Epoxides, microbial hydrolysis

One of the first applications of the microbial hydrolysis of epoxides for the synthesis of a bioactive compound is based on the resolution of a 2,3-disub-stituted oxirane having a cis-configuration (Scheme 14). Thus, by using an enzyme preparation derived from Pseudomonas sp., the (91 ,10S)-enantiomer was hydrolyzed in a frans-specific fashion (i.e. via inversion of configuration at C-10) yielding the 9R,10R)-threo-diol. The remaining (9S,101 )-epoxide was converted into (-1-)-dispar lure, the sex pheromone of the gypsy moth in >95% ee [101]. 

The microbial hydrolysis of epoxides can also be a useful synthetic method for introducing chirality. The example shown in Scheme 10.3 involved epoxidation of the 6,7-double bond of geraniol A -phenylcarbamate (10.4) by the fungus, Aspergillus niger to give the (65 )-epoxide (10.5). This epoxide underwent a spontaneous acid-catalysed hydrolysis at pH 2 to form the (6S)-diol (10.6). However, at pH 5-6 this underwent enzymatic hydrolysis to give the (6i )-diol (10.7). 

Figure 11.2-6. Microbial hydrolysis of epoxides proceeding with retention or inversion of configuration.

Fig. 4. (A) Synthesis of chiral intermediates for melatonin receptor agonist Enantioselective microbial hydrolysis of racemic epoxide US) to the corresponding (/ )-diol (14) and unreacted (5)-epoxide (12). (B) Stereoinversion of racemic diol (16) to 5-diol (15) by Candida boidinii and Pichia methanolica.

Mischitz, M., Kroutil, W., Wandel, U., and Faber, K. (1995) Asymmetric Microbial Hydrolysis of Epoxides. Tetrahedron Asymmetry 6,1261-1272. 

Asymmetric microbial oxidation afforded the (-)-epoxide which has been explored as a building block ring opening reactions with organometallic nucleophiles, and via Friedel-Crafts reactions have been reported. [226,227]. A non-biotransformative approach to this epoxide has also been described [228]. Copper(II)-catalysed oxidative hydrolysis (Eq. 72) affords a lactic acid analogue in high enantiomeric purity [229]. 

Active hits were found for every type of substrate screened, including those for which other known microbial epoxide hydrolases were ineffective. For example, hydrolysis of m-stilbenc oxide was not successful with several microbial EHs tested previously.4243 By contrast, several of our new enzymes actively hydrolyzed this substrate and exhibited excellent enantioselectivities (>99% ee). It is important to note that these enzymes were found to be capable of selectively hydrolyzing a wide range of mc.vo-cpoxidcs, including cyclic and acyclic alkyl- and aryl-substituted substrates. 

Lewisite in soil may rapidly volatilize or may be converted to lewisite oxide due to moisture in the soil (Rosenblatt et al, 1975). The low water solubility suggests intermediate persistence in moist soil (Watson and Griffin, 1992). Both lewisite and lewisite oxide may be slowly oxidized to 2-chlorovinylarsonic acid (Rosenblatt et al, 1975). Possible pathways of microbial degradation in soil include epoxidation of the C=C bond and reductive deha-logenation and dehydrohalogenation (Morrill et al, 1985). Due to the epoxy bond and arsine group, toxic metabolites may result. Additionally, residual hydrolysis may result in arsenic compounds. Lewisite is not likely to bioaccumulate. However, the arsenic degradation products may bioaccumulate (Rosenblatt et al, 1975). 

Similar results were described by Mamouhdian and Michael, who isolated 18 bacterial strains able to produce optically enriched epoxides with excellent ee s (up to 98%) [104, 105]. However, in the case of trflns-(2J ,3J )-epoxybutane, it was shown that the enantiomeric enrichment is in fact due to a second-step enantioselective hydrolysis of the epoxide, which is first produced in racemic form. This, interestingly, is an unexpected example of the possible use of microbial epoxide hydrolases for the synthesis of enantiopure epoxides (see below). 

In 1974 Marumo and coworkers synthesized the enantiomers of JH II by microbial asymmetric hydrolysis of the epoxy ring of ( )-JH II (prepared by Mori) with a fungus Helminthosporium sativum 23 The hydrolysed diol was converted to (+)-JH II, while the epoxide remained intact was (—)-JH II. Their enantiomeric purities, however, were rather low (66-73% ee), and no definite biological data could be obtained. 

In spite of the considerable value of epoxide hydrolases for fine chemical synthesis, it was only recently that a detailed search for epoxide hydrolases from microbial sources was undertaken by the groups of Furstoss185, 901 and Faber123, 79, 911, bearing in mind that the use of microbial enzymes allows an (almost) unlimited supply of biocatalyst. The screening was based along the following considerations on the one hand, the catabolism of alkenes often implies the hydrolysis of an epoxide inter-... 

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]. 

The hydrolysis of epoxides to give 1 -diols is an area that is ripe for development. Some work has been published showing that epoxides such as cyclohexane epoxide (36) form optically active diols, in this case cyclo-hexane-(lR,2i )-diol (37). The research has concentrated on the use of enzymes present in liver microsomes, and while this elegant work has indicated what can be achieved, it is clear that rapid progress and the involvement of non-experts in this particular area must await the discovery of readily available epoxide hydrolase enzyme(s) from microbial sources. 

The asymmetric hydrolysis of epoxides, which was impeded by the lack of readily available sources of microbial enzymes, is now possible on a preparative scale. This method offers a valuable alternative to the asymmetric epoxidation of olefins, particularly for those substrates where chemical methods fail due to the absence of directing functional groups. 

This process only becomes possible when both enantiomers are converted by two independent enantioselective reactions to the same enantiomeric product. Both pathways must exhibit an opposite sense of enantioselectivity. For example, as shown in Scheme 5.58, whole-cell microbial transformation of a racemic epoxide using two different organisms, each harbouring a hydrolase that performs the enantioselective hydrolysis of the epoxide ring (with opposite stereocontrol), to give a single enantiomeric 1,2-diol as the sole product in high yield with excellent enantiomeric excess [148]. 

The conversion of the chromene 54 to epoxide 55 (a synthetic precursor for potassium channel modulators) and diol 56 (Fig. 40) may be achieved by a number of microbial catalysts, notably Mortierella rammaniana SC 13840 which gives the diol 56 in 65% yield m. 91% optical purity [62]. In an analogous conversion, the related microorganism Mortierella isabellina ATCC 42613 converted both chromenes 57 and 58 (Fig. 41) to a mixture of the corresponding cis- and tran -diols, presumably the result of regio- but nonstereo-selective acid-catalyzed hydrolysis of an intermediate epoxide, with both isomeric diols being formed in identical enantiomeric purities [34]. 

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