Epoxide hydrolase 2-diols

Furstoss et al. have reported their studies on the use of an epoxide hydrolase with four styrene oxide derivatives (Figure 5.26) [39]. The (R)-diol (43) was obtained in 91% ee at 100% conversion from racemic (42), demonstrating an enantioconvergent... 

Figure 5.25 Enantioconvergent hydrolysis of epoxides (35) to the corresponding diols (36) using mung bean epoxide hydrolase.

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... 

Epoxide hydrolase A type of enzyme that converts epoxides to diols by the addition of water. 

The activities of both haloalkanol dehalogenase (halohydrin hydrogen lyase) that catalyzes the formation of epoxides from alkanes with vicinal hydroxyl and halogen groups, and epoxide hydrolase that brings about hydrolysis of epoxyalkanes to diols are involved in a number of degradations that involve their sequential operation. 

In addition to the enzymatic hydrolysis of esters, there also ample examples where an epoxide has been cleaved using a biocatalyst. As described by the Faber group [19], reaction of the ( )-2,3-disubstituted ds-chloroalkyl epoxide roc-8-40 with a bacterial epoxide hydrolase (BEH), led to the formation of vie-diol (2 ,3S)-8-41 (Scheme 8.11). The latter underwent a spontaneous cydization to give the desired product (2i ,3i )-8-42 in 92 % ee and 76 % yield. The same strategy was used with the homologous molecule rac-8-43, which afforded the THF derivative (2R,3R)-S-4S in 86% ee and 79% yield. 

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,ds,weso-8-51 with the epoxide hydrolase Rhodococcus sp. CBS 71773 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. 

Chiappe, C., Leandri, E., Hammock, B.D. and Morisseau, C. (2007) Effect of ionic liquids on epoxide hydrolase-catalyzed synthesis of chiral 1,2-diols. Green Chemistry, 2007 (9), 162-168. 

As with several other functional groups considered earlier, epoxides are most commonly found in alkaloid metabolites rather than the original compound. These epoxides may arise via oxidation of alkenes or aromatic hydrocarbons. Epoxide hydrolase catalyzes hydrolysis of epoxides to the more hydrophilic diol. As seen in Scheme 6, this is usually a stereospecific reaction that always yields a... 

This chapter begins, thus, with a short introduction to the chemical reactivity of epoxides. We continue with a description of the epoxides hydrolases and their biochemistry, and devote most of its length to a systematic discussion of the substrates hydrated by these enzymes. Arene oxides and diol epoxides will be presented first, followed by a large variety of alkene and cy-cloalkene oxides. 

Cholesterol epoxide hydrolase, which is expressed in the endoplasmic reticulum and catalyzes the trans-addition of H20 to cholesterol 5,6a-ox-ide and cholesterol 5,6/3-oxide, as well as to a number of other steroid 5,6-oxides. The products are the corresponding vicinal diols [54][55],... 

The cytosolic enzyme leukotriene A4 hydrolase (EC 3.3.2.6), which ster-eoselectively converts leukotriene A4 (LTA4) to leukotriene B4 [56], This enzyme catalyzes the hydrolytic cleavage of the 5,6-epoxide ring in LTA4, but, in contrast to what happens with other EHs, the product is not a vicinal diol but a 5,12-diol. As a zinc metalloenzyme, LTA4 hydrolase does not appear to be related to any other epoxide hydrolase. 

The overall reaction catalyzed by epoxide hydrolases is the addition of a H20 molecule to an epoxide. Alkene oxides, thus, yield diols (Fig. 10.5), whereas arene oxides yield dihydrodiols (cf. Fig. 10.8). In earlier studies, it had been postulated that epoxide hydrolases act by enhancing the nucleo-philicity of a H20 molecule and directing it to attack an epoxide, as pictured in Fig. 10.5, a [59] [60], Further evidence such as the lack of incorporation of 180 from H2180 into the substrate, the isolation of an ester intermediate, and the effects of group-selective reagents and carefully designed inhibitors led to a more-elaborate model [59][61 - 67]. As pictured in Fig. 10.5,b, nucleophilic attack of the substrate is mediated by a carboxylate group in the catalytic site to form an ester intermediate. In a second step, an activated H20... 

Fig. 10.10. Metabolic route to the formation of <-)-(9S, 70S)-9,10-iiihydrophenanthrene-9,10-diol (10.30). The arrow indicates the direction of nucleophilic attack by epoxide hydrolase.

Turning to enzymatic hydration, we see from the data in Table 10.1 that phenanthrene 9,10-oxide Fig. 10.10, 10.29) is an excellent substrate for epoxide hydrolase. Comparison of enzymatic hydration of the three isomeric phenanthrene oxides shows that the Vmax with the 9,10-oxide is greater than with the 1,2- or the 3,4-oxide the affinity was higher as well, as assessed by the tenfold lower Km value [90]. Furthermore, phenanthrene 9,10-oxide has a plane of symmetry and is, thus, an achiral molecule, but hydration gives rise to a chiral metabolite with high product enantioselectivity. Indeed, nucleophilic attack by epoxide hydrolase occurs at C(9) with inversion of configuration i.e., from below the oxirane ring as shown in Fig. 10.10) to yield the C-H9.S, 10.S )-9,10-dihydro-9,10-diol (10.30) [91],... 

Fig. 10.16. Comparison of the metabolism of A4-valproic acid (10.54), a metabolite of valproic acid, with that of ethyl A4-valproate (10.57), a synthetic analogue. Both compounds undergo cytochrome P450 catalyzed oxygenation to form the corresponding epoxides (10.55 and 10.58, respectively). The former reacts intramolecularly to form the lactone 10.56 and is not detectably a substrate for epoxide hydrolase. Epoxide 10.58, in contrast, is a substrate for epoxide hydrolase, forming the diol 10.59, which, in turn, carries out an intramolecular nucleophilic attack to form lactone 10.56 [136],...

Both the mono- and diepoxides of butadiene are substrates for epoxide hydrolase [163], In rat liver microsomes, (R)- and (S)-butadiene monoepoxides were hydrolyzed to but-3-ene-l,2-diol (10.104, Fig. 10.24) with complete retention of configuration at C(2), indicating attack at C(l) [164], In mouse liver microsomes, in contrast, 15 - 25% inversion of configuration was observed, suggesting partial attack at C(2). Preliminary results indicate that human liver microsomes are more efficient than mouse or rat liver microsomes in hydrolyzing butadiene monoepoxide [165]. The hydrolysis of diepoxybutane (10.103) yields 3,4-epoxybutan-l,2-diol (10.105), which can be further hydrated to erytritol (10.106) [163]. 

The base-catalyzed hydration of 2-phenyloxirane involves nucleophilic attack preferentially at C(3) (0-C(3) cleavage), but with only partial regio-selectivity. Acid-catalyzed hydration is mainly by 0-C(2) cleavage. The hydration of 2-phenyloxirane catalyzed by epoxide hydrolase is characterized by its very high regioselectivity for the less-hindered, unsubstituted C(3) [175] [176], involving retention of configuration at C(2). In other words, (R)-and (5)-2-phenyloxirane are metabolized to (/ )- and (S)-l-phenylethane-l,2-diol (10.118), respectively. Substrate enantioselectivity was also character-... 

Fig. 10.27. Mechanistic and stereochemical aspects of the hydration of trans-(R,K)-anethole epoxide in the absence (Reactions a and b) and presence of epoxide hydrolase (Reaction c) [178][179]. The ratio of Reaction a to bis ca. 4 1 in acid- and base-catalyzed hydration to yield the threo- and erythro-diols, respectively. The enzymatic Reaction c also yields preferentially the erythro-diol.

R. J. Krause, J. E. Sharer, A. A. Elfarra, Epoxide Hydrolase-Dependent Metabolism of Butadiene Monoxide to 3-Butene- 1,2-diol in Mouse, Rat, and Human Liver , Drug Metab. Dispos. 1997, 25, 1013 - 1015. 

Epoxide Hydrolase-catalyzed Synthesis of (/ )-3-Benzyloxy-2-methylpropane-l,2-diol... 

Bacillus subtilis, engineered to overproduce epoxide hydrolase, was used as a whole-cell biocatalyst to resolve racemic 1-benzyloxymethyl-1-methyloxirane with high (5)-selectivity. The remaining (/ )-epoxide was subsequently ring opened in situ, with inversion of stereochemistry, to obtain highly enantiomerically enriched (/ )-3-benzyloxy-2-methylpropane-l,2-diol in greater than 50 % theoretical yield (Figure 5.2). 

Benzyloxy-2-methylpropane-l,2-diol, a desymmetrized form of 2-methylpropane-1,2,3-triol with its terminal hydroxy being protected as a benzyl ether, was prepared using the B. subtilis epoxide hydrolase-catalyzed enantioselective hydrolysis of the racemic benzyloxymethyl-l-methyloxirane readily available from methallyl chloride and benzyl alcohol. The preparation of the racemic epoxide, a key intermediate, was described in Procedures 1 and 2 (Sections 5.6.1 and 5.6.2), its overall yield being 78 %. The combined yield of enantiomerically pure (7 )-3-benzyloxy-2-methylpropane-l,2-diol was 74 % from ( )-benzyloxymethyl-l-methyloxirane, as described in Procedures 3-5 (Sections 5.6.3 and 5.6.5), with the overall procedures leading to the biocatalytic dihydroxylation of benzyl methallyl ether . 

Keywords Epoxide, Vicinal diol. Epoxide hydrolase. Biocatalysis, Enantio-convergent. 

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