Steroids microbial hydroxylation

Many examples of microbial hydroxylation of sterols/steroids have been reported. These hydroxylations usually involve mixed function oxidases which utilise molecular oxygen and cytochrome P-450. The reaction can be represented by ... 

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. 

A large range of microbial hydroxylations of aliphatic and aromatic hydrocarbons, terpenes, and steroids is of flavor and fragrance, pharmaceutical, and... 

Virtually eveiy site in the steroid molecule is accessible for microbial hydroxylation, and almost all positions have been hydroxylated by various microbial strains. Of particular importance are hydroxylated products with the unnatural a-configuradon, and of those the derived 1 la- and the 16a-alcohols are of greatest synthetic interest. 

The greatest contribution towards understanding the structural features in the substrate that effect, or indeed control, the selectivity in microbial hydroxylation of steroids was carried out by Jones and Meakins and their work is recorded in a series of papers entitled Microbiological Hydroxylation . Their last report was published in 1980, and serves as a source of references to earlier material. ... 

Aspects of regioseleetivity in the microbial hydroxylation of steroids have been reviewed most recently by Kieslich. ... 

Hydroxy steroids are important as direct precursors to 19-norsteroids, but are not readily obtainable by microbial hydroxylation without additional nuclear substitution. However a strain of Pellicularia filamentosa will 19-hydroxylate coitexolone (82 equation 28) successfully. lip-Hydroxylation of cortexolone derivatives can be accomplished in up to 86% yield using a mutam strain of Curvularia lu /WM (PERM P-8515). 

Practical aspects are much as discussed earlier, and a good laboratory steroid hydroxylation procedure has been published. Numerous additives (e.g. surfactants, antibiotics and fungicides) have been used to improve microbial hydroxylation of steroids, and some of these have been discussed previously... 

The value of microbial hydroxylation of steroids has long been that they allow functionalization of positions not easily accessible by normal chemical methods. For this reason yields of a few percent have often been tolerated, but yields approaching quantitative can be achieved in many instances. The objective of much current work is to improve the selectivity of microbial hydroxylations. This can be achieved in a number of ways, for example by structural modifications of the substrate, optimization of fermentation conditions and by strain improvement. The latter is most likely to yield the most significant improvements in selectivity, and has previously been discussed at length. ... 

Microbial hydroxylations at most positions in the steroid nucleus 1 are known, including 10j6-hydroxylation of 19-norsteroids, 14j8-hydroxylation of synthetic 14/i-steroids and 3-hy-droxylation of 3-deoxysteroids. In addition, hydroxylations of both angular methyl groups and of side-chain positions have been reported. 

The first recorded microbial hydroxylation of a steroid was that at the 7-position of cholesterol using Proactinomyces sp.2. Although this may have been 7/J-hydroxylation, an unambigious assignment cannot be made. Subsequently, unequivocal 7/ -hydroxylation by Rhizopus strains was observed. The mold Rhizopus arrhizus, which induces 6/f- and 1 la-hydroxylations in 4-en-3-one-steroids (e.g., androst-4-ene-3,17-dione), hydroxylates 3/ -hydroxypregn-5-en-20-one in positions Iji and 11a, and compounds of the 5a-pregnane series in the 7/i-position (see Table 8)154,4. 

The hydroxylation of nonactivated centers in hydrocarbons is one of the most useful biotransformations [1040,1074—1079] due to the fact that this process has only very few counterparts in traditional organic synthesis [1080-1082]. In general, the relative reactivity of carbon atoms in bio-hydroxylation reactions declines in the order of secondary > tertiary > primary [1083], which is in contrast to radical reactions (tertiary > secondary > primary) [1084]. There are two main groups of hydrocarbon molecules, which have been thoroughly investigated with respect to microbial hydroxylation, i.e., steroids and terpenoids. Both have in common, that they possess a large main framework, which impedes the metabolic degradation of their hydroxylated products. 

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

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