Nocardia epoxide hydrolase

Although the use of an epoxide hydrolase was already claimed for the industrial synthesis of L- and meso-tartaric acid in 1969 [51], it was only recently that applications to asymmetric synthesis appeared in the hterature. This fact can be attributed to the limited availabihty of these biocatalysts from sources such as mammals or plants. Since the production of large amounts of crude enzyme is now feasible, preparative-scale applications are within reach of the synthetic chemist. For instance, fermentation of Nocardia EHl on a 701-scale afforded > 700 g of lyophilized cells [100]). 

Fig. 15. Bacterial epoxide hydrolase. Deracemization of ( )-d5-2,3-epoxyheptane via enantio-convergent biohydrolysis using Nocardia EHl [187]...

Among the sterically more demanding substrates, 2,2-disubstituted oxiranes were hydrolyzed in virtually complete enantioselectivities using enzymes from bacterial sources (E > 200), in particular Mycobacterium NCIMB 10420, Rhodococcus (NCIMB 1216, DSM 43338, IFO 3730) and closely related Nocardia spp. (Scheme 2.93) [608, 609]. All bacterial epoxide hydrolases exhibited a preference for the (S)-enantiomer. In those cases where the regioselectivity was determined, attack was found to exclusively occur at the unsubstituted oxirane carbon atom. 

Another example of an enantioconvergent transformation of racemic epoxides was reported by Faber and coworkers who employed a chemoenzymatic approach for the synthesis of optically enriched diols. They used an (S)-selective epoxide hydrolase from Nocardia sp. in connection with an add catalyzed hydrolytic step to convergently prepare (S)-diols 41 in good to excellent yields (71-98%) with enantiomeric excess values up to 99% (Scheme 9.13) [75]. 

R. V.A. Orm, W. Kroutil, K. Faber, Deracemization of (+/-)-2,2-disubstituted epoxides via enantioconvergent chemoenzymatic hydrolysis using Nocardia EHl epoxide hydrolase and sulfuric acid. Tetrahedron Lett. 38 (1997) 1753-1754. 

Epoxide hydrolases have been purified from mammalian liver cells [63-66] but also from microbial sources such as Bacillus megaterium [67], Corynebacterium [45,46,68], Pseudomonas sp. [46,69], and dematiaceous fungi such as Ulocladium atrum and Zopfiella karachiensis [70]. However, some of these enzymes were only partially purified [67,68], or their enantioselectivity was not investigated [69] or was very low [45,46]. More recently, two bacterial epoxide hydrolases with high activity and high enantioselectivity were purified and characterized from Rhodococcus sp. NCIMB 11216 [71] and Nocardia TBl [72]. 

More recently, the deracemization of ( )-2.6 (Scheme 18) through combination of Nocardia EHl epoxide hydrolase and sulfuric acid in dioxane containing a trace amount of water (vide supra) was employed for a short total synthesis of (S)-(—)-frontalin, a central aggregation pheromone of pine beetles of the Dendroctonus family [131]. The product [(S)-2-methyl-hept-6-ene-l,2-diol], which was obtained in 97% yield and 99% e.e. from... 

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