Ene‐Reductase Catalyzed Regio‐ and Stereoselective 1,4‐Mono‐Reduction of Pseudoionone to Geranylacetone

Abstract

The regio-and stereoselective mono-reduction of a particular C=C bond of conjugated C=C double bonds is a very challenging task. Here the regio-and stereoselective 1,4reduction of pseudoionone, an α,β,γ,δ-bisunsaturated ketone, was demonstrated to give geranylacetone, an industrially relevant molecule. OYE1 from Saccharomyces pastorianus was identified as the most suitable biocatalyst for this reaction. Elevated substrate concentrations of up to 200 mM were tolerated allowing still to reach excellent conversions (> 99 % and 80 % for 100 or 200 mM pseudoionone concentration, respectively). Interestingly, the organic cosolvent often required for substrate solubilization in aqueous buffer can be avoided for pseudoionone when using permeabilized E. coli cells containing the overexpressed enzyme instead of purified enzyme, reaching still > 99 % conversion at 100 mM (19.2 g/L) substrate concentration. Performing this reaction at a 0.5 g scale allowed to run the reaction to completion (> 99 %) and pure product was isolated with 80 % yield. Additionally, the bis-unsaturated ketone 6-methyl-3,5-heptadien-2-one was transformed under similar conditions giving the floral compound sulcatone with excellent conversion (97 %) and 77 % isolated yield. Finally, the stereoselective reduction of the (E,E)-over the (E,Z)-pseudoionone isomer was enabled by the ene-reductase from Zymomonas mobilis (NCR). Thus, both (E)-geranylacetone and (E,Z)pseudoionone were obtained with isomeric excess above 60 %.

Site‐Selective Hydrogenation of Electron‐Poor Alkenes and Dienes Enabled by a Rhodium‐Catalyzed Hydride Addition/Protonolysis Mechanism**

“…The C2,C3‐reduction of electron‐poor conjugated dienes to generate C4,C5‐unsaturated products using H 2 is limited to a specific substrate class of C2‐disubstituted dienamide esters [8] . The C2,C3‐reduction of acyclic dienones with H 2 is not established and instead requires the use of stoichiometric silane reductant [9, 10] …”

“…[8] The C2,C3-reduction of acyclic dienones with H 2 is not established and instead requires the use of stoichiometric silane reductant. [9,10] We were interested in developing a homogeneous catalyst system for the C2,C3-hydrogenation of conjugated dienones as an approach to prepare the intermediate geranylacetone (2) directly from pseudoionone (1) (Figure 1B), a readily available precursor involved in the industrial synthesis of Vitamin A. [11] This route to geranylacetone via pseudoionone would be shorter compared to classical synthetic routes that proceed through linalool, with both routes starting from citral.…”

Engineering aBacillus subtilisesterase for selective hydrolysis ofd,l-menthyl acetate in an organic solvent-free system

“…6 In the other case, the organic cosolvent for the solubilization of high-concentration substrate in an aqueous buffer can be omitted. 24 In addition, the substrate-feeding strategy was effective in increasing the substrate loading when substrate inhibition occurred. 25,26 Herein, a para-nitrobenzyl esterase from B. subtilis 168 (pnbA-BS) was cloned and over-expressed in E. coli BL21(DE3), which harbored high activity but low L-enantioselectivity in the resolution of D, L-menthyl acetate.…”

“… 6 In the other case, the organic cosolvent for the solubilization of high-concentration substrate in an aqueous buffer can be omitted. 24 In addition, the substrate-feeding strategy was effective in increasing the substrate loading when substrate inhibition occurred. 25,26 …”

Fop

“…Carboxylic acids, esters and nitriles are relatively poor EWGs and need an additional EWG for double bond activation. [354][355][356][357][358][359][360] The reduction of lesser activated alkenesthus lower redox potentials -however, would be attractive for organic synthesis. Therefore, enzymes with a lower redox potential are needed.…”