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

With the elaboration of high-yielding, high-titer syntheses of 3-dehydroshikimic acid from glucose using recombinant Escherichia coli, oxidation of this hydroaromatic becomes a potential route for synthesis of gallic acid. Conversion of 3-dehydroshikimic acid into gallic acid likely proceeds via initial enolization of an alpha -hydroxycarbonyl and oxidation of the resulting enediol. 3-Dehydroshikimate enolization in water was catalyzed by inorganic phosphate while Zn2+ was used to catalyze enolization in acetic acid. Enediol oxidation employed Cu2+ as either the stoichiometric oxidant or as a catalyst in the presence of a cooxidant. Gallic acid was produced in a yield of 36% when 3-dehydroshikimic acid in phosphate-buffered water reacted for 35 h with H2O2 and catalytic amounts of CuSO4. 3-Dehydroshikimate-containing, phosphate-buffered culture supernatants reacted with stoichiometric amounts 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 afford a 74% yield of gallic acid in 36 h, Acetic acid solutions of 3-dehydroshikimic acid could also be oxidized by air using catalytic quantities of Cu(OAc)(2), ZnO accelerated these oxidations leading to a 67% yield of gallic acid in 4 h when an acetic acid solution of 3-dehydroshikimic acid was reacted with O-2 and a catalytic amount of Cu(OAc)(2).