Directed mutagenesis alters the stereochemistry of catalysis by isolated ketoreductase domains from the erythromycin polyketide synthase

The ketoreductase (KR) domains eryKR(1) and eryKR(2) from the erythromycin-producing polyketide synthase (PKS) reduce 3-ketoacyl-thioester intermediates with opposite stereospecificity. Modeling of eryKR(1) and eryKR(2) showed that conserved amino acids previously correlated with production of alternative alcohol configurations lie in the active site. eryKR(1) domains mutated at these positions showed an altered stereochemical outcome in reduction of (2R, S)-2-methyl-3-oxopentanoic acid N-acetylcysteamine thioester. The wild-type eryKR(1) domain exclusively gave the (2S, 3R)-3-hydroxy-2-methylpentanoic acid N-acetylcysteamine thioester, while the double mutant (F141W, P144G) gave only the (2S, 3S) isomer, a switch of the alcohol stereochemistry. Mutation of the eryKR(2) domain, in contrast, greatly increased the proportion of the wild-type (2R, 3S)-alcohol product. These data confirm the role of key residues in stereocontrol and suggest an additional way to make rational alterations in polyketide antibiotic structure.     

Highly asymmetric synthesis of (+)-corsifuran A. Elucidation of the electronic requirements in the Ruthenium–NHC catalyzed hydrogenation of benzofurans

A short and efficient synthesis of ent-corsifuran A by a highly asymmetric hydrogenation of a benzofuran precursor is reported. In addition, the electronic influence of the substituents on the asymmetric hydrogenation of benzofurans is provided. Whereas the hydrogenation of electron-deficient benzofurans was achieved under very mild conditions, the presence of electron-donating groups in the benzofuran required harsher reaction conditions for achieving full conversion to the 2,3-dihydrobenzofuran.