Biocatalytic epoxidation

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... 

Fig. 4.106 Biocatalytic epoxidation with styrene monooxygenase including cofactor regeneration.

Buhler B, Park JB, Blank LM, Schmid A (2008) NADH availability limits asymmetric biocatalytic epoxidation in a growing recombinant Escherichia coli strain. Appl Environ Microbiol 74 1436-1446... 

On the other hand, the biocatalytic epoxidation of styrene and derivatives can be achieved with excellent stereoselectivity using SMOs of various origins (Scheme 13.9). Although isolated SMO has been successfully applied in combination with enzymatic NADH regeneration [92,97] or reductive electrochemical cofactor regeneration [98-101], the process based on E. coli whole cells expressing those SMOs has been proven superior in terms of productivity due to the limited stability of cell-free enzymes, and consequently it has been applied in the majority of reported studies. 

Examples of Engineered Enzymes for Biocatalytic Epoxidation Reactions... 

Archelas A, R Furstoss (1997) Synthesis of enantiopure epoxides through biocatalytic approaches. Annu Rev Microbiol 51 491-525. 

In Case study 2, mass balancing is used to compare a biocatalytic (a) and a chemical catalytic (b) enantioselective epoxidation reaction (Scheme 5.3). 

Scheme 5.3 Biocatalytic (a) and chemical catalytic (b) styrene epoxidation.

Another very recent development in the field of enzymatic domino reactions is a biocatalytic hydrogen-transfer reduction of halo ketones into enantiopure epoxides, which has been developed by Faber, Bornscheuer and Kroutil. Interestingly, the reaction was carried out with whole lyophilized microbial cells at pH ca. 13. Investigations using isolated enzymes were not successful, as they lost their activity under these conditions [26]. 

Poessl, T.M., Kosjek, B., Ellmer, U. et al. (2005) Non-racemic halohydrins via biocatalytic hydrogen-transfer reduction of halo-ketones and one-pot cascade reaction to enantiopure epoxides. Advanced Synthesis and Catalysis, 347 (14), 1827-1834. 

Biocatalytic hydrolysis or transesterification of esters is one of the most widely used enzyme-catalyzed reactions. In addition to the kinetic resolution of common esters or amides, attention is also directed toward the reactions of other functional groups such as nitriles, epoxides, and glycosides. It is easy to run these reactions without the need for cofactors, and the commercial availability of many enzymes makes this area quite popular in the laboratory. 

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 . 

On the other hand, a number of biocatalytic methods provide a useful arsenal of methods as valuable alternatives to the above-mentioned techniques [9-14]. One is where prochiral or racemic synthetic precursors of epoxides, such as halohydrins, can be asymmetrized or resolved using hydrolytic enzym-... 

Another illustration of the use of such a biocatalytic approach was the synthesis of either enantiomer of a-bisabolol, one of these stereoisomers (out of four) which is of industrial value for the cosmetic industry. This approach was based on the diastereoselective hydrolysis of a mixture of oxirane-diastereoiso-mers obtained from (R)- or (S)-limonene [68]. Thus,starting from (S)-hmonene, the biohydrolysis of the mixture of (4S,81 S)-epoxides led to unreacted (4S,8S)-epoxide and (4S,8i )-diol. The former showed a diastereomeric purity (> 95%) and was chemically transformed into (4S,8S)-a-bisabolol. The formed diol... 

Advantageous use of homochiral cyclohexadiene-cis-l,2-diol, available by means of biocatalytic oxidation of chlorobenzene with toluene dioxygenase, has enabled the synthesis of all four enantiomerically pure C18-sphingosines (Nugent, 1998), which are known inhibitors of protein kinase C and important in cellular response mediation for tumor promoters and growth factors. The four requisite diastere-omers of azido alcohol precursors were accessed by regioselective opening of epoxides with either azide or halide ions. 

The deracemization of a number of pharmaceutically valuable building blocks has been carried out by biocatalytic processes. This includes epoxides, alcohols, amines and acids. DKR involves the combination of an enantioselective transformation with an in situ racemisation process such that, in principle, both enantiomers of the starting material can be converted to the product in high yield and ee. The racemization step can be catalysed either enzymatically by racemases, or non-enzymatically by transition metals. 

A variety of monooxygenases (see above) can perform epoxidations. Some biocatalytic methods come into sight, which will become attractive for industrial use. In these cases chiral epoxides are the targeted products, and therefore this subject will be dealt with in Section 4.6. 

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