Synthesis of pyrazine via chemoselective reduction of β-keto-α-oximino ester using baker’s yeast

Mechanistic Insights into the Ene-Reductase-Catalyzed Promiscuous Reduction of Oximes to Amines

The biocatalytic reduction of the oxime moiety to the corresponding amine group has only recently been found to be a promiscuous activity of ene-reductases transforming α-oximo β-keto esters. However, the reaction pathway of this two-step reduction remained elusive. By studying the crystal structures of enzyme oxime complexes, analyzing molecular dynamics simulations, and investigating biocatalytic cascades and possible intermediates, we obtained evidence that the reaction proceeds via an imine intermediate and not via the hydroxylamine intermediate. The imine is reduced further by the ene-reductase to the amine product. Remarkably, a non-canonical tyrosine residue was found to contribute to the catalytic activity of the ene-reductase OPR3, protonating the hydroxyl group of the oxime in the first reduction step.

C=C-Ene-Reductases Reduce the C=N Bond of Oximes

Although enzymes have been found for many reactions, there are still transformations for which no enzyme is known. For instance, not a single defined enzyme has been described for the reduction of the C=N bond of an oxime, only whole organisms. Such an enzymatic reduction of an oxime may give access to (chiral) amines. By serendipity, we found that the oxime moiety adjacent to a ketone as well as an ester group can be reduced by ene-reductases (ERs) to an intermediate amino group. ERs are well-known enzymes for the reduction of activated alkenes, as of α,β-unsaturated ketones. For the specific substrate used here, the amine intermediate spontaneously reacts further to tetrasubstituted pyrazines. This reduction reaction represents an unexpected promiscuous activity of ERs expanding the toolkit of transformations using enzymes.