavenge more than 4 equiv. of radical (C(2) refers to the protocatechuic acid numbering). Therefore, to elucidate the oxidation mechanism beyond the formation of the C(2) alcohol adduct, 3,4-dihydroxy-2-methoxybenzoic acid methyl ester (4), the C(2) MeOH adduct, which is an oxidation product of methyl protocatechuate (1) in MeOH, was oxidized by the DPPH radical (=2,2-diphenyl-1-picrylhydrazyl) or o-chloranil (=3,4,5,6-tetrachlorocyclohexa- 3,5-diene-1,2-dione) in CD3-OD/(D6)acetone 3:1). The oxidation mixtures were directly analyzed by NMR. Oxidation with both the DPPH radical and o-chloranil produced a C(2),C(6) bis-methanol adduct (7), which could scavenge additional 2 equiv. of radical. Calculations of LUMO electron densities of o-quinones corroborated the regioselective nucleophilic addition of alcohol molecules with o-quinones. Our results strongly suggest that the regeneration of a catechol structure via a nucleophilic addition of an alcohol molecule with a o-quinone is a key reaction for the high radical-scavenging activity of protocatechuic acid esters in alcoholic solvents.
