Synthesis of gallic acid: Cu2+-mediated oxidation of 3-dehydroshikimic acid

With the elaboration of high-yielding, high-titer syntheses of 3-dehydroshi kimic acid from glucose using recombinant Escherichia coli, oxidation of th is hydroaromatic becomes a potential route for synthesis of gallic acid. Co nversion of 3-dehydroshikimic acid into gallic acid likely proceeds via ini tial enolization of an alpha -hydroxycarbonyl and oxidation of the resultin g enediol. 3-Dehydroshikimate enolization in water was catalyzed by inorgan ic phosphate while Zn2+ was used to catalyze enolization in acetic acid. En ediol oxidation employed Cu2+ as either the stoichiometric oxidant or as a catalyst in the presence of a cooxidant. Gallic acid was produced in a yiel d of 36% when 3-dehydroshikimic acid in phosphate-buffered water reacted fo r 35 h with H2O2 and catalytic amounts of CuSO4. 3-Dehydroshikimate-contain ing, phosphate-buffered culture supernatants reacted with stoichiometric am ounts of CuCO3Cu(OH)(2) and Cu-x(H3-xPO4)(2) to give gallic acid in yields of 51% in 5 h and 43% in 12 h, respectively. Solutions of 3-dehydroshikimic acid in acetic acid reacted with stoichiometric amounts of Cu(OAc)(2) to a fford a 74% yield of gallic acid in 36 h, Acetic acid solutions of 3-dehydr oshikimic acid could also be oxidized by air using catalytic quantities of Cu(OAc)(2), ZnO accelerated these oxidations leading to a 67% yield of gall ic acid in 4 h when an acetic acid solution of 3-dehydroshikimic acid was r eacted with O-2 and a catalytic amount of Cu(OAc)(2).