Sequential Enzymatic Conversion of α‐Angelica Lactone to γ‐Valerolactone through Hydride‐Independent C=C Bond Isomerization

Abstract

A case of hydride‐independent reaction catalyzed by flavin‐dependent ene‐reductases from the Old Yellow Enzyme (OYE) family was identified. α‐Angelica lactone was isomerized to the conjugated β‐isomer in a nicotinamide‐free and hydride‐independent process. The catalytic cycle of C=C bond isomerization appears to be flavin‐independent and to rely solely on a deprotonation–reprotonation sequence through acid–base catalysis. Key residues in the enzyme active site were mutated and provided insight on important mechanistic features. The isomerization of α‐angelica lactone by OYE2 in aqueous buffer furnished 6.3 mm β‐isomer in 15 min at 30 °C. In presence of nicotinamide adenine dinucleotide (NADH), the latter could be further reduced to γ‐valerolactone. This enzymatic tool was successfully applied on semi‐preparative scale and constitutes a sustainable process for the valorization of platform chemicals from renewable resources.

Enantioselective Enzymatic Naphthoyl Ring Reduction

“…The mechanism of DHNCR is unlikely to proceed through hydride transfer to C7 as the resulting anionic intermediate ( 7 b ) cannot be sufficiently stabilized (Scheme a). However, a preceding isomerization would allow for a direct hydride transfer to C8 (Scheme b); enzymatic isomerization has already been reported for an OYE . In this scenario, DHNCR reaction would involve the abstraction of a proton at C5 and isomerization to a conjugated tetraene ( 7 c ) intermediate.…”

“…However,a precedingi somerization would allow for ad irect hydridet ransfer to C8 (Scheme3b);e nzymatic isomerization has already been reportedf or an OYE. [19] In this scenario, DHNCR reaction would involvet he abstraction of ap roton at C5 and isomerization to ac onjugated tetraene (7c)i ntermediate. Substrates lacking this prerequisite such as 3-vinylbenzoyl-CoA (13 a) should therefore not be converted (Scheme 3c).…”

Angelica Lactones: From Biomass‐Derived Platform Chemicals to Value‐Added Products

“…Another very promising approach for this transformation relies on the use of enzymes as catalysts. In a seminal paper, Hall's group describes the enzymatic conversion of α‐AL into GVL through hydride‐independent C=C bond isomerization catalyzed by ene‐reductase enzymes from the old yellow enzyme (OYE) family followed by a cascade reduction in the presence of NADH (Scheme ) …”

“…enzyme (OYE)f amily followed by ac ascade reduction in the presenceo fN ADH (Scheme 9). [29] In this way, a-AL is initially isomerized to b-AL, and then the reduction step can take place. Remarkably,a fter 15 min of reaction at 30 8C, conducted with as ubstrate solutioni naTris-HCl buffer [tris(hydroxymethyl)aminomethane]( pH 7.5) in the presence of OYE from Saccharomyces cerevisiae and NADH, a-AL is not detected in the medium, and only GVL and at race amount of b-AL are present, which suggests full conversion into GVL.…”

Discovery, Characterization, Engineering, and Applications of Ene-Reductases for Industrial Biocatalysis

“…114 This latter reaction is in effect converting non-activated compounds (C=C not conjugated to the activating group) into OYE substrates. Therefore, a two-step, one enzyme method was applied for the production of γ-valerolactone from α-Angelica lactone, via the initial generation of the OYE substrate 3-methylfuran-2(5 H )-one 115. This approach opens up new possibilities of OYE-catalysed bioreductions on non-activated compounds, as the same biocatalyst catalyses both the activation and bioreduction steps.…”