Epoxide Hydrolase‐Catalysed Kinetic Resolution of a Spiroepoxide, a Key Building Block of Various 11‐Heterosteroids

Abstract

Two microbial epoxide hydrolases -i.e., Aspergillus niger (AnEH) and Rhodococcus erythropolis (the so-called "Limonene EH": LEH) were used to achieve, for the first time, the biocatalysed hydrolytic kinetic resolution (BHKR) of spiroepoxide rac-1. This compound is a strategic key building block allowing the synthesis of 11-heterosteroids. Interestingly enough, the two enzymes exhibited opposite and therefore complementary enantioselectivity allowing us to isolate the residual (R,R)-1 (from AnEH) and the residual (S,S)-1 (from LEH) in nearly enantiopure forms (> 98 %). Their absolute configurations were determined by X-ray crystallography. An opposite regioselectivity of the oxirane ring opening for both enantiomers of substrate 1, determined using H 2 18O labelling and chiral GC-MS analysis, was also observed, corresponding to an attack at the less substituted carbon atom using AnEH, and at the most substituted carbon atom using LEH. A chemical process-improving methodology was also developed. This allowed us to obtain both enantiomers of the substrate in high enantiomeric purity (99 %) and optimised quantity. In the case of the AnEH, the use of a biphasic (water/isooctane) reaction medium allowed us to increase the global substrate concentration up to 200 g/ L. The preparation of both enantiomers of 1 clearly paves the way to the preparative scale synthesis and biochemical evaluation of the corresponding 11-heterosteroid enantiomers.

Efficient Epoxide Hydrolase Catalyzed Resolutions of (+)‐ and (−)‐cis/trans‐Limonene Oxides

“…It is worth mentioning that the exploitation of LEHs at comparably high substrate concentrations (76 g L −1 ) has never been reported before; the closest example was the application of Re ‐LEH to the resolution of trans ‐spiroepoxide, which, after process optimization, was performed at a 10 g L −1 substrate concentration.…”

“…This fact, together with the low enantioselectivity shown in the kinetic resolution of typical EH substrates, for example, styrene oxide, has partly limited the interest toward its synthetic exploitation. Only a few examples of practical applications of Re ‐LEH in synthetic organic chemistry have been reported to date . In contrast, Re ‐LEH has been recently the subject of different protein engineering studies aimed at the improvement of useful applicative features, such as the stereoselectivity and thermostability …”

Epoxide Hydrolases and their Application in Organic Synthesis

“…Enantiopure epoxides and diols are chiral intermediates or building blocks, which can be used for the synthesis of biologically active compounds such as antibi otics [4], antifungal drugs [5], antiandrogens [6], receptor agonists and antagonists [7][8][9], 11-heterosteroids [10], HIV protease inhibitors [11,12], nonsteroidal antiinflammatory drugs [13,14], and other natural products [15][16][17][18][19]. An EH-triggered cascade reaction enabled the asymmetric synthesis of two antitumor agents, which occur naturally in Panax ginseng [20].…”

“…10). Enantioselective biohydrolysis of the (S)-epoxide proceeded with the retention of configuration, resulting in the formation of the corresponding (S)-1,2-diol.…”

Grafting of Copper(I)–NHC Species on MCM‐41: Homogeneous versus Heterogeneous Catalysis

“…Only af ew examples of practical applicationso fRe-LEH in synthetic organic chemistry have been reported to date. [24,25] In contrast, Re-LEH has been recently the subjecto fd ifferent protein engineering studies aimed at the improvement of useful applicativef eatures, such as the stereoselectivity [26] and thermostability. [27] Interestingly, Re-LEH, as well as other EHs, [28] is capable of performing enantioconvergent processes, thanks to complementaryr egioselectivity with respectt ow hich oxirane carbon atom is attacked on each substrate enantiomer.T his ability was first observed during studies into the Re-LEH-catalyzed hydrolysis of the naturals ubstrate limonene-1,2-oxide.…”