Overcoming co‐product inhibition in the nicotinamide independent asymmetric bioreduction of activated CC‐bonds using flavin‐dependent ene‐reductases

Abstract

Eleven flavoproteins from the old yellow enzyme family were found to catalyze the disproportionation (“dismutation”) of conjugated enones. Incomplete conversions, which were attributed to enzyme inhibition by the co-product phenol could be circumvented via in situ co-product removal by scavenging the phenol using the polymeric adsorbent MP-carbonate. The optimized system allowed to reduce an alkene activated by ester groups in a “coupled-substrate” approach via nicotinamide-free hydrogen transfer with >90% conversion and complete stereoselectivity.

Ene‐reductases and their Applications

“…However, electron-rich phenols are often inhibitors of OYEs as they can form stable charge-transfer complexes with the electrondeficient flavin in the active site [14]. Recently, Faber and coworkers have overcome this inhibition by optimizing the reaction parameter (pH and temperature) and employing phenolic coproduct removal using a solid-phase organic resin (Figure 18.7a) [62]. After optimizing the reaction conditions and adsorbing the phenolic product using macroporous triethylammonium methylpolystyrene carbonate (MP-carbonate), the conversion for 2-cyclohexenone (23a) could be increased from 55 to 97% using the enzyme chromate reductase (CrS) from Thermus scotoductus SA-01.…”

“…The moderate conversion (48%) of dimethyl citraconate to dimethyl (R)-2-methylsuccinate using the cosubstrate 1,4-cyclohexanedione was optimized to 92% with an enantioselectivity of >99%. However, this technique is not applicable to all substrates as, in some cases, substrate and product were sequestered by MP-carbonate, and there were examples of undesired racemization of chiral sensitive α-substituted ketones [62]. Further investigations identified cheap and commercially available cosubstrates that resulted in conversions and enantioselectivities comparable to those obtained in the presence of an excess of a nicotinamide coenzyme or in combination with traditional NAD(P)H recycling systems [63].…”

Old Yellow Enzyme-Catalysed Asymmetric Hydrogenation: Linking Family Roots with Improved Catalysis

“…Although NADPH is the preferred physiological coenzyme for OYE, the dependency on this commercially expensive compound can be circumvented by established recycling systems with dehydrogenases [12,18,[42][43][44][45], with alternative sources of hydride [46,47], or through a nicotinamide-independent disproportionation coupling reaction [48][49][50][51]. A highly promising and elegant alternative is the use of relatively inexpensive nicotinamide coenzyme biomimetics (NCBs) [52][53][54][55][56].…”

Alternative Hydride Sources for Ene‐Reductases: Current Trends

“…However phenolic compounds are often potent inhibitors of OYEs, and may lead to a significant reduction in alkane product yields . A comprehensive study of the disproportionation of 1 a (alkene acts as the hydride donor and substrate) by eleven OYEs showed poor conversions (<15 %) in the majority of cases . Recently, Faber and co‐workers have overcome this phenomenon by the inclusion of an in situ phenolic co‐product scavenging system using the polymeric adsorbent macroporous triethylammonium methylpolystyrene carbonate (MP‐carbonate) .…”

“…A comprehensive study of the disproportionation of 1 a (alkene acts as the hydride donor and substrate) by eleven OYEs showed poor conversions (<15 %) in the majority of cases . Recently, Faber and co‐workers have overcome this phenomenon by the inclusion of an in situ phenolic co‐product scavenging system using the polymeric adsorbent macroporous triethylammonium methylpolystyrene carbonate (MP‐carbonate) . For example, the OYE homologue chromate reductase (CrS) from Thermus scotoductus SA‐01 reduced dimethyl citraconate 2 a to dimethyl ( R )‐2‐methylsuccinate 2 b with moderate conversions (48 %) and absolute enantioselectivity (Figure a).…”