Biocatalytic Enantioselective Oxidation of Sec‐Allylic Alcohols with Flavin‐Dependent Oxidases

Abstract

The oxidation of allylic alcohols is challenging to perform in a chemo‐ as well as stereo‐selective fashion at the expense of molecular oxygen using conventional chemical protocols. Here, we report the identification of a library of flavin‐dependent oxidases including variants of the berberine bridge enzyme (BBE) analogue from Arabidopsis thaliana (AtBBE15) and the 5‐(hydroxymethyl)furfural oxidase (HMFO) and its variants (V465T, V465S, V465T/W466H and V367R/W466F) for the enantioselective oxidation of sec‐allylic alcohols. While primary and benzylic alcohols as well as certain sugars are well known to be transformed by flavin‐dependent oxidases, sec‐allylic alcohols have not been studied yet except in a single report. The model substrates investigated were oxidized enantioselectively in a kinetic resolution with an E‐value of up to >200. For instance HMFO V465S/T oxidized the (S)‐enantiomer of (E)‐oct‐3‐en‐2‐ol (1 a) and (E)‐4‐phenylbut‐3‐en‐2‐ol with E>200 giving the remaining (R)‐alcohol with ee>99% at 50% conversion. The enantioselectivity could be decreased if required by medium engineering by the addition of cosolvents (e. g. dimethyl sulfoxide).

Positive Impact of Natural Deep Eutectic Solvents on the Biocatalytic Performance of 5-Hydroxymethyl-Furfural Oxidase

“…Thus, cinnamaldehyde (7b) was obtained at 96% after 14 h, as shown in Entry 9 of Table 3. Finally, as recently shown with a set of allylic secondary alcohols [12], HMFO was able to catalyze the kinetic resolution of the secondary racemic alcohol 1indanol, (±)-12a (Entry 10, Table 3). The biocatalyst selectively oxidizes one of the enantiomers of the starting alcohol to 1-indanone (12b), whereas the remaining enantiomer of 12a gets enantioenriched, as shown in Scheme 2.…”

“…With these values, it can be estimated that the enantioselectivity (E) [44] for this kinetic resolution was 21. This is a moderate value when compared with the previous report in which allylic secondary alcohols were studied as substrates [12]. This different behavior can be explained by the different structure of the starting alcohol, as 1-indanol is sterically constrained, complicating enantiodiscrimination by the biocatalyst.…”

“…This represents an important increase in HMFO performance when compared with aqueous buffer as a medium where only 7% of 1-indanone was obtained (see Entry 8, Table 1). The optical purity of the remaining substrate at the end of the reaction was measured by HPLC, which revealed that (R)-1-indanol was recovered with 17% ee, showing a similar enantiopreference of HMFO when catalyzing the kinetic resolution of allylic alcohols [12]. With these values, it can be estimated that the enantioselectivity (E) [44] for this kinetic resolution was 21.…”

“…This different behavior can be explained by the different structure of the starting alcohol, as 1-indanol is sterically constrained, complicating enantiodiscrimination by the biocatalyst. Still, the kinetic resolution of 1-indanol represents the first example of enantioselective oxidation of a benzofused racemic secondary alcohol catalyzed by HMFO [12]. With the available crystal structure of HMFO, enzyme engineering may be performed to improve the enantioselective behavior towards 1-indanol and related compounds [1].…”

“…To improve the efficiency of HMFO-based biocatalytic processes, HMFO mutants have been engineered to display increased (thermo)stability [11]. Recently, wild type HMFO and some of its mutants have been employed for the first time in the enantioselective oxidation of sec-allylic alcohols, achieving excellent enantioselectivities [12].…”