Epoxide hydrolases meso-epoxides

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

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]). 

In addition, new some epoxide hydrolases have also shown a great utility for the desymmetrization of meso-epoxides. An interesting example is the synthesis of nelfinavir-the active pharmaceutical ingredient (API) of the anti-human immunodeficiency virus drug Viracept-where the (R,R)-diol obtained by opening the meso-epoxide is a suitable starting material. Scheme 10.6 shows a synthetic route to nehinavir [14]. 

In a similar manner, and as shown again by the Faber group, the catalyzed reaction of bis-epoxides led to THFs containing four stereocenters [22], Thus, treatment of cis,cis,meso-8-51 with the epoxide hydrolase Rhodococcus sp. CBS 717.73 predominantly yielded the THF derivative 8-53a in 94% ee and 89% de, whereas the use of other biocatalysts has shown only low to moderate stereoselectivity (Scheme 8.14). As intermediate, the diol 8-52 can be assumed, whereby for the further transformation path A is always favored. 

Fig. 2.11 Enzyme-catalyzed desymmetrization of meso-epoxides using epoxide hydrolases of microbial origin that were screened for maximum activity. Note that the relative rate (measured as the turnover frequency, TOE) offormingthe (R,R)-stereoisomer is about 250 times higher than for the (S,S) one.

Fig. 14. Mammalian epoxide hydrolase. Stereoselective hydrolysis of meso-epoxides [162]...

Unexpectedly, as compared with microsomal mammalian epoxide hydrolases, all the meso-epoxides tested with the two above mentioned bacterial enzymes were not substrates. 

Recently, encouraging progress was made in the hydrolysis of cyclopentene oxide and cyclohexene oxide using the yeast Rhodotonda glutinis195L The corresponding (R,R)-trans-diols were obtained in over 90% optical and chemical yields. However, asymmetric hydrolysis of meso-epoxides by bacterial and fungal epoxide hydrolases is still impeded by insufficient selectivities. 

For the enantioselective preparations of chiral synthons, the most interesting oxidations are the hydroxylations of unactivated saturated carbons or carbon-carbon double bonds in alkene and arene systems, together with the oxidative transformations of various chemical functions. Of special interest is the enzymatic generation of enantiopure epoxides. This can be achieved by epoxidation of double bonds with cytochrome P450 mono-oxygenases, w-hydroxylases, or biotransformation with whole micro-organisms. Alternative approaches include the microbial reduction of a-haloketones, or the use of haloperoxi-dases and halohydrine epoxidases [128]. The enantioselective hydrolysis of several types of epoxides can be achieved with epoxide hydrolases (a relatively new class of enzymes). These enzymes give access to enantiopure epoxides and chiral diols by enantioselective hydrolysis of racemic epoxides or by stereoselective hydrolysis of meso-epoxides [128,129]. 

Epoxide hydrolase has emerged as an important enzyme for the asymmetric synthesis of enantiopure epoxides and diols [24]. The hydrolase HXN-200 has been shown to catalyse the enantioselective hydrolysis of meso epoxides to give optically active diols (Scheme 4.14) [25]. A related group of enzymes is the haloalkane dehalogenases that display epoxide hydrolase activity with nucleophiles other than water (Scheme 4.15) [26]. 

Chang, D., Wang, Z., Heringa, M.R, Wirthner, R., Witholt, B. and Li, Z. (2003) Highly enantioselective hydrolysis of alicyclic meso-epoxides with a bacterial epoxide hydrolase from Sphingomonas sp. HXN-200 simple synthesis of alicyclic vicinal frans-diols. Chem. Commun., 960-961. 

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