Carboxylation of phenols and asymmetric nucleophile addition across C=C bond
September 24-28, 2017
The regioselective carboxylation of electron-rich (hetero)aromatics employing decarboxylases in the redox-neutral (reverse) carboxylation reaction using bicarbonate or CO2(g) is currently exploited for the biocatalytic synthesis of carboxylic acids.1 Three enzyme classes exert complementary regioselectivities through diverse mechanisms: (i) Whereas the o-carboxylation of phenols (an equivalent to the Kolbe-Schmitt reaction) is mediated by Zn2+-dependent o-benzoic acid (de)carboxylases,2 (ii) the -carboxylation of hydroxystyrenes is catalysed by phenolic/ferulic acid (de)carboxylases acting via a pair of Tyr-Arg residues.3 (iii) Surpringly, these enzymes also exhibit a catalytic promiscuity for the nucleophile addition of H2O,4 NH2-OMe, cyanide and n-Pr-SH across the vinyl C=C bond via a quinone-methide intermediate, which yields the corresponding (S)-configurated adducts in up to 91% e.e.5 (iv) In search of ATP-independent regio-complementary p-benzoic acid (de)carboxylases, we discovered that 3,4-dihydroxybenzoic acid decarboxylase from Enterobacter cloacae6 (DHBDC_Ec) surprisingly depends on prenylated FMN7 as cofactor. In an attempt to propose a mechanism for the carboxylation of catechol by DHBDC_Ec, QM calculations revealed that the transient formation of a 1,3-dipolar cycloaddition product (as suggested for the decarboxylation of cinnamic acid with ferulic acid decarboxylase from S. cerevisiae8) was highly disfavored (>30 kcal/M). As an alternative, we propose a mono-covalent nucleophile adduct involving a prFMN iminium electrophile (~14 kcal/M).
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Kurt Faber, S. Payer, K. Plasch, T. Reiter, V. Resch, C. Wuensch, S. M. Glueck, K. Gruber, G. Steinkellner, T. Pavkov-Keller, F. Himo, and X. Sheng, "Carboxylation of phenols and asymmetric nucleophile addition across C=C bond" in "Enzyme Engineering XXIV", Pierre Monsan, Toulouse White Biotechnology, France Magali Remaud-Simeon, LISBP-INSA, University of Toulouse, France Eds, ECI Symposium Series, (2017). http://dc.engconfintl.org/enzyme_xxiv/136