Epimerization of hydroxyl groups

HSDH activities are found rather widely distributed among intestinal bacteria including members of the genera Bacteroides, Eubacterium, Clostridium, Bifidobacterium and Escherichia [25-29]. In the human intestinal microflora, 7a-HSDH appears to be much more widely distributed than 3a-HSDH or 12a-HSDH [17]. Individual species may contain from 1 to 3 different stereospecific HSDHs. However, there is considerable variation between strains regarding the presence and extent of HSDH activity. 

3jS-HSDH has not been measured in cell extracts of either C perfringens or E. lentum. However, whole cells of these bacteria produced 3a- and 3/8-hydroxy bile acids from 3-oxo bile acid substrates. Aeration of whole cell suspensions or growing cultures favored the formation of 3-oxo bile acids whereas, anaerobic conditions increased the rate of formation of 3 S-hydroxy bile acids [34]. 

---

Epimerhation of hydroxyl groups. Epimerization of hydroxyl groups is often effected by S 2 displacement of the tosylates of the alcohol with tetraethylammonium acetate (1, 1136-11.37 2. 397) or with tetra-n-butylammonium acetate (3, 277). In connection with a study of configuration and biological activity in the prostaglandins, Corey and Terashima examined the reaction of the tosylatc of the model (-t)-hydroxy-laclone (1) with 5.0 eq. of tetra-n-butylammonium acetate in acetone at 25" for 2 hr. 

In the CyS/Cy6 case a reversible addition process has been suggested by Suginome and Nickon to explain epimerization of hydroxyl group when photolyzing nitrite esters of steroidal or terpenic compounds such as 175 which gives 176 (Scheme 79). ... 

Fig. 8.33 DYKAT of 1,3-diols via lipase-catalyzed acyl-transfer in combination with Ru-catalyzed epimerization of hydroxyl groups. G=chiral carbon, convertible for equilibration and acyl migration, but not for the irreversible step H=chiral carbon, convertible for equilibration, acyl migration and the irreversible step l=chiral carbon, convertible for acyl migration, stable chirality. (From J. Steinreiber, K. Faber, H. Griengl, De-racemization of enantiomers versus de-epimerization of diastereomers-chssification of dynamic kinetic asymmetric transformations (DYKAT), Chemistry 14 (2(X)8), 8060. Copyright 2008 Wiley).

The intestinal microflora of man and animals can biotransform bile acids into a number of different metabolites. Normal human feces may contain more than 20 different bile acids which have been formed from the primary bile acids, cholic acid and chenodeoxycholic acid [1-5], Known microbial biotransformations of these bile acids include the hydrolysis of bile acid conjugates yielding free bile acids, oxidation of hydroxyl groups at C-3, C-6, C-7 and C-12 and reduction of oxo groups to give epimeric hydroxy bile acids. In addition, certain members of the intestinal microflora la- and 7j8-dehydroxylate primary bile acids yielding deoxycholic acid and lithocholic acid (Fig. 1). Moreover, 3-sulfated bile acids are converted into a variety of different metabolites by the intestinal microflora [6,7]. 

Addition of 1-ethoxyvinyllithium to aldehyde 10 gives a 1 1 epimeric mixture of alcohols 55. Mesylation of hydroxyl group, followed by reaction with lithium azide, leads to azides 56. Hydrolysis of compound 56 gives 1 1 epimeric mixture of azidoketones 49 and 50. The azido-ketone 50 can be equilibrated into a mixture of diastereoisomers 49 and 50, via a base-catalyzed process [26]. 

The changes of the cholesterol molecule that occur in its conversion into bile acids include epimerization of the 3 -hydroxyl group, reduction of the double bond, introduction of hydroxyl groups in positions C-7 (cheno-deoxycholic acid), C-7 and C-12 (cholic acid), or C-6 and C-7 (a- and P-muricholic acids, hyocholic acid), and transformation of the C27 side chain into a C24-carboxylic acid. 

The nuclear changes involved in the conversion of cholesterol to cholic acid (reduction of the 5,6 double bond, epimerization of the hydroxyl group at carbon-3, and insertion of hydroxyl groups at positions 7 and 12) appear to be carried out by microsomal enzyme systems. Side-chain oxidation is effected by mitochondrial enzymes. 

Oxidoreduction of hydroxyl groups by intestinal organisms is associated with the formation of epimerized bile acids via ketonic intermediates (Figure 1). 3a-, 63-, 7a-, 73- and 12a-hydroxysteroid... 

The facile and selective oxidation of both primary and secondary hydroxy groups with certain nucleotides led Pfitzner and Moffatt (48) to explore the scope of the carbodiimide-methyl sulfoxide reagent with steroid and alkaloid alcohols. Relatively minor differences were apparent in the rate of oxidation of epimeric pairs of 3- and 17- hydroxy steroids whereas the equatorial lLx-hydroxyl group in several steroids was readily oxidized under conditions where the axial epimer was unreactive [cf. chromic oxide oxidation (51)]. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

To obtain this compound the key step consisted in the epimerization of the C-5 in compound 6. This was acomplished by triflation of the alcohol 6 and nucleophilic substitution of the triflate by a large excess of tetrabutylammonium acetate in dichloromethane. A controlled (4 °C, 3 h) basic methanolysis of the enol benzoate led to the keto-ester 11" whose hydroxyl functions at C-4 and C-6 were simultaneously deprotected under acidic conditions to furnish 12. Finally a Zemplen deprotection of the 5-acetoxy group led to 13 obtained in five steps and 11% overall yield from 6 (figure 4). 

The synthesis of aldehydes and ketoamides was performed on solid phase as well as in solution (Scheme 2.2). A semicarbazone linker (6) was employed for the assembly of the aldehydes on solid phase whereas the corresponding aminoalcohol was coupled in solution to the tripeptide and oxidized to the aldehyde, which produced epimeric mixtures [137]. For the synthesis of the ketoamides, hydroxyester THP resins were used as solid support ((7), Scheme 2.2) [138]. In solution the peptide bond was formed using an aminohydroxycarboxylic acid building block [138, 147]. Oxidation of the free hydroxyl group yielded the final inhibitors ((8), Scheme 2.2). 

---