Hydroxylation microbial, stereoselectivity

In every case the information provided has been obtained by collating public domain sources of information, but unfortunately very often little data is available, particularly on commercial aspects, even for products that have proved to be big successes. Thus microbial biotransformations for steroid modification, particularly stereoselective hydroxylations, such as the use of Rhizopus arrhizus to convert progesterone into antiinflammatory and other dmgs via 11- -hydroxyprogestrone, have proved to be very successful. However, comparatively little useful information exists from public domain sources, despite (or perhaps because) a market of hundreds of millions /a exists for such microbially transformed steroids (cortisone, aldosterone, prednisolone and prednisone etc.) produced by microbial hydroxylation and dehydrogenation reactions coupled with complimentary chemical steps. 

Another process patented by Givaudan uses Mucor circinelloides as a biocatalyst for the production of 4-decanolide [228]. Here the natural substrate is the ethyl ester of decanoic acid which is isolated from coconut oil. The key microbial activity harnessed in this process is the stereoselective and regioselective hydroxylation of the fatty acid in the y-position, which is followed by spontaneous lactonisation of the hydroxy fatty acid under acidic conditions and results in yields of up to 10.5 g 4-decanolide after 60 h. 

Figure 16 Stereoselective microbial hydroxylation of ML-236B (49) to Pravastatin (47).

Microaganisms have the ability to effect chemical oxidations on a wide variety of substrates, many of which occur with chemo-, regio- or steieo-selectivities unattainable by conventional chemical methods. This is paiticulariy true in the case of unactivated C—H oxidation. What chemical oxidation system, for example, would be c )iable of the stereoselective hydroxylation of the diol (1) to the 3P-hydioxy derivative (2) This transformation has been reported to occur in 80% yield when a microbial oxidation is employed. ... 

Epoxidation of various olefins by cytochrome P-450 enzymes has been studied using rat liver microsomes [29,30] as well as using enzymes from microbial origin. For example, Ruettinger and Fulco [31] reported the epoxidation of fatty acids such as palmitoleic acid by a cytochrome P-450 from Bacillus megaterium. Their results indicate that both the epoxidation and the hydroxylation processes are catalyzed by the same NADPH-dependent monooxygenase. More recently, other researchers demonstrated that the cytochrome P-450cam from Pseudomonas putida, which is known to hydroxylate camphor at a non-activated carbon atom, is also responsible for stereoselective epoxidation of cis- -methylstyrene [32]. The (lS,2R)-epoxide enantiomer obtained showed an enantiomeric purity (ee) of 78%. This result fits the predictions based on a theoretical approach (Fig. 2). 

R Azerad, Regio- and stereoselective microbial hydroxylation of terpenoid compounds, in Stereoselective Biocatalysis, R N Patel (ed), 2000, Marcel Dekker New York. p. 152-180. 

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]. 

Scheme 2.148 Regio- and stereoselective microbial hydroxylation of steroids...

Microbial hydroxylation of a methyl group has the potential to be stereoselective in cases where the substrate possesses enantiotopic CH3 substituents. A classic example of such a process is the conversion of isobutyric acid to P-hydroxyisobutyric acid (Fig. 4), where the use of Candida rugosa IFO 0750 leads to formation of the D-(—) (R) isomer [9], and the L-(+) (S) product is obtained from oxidation using Bullera alba IFO 1030 [10]. Similar stereoselectivity is also observed in the oxidation of homologous acids and hydrocarbons by Rhodococcus species [11], and in the oxidation of cumene (1) to (jR)-2-phenylpropionic acid (2) by Pseudomonas oleovorans NRRL B-3429 (Fig. 5) [12]. 

Stereoselective microbial hydroxylations occurring at methylene groups of chiral substrates are frequently reported [4]. The present discussion will focus on the more unusual situation of stereoselective hydroxylations that occur at methylene groups of achiral, prochiral, or racemic substrates, resulting in the introduction of chirality to the product. 

Regio- and Stereoselective Microbial Hydroxylation of Terpenoid Compounds... 

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