Enantioselektive allylische Hydroxylierung von ω‐Alkensäuren und ‐estern mittels der P450‐BM3‐Monooxygenase

Abstract

Chirale Allylalkohole der w-Alkensäuren und entsprechende Derivate sind essentielle Bausteine für die Synthese biologisch aktiver Verbindungen. Die direkte enantioselektive C-H-Oxidation von linearen terminalen Olefinen ermçglicht den effizientesten Zugang zu diesen Strukturen. Synthetische Methoden für diese Umsetzungen stehen jedoch nur begrenzt zur Verfügung und liefern unzureichende Selektivitäten. Hier wird ein enzymatischer Ansatz zu diesen interessanten Zielverbindungen mittels der P450-BM3-Monooxygenase vorgestellt, der allylische Hydroxylierungen mit hohen bis exzellenten Chemo-und Enantioselektivitäten ermçglicht.

Chemoenzymatic Total Synthesis of the Proposed Structures of Putaminoxins B and D

“…Both enantiomers were obtained with an excellent ee of >99% (Scheme A). Next to the application of ADHs for the synthesis of allylic alcohols, an elegant alternative is the P450 BM3 monooxygenase-catalyzed enantioselective allylic hydroxylation of ethyl 6-heptenoate ( 6 ), where the best results were obtained with the P450 BM3 variant A74G L188Q . Ester 6 was synthesized by esterification of the acid 17 with EtOH with catalytic amounts of H 2 SO 4 and microwave irradiation in a quantitative yield.…”

“…The P450 BM3 A74G L188Q-catalyzed hydroxylation of ester 6 yielded allylic alcohol ( S )- 4 in 25% yield (40% conversion) and 91% ee (Scheme B). A possible explanation of the low conversion of substrate 6 could stem from the enzyme’s instability and decreased activity in purified samples . However, so far this monooxygenase strategy only gives access to the ( S )-configured allylic alcohol 4 .…”

“…For heterologous protein expression, the Escherichia coli strain BL21 (DE3) was used. The P450 BM3 A74G L188Q (GQ) variant was obtained as described before . Expression, purification, and quantification of the monooxygenase, and production and activity measurements of LbADH and TbADH, were performed as described earlier. , …”

“…Chiral homoallylic alcohols are well-known motifs for a variety of natural products and can be synthesized by “chemical” allyl addition to an aldehyde or by enzymatic transformations in terms of asymmetric reduction of a prochiral ketone or kinetic resolution of the respective racemic alcohol. − A decision in favor of one route over the other in terms of practicability, efficiency, and convenience is often not obvious. Chemical methods for the synthesis of chiral homoallylic alcohols often rely on complex ligands. − Additionally, excellent enantioselectivity is still not easy to achieve when dealing with linear alkyl-substituted aldehydes (i.e., hexanal). , In previous studies, we presented alcohol dehydrogenases (ADHs) as convenient biocatalysts for the synthesis of chiral alcohols (e.g., homoallylic and allylic alcohols) via oxidative kinetic resolutions (OKRs) as well as asymmetric reductions. , In addition, aiming at the synthesis of chiral allylic alcohols, the P450 BM3 monooxygenase variant A74G L188Q was described for stereoselective hydroxylations of linear terminal olefins representing a short and elegant route toward these compounds . At the same time, known chemical methods for direct C–H activations are limited and often result in low selectivity.…”