7 beta-hydroxy bile alcohols: facile synthesis and 2D 1H NMR studies of 5 beta-cholestane-3 alpha, 7 beta, 12 alpha, 25-tetrol

Synthesis of 12β-Methyl-18-nor-bile Acids

Decoupling the roles of the farnesoid X nuclear receptor and Takeda G-protein-coupled bile acid receptor 5 is essential for the development of novel bile acid therapeutics targeting metabolic and neurodegenerative diseases. Herein, we describe the synthesis of 12β-methyl-18-nor-bile acids which may serve as probes in the search for new bile acid analogues with clinical applicability. A Nametkin-type rearrangement was applied to protected cholic acid derivatives, giving rise to tetra-substituted Δ13,14- and Δ13,17-unsaturated 12β-methyl-18-nor-bile acid intermediates (24a and 25a). Subsequent catalytic hydrogenation and deprotection yielded 12β-methyl-18-nor-chenodeoxycholic acid (27a) and its 17-epi-epimer (28a) as the two major reaction products. Optimization of the synthetic sequence enabled a chromatography-free route to prepare these bile acids at a multi-gram scale. In addition, the first cis-C-D ring-junctured bile acid and a new 14(13 → 12)-abeo-bile acid are described. Furthermore, deuteration experiments were performed to provide mechanistic insights into the formation of the formal anti-hydrogenation product 12β-methyl-18-nor-chenodeoxycholic acid (27a).

Microwave-Assisted Synthesis of Bile Acids Derivatives: An Overview

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The first attempts at microwave-assisted (MW) syntheses of bile acid derivatives were performed in domestic MW appliances. However, the reproducibility of these syntheses, which were performed in uncontrolled conditions, was very low. In the first part of this overview, compounds synthesized under such conditions are presented. Consequently, with the development of MW technology, MW-assisted reactions in MW reactors became reproducible. Thus, in the second part of this review, syntheses of bile acidsbased compounds in MW reactors are presented. Among others, publications dealing with the following topics will be covered:

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− Chemical transformations of hydroxyl and/or carboxyl functions of bile acids into esters or amides,

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− Hydroxyl group oxidations,

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− Derivatization of oxo-compounds with different nitrogen-containing compounds (e.g. 4-amino-3- substituted-1H-1,2,4-triazole-5-thiones, thiocarbohydrazides and thiosemicarbazides)

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Bile acid-based molecular tweezers, capable of stereospecific molecular recognition

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Reactions of hydroxyl functions to give chlorine derivatives, presenting reactive intermediates in substitution reactions with N- or O-containing nucleophilic arylhydrazides, urea derivatives, substituted thiadiazoles or triazoles or amino acid methyl esters, mainly in solvent-free conditions.

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Some of the synthesized compounds expressed antimicrobial potential and/or good recognition properties as artificial receptors for specific amino acids or anions.

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Detailed comparisons between conventional and MW-assisted procedures for chemical transformations of bile acids are given in most of the presented publications. Based on these results, MW irradiation methods are simpler, more efficient, cleaner and faster than conventional synthetic methods, meeting the requirements of green chemistry.

Regio- and stereoselective reductions of dehydrocholic acid

Dehydrocholic acid (DHCA), an unnatural bile acid, is manufactured by oxidation of cholic acid. Its biotransformation by two basidiomycetes (Trametes hirsuta and Collybia velutipes) is reported. These mycelia showed different affinities for the substrate and selectivities of attack: T. hirsuta in particular regio- and stereoselectively reduced the 3-keto group to yield 3 alpha-hydroxy-7,12-diketo-5 beta-cholan-24-oic acid (7,12-diketolithocolic acid) as the main product. A number of different chemical reductions were carried out on DHCA; among them hydrogenation with Raney Nickel in water under high-intensity ultrasound proved highly regio- and stereoselective, yielding 7,12-diketolithocolic acid exclusively. (1)H and (13)C resonances were assigned in details thanks to a series of 1D and 2D NMR runs including DEPT, NOESY, H-H COSY, gHSQC and gHMBC.

Chemical modifications of bile acids under high-intensity ultrasound or microwave irradiation

High-intensity ultrasound (HIU) and microwave (MW) irradiation, having emerged as effective promoters of organic reactions, were exploited for the synthesis of bile acids derivatives. Esterification, amidation, hydrolysis, oxidation, and reduction were investigated. Compared to conventional methods, both techniques proved much more efficient, increasing product yields and dramatically cutting down reaction times. Scaled-up studies are now under way.

Bile acids are new products of a marine bacterium, Myroides sp. strain SM1

Strain SM1 was isolated as a biosurfactant-producing microorganism from seawater and presumptively identified as Myroides sp., based on morphology, biochemical characteristics and 16S rDNA sequence. The strain produced surface-active compounds in marine broth, which were purified, using emulsification activity for n-hexadecane as an indicator. The purified compounds were identified by thin-layer chromatography, (1)H- and (13)C-NMR spectra and fast atom bombardment mass spectrometry as cholic acid, deoxycholic acid and their glycine conjugates. Type strains of the genus Myroides, M. odoratus JCM7458 and M. odoramitimus JCM7460, also produced these compounds. Myroides sp. strain SM1 possessed a biosynthetic route to cholic acid from cholesterol. Thus, bile acids were found as new products of prokaryotic cells, genus Myroides.

Anomalous enantioselectivity in the sharpless asymmetric dihydroxylation reaction of 24-nor-5beta-cholest-23-ene-3alpha,7alpha,12alpha-triol: synthesis of substrates for studies of cholesterol side-chain oxidation

Recently we described a block in bile acid synthesis in cerebrotendinous xanthomatosis (CTX), a lipid storage disease related to an inborn error of bile acid metabolism. In this disease a defect in hepatic microsomal (24S) hydroxylation blocks the transformation of 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol into (24S) 5beta-cholestane-3alpha,7alpha,12alpha,24,25- pentol and cholic acid. Mitochondrial cholesterol 27-hydroxylation has also been reported to be abnormal in CTX subjects, but the relative importance of the enzymatic defect in this alternative microsomal pathway (namely, the 24S hydroxylation of 5beta-cholestane-3alpha,7alpha, 12alpha,25-tetrol relative to the abnormality in mitochondrial 27-hydroxylase) has not been established in CTX. To delineate the sequence of side-chain hydroxylations and the enzymatic block in bile acid synthesis, we synthesized the (23R and 23S) 24-nor-5beta-cholestane-3alpha,7alpha, 12alpha,23,25-pentols utilizing a modified Sharpless asymmetric dihydroxylation reaction on 24-nor-5beta-cholest-23-ene-3alpha, 7alpha, 12alpha-triol, a C26 analog of the naturally occurring C27 bile alcohol, 5beta-cholest-24-ene-3alpha,7alpha,12alpha-triol . Stereospecific conversion of the unsaturated 24-nor triol to the corresponding chiral compounds (23R and 23S), 24-nor-5beta-cholestane-3alpha,7alpha,12alpha,23 ,25-pentols, was quantitative. However, conversion of the unsaturated 24-nor triol to the chiral nor-pentols had absolute stereochemistry opposite to the products predicted by the Sharpless steric model. The absolute configurations and enantiomeric excess of the C26 nor-pentols and the C27 pentols (synthesized from 5beta-cholest-24-ene-3alpha,7alpha,12alpha-triol for comparison) were confirmed by nuclear magnetic resonance and lanthanide-induced circular dichroism Cotton effect measurements. These results may contribute to a better understanding of the role of the 24S-hydroxylation vs. 27-hydroxylation step in cholic acid biosynthesis.

Rapid hydrolysis of bile acid conjugates using microwaves: retention of absolute stereochemistry in the hydrolysis of (25R) 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oyltaurine

In recent years, defects of bile acid synthesis caused by disorders of peroxisome biogenesis have led to increased interest in C27 bile acids. In humans, while the majority of bile acids are C24 carboxylic acids, the presence of increased concentrations of C27 bile acids and their metabolites in hereditary diseases associated with peroxisomal dysfunction can serve as a useful marker for the intensity of the metabolic disorder. Our present studies describe an efficient method for the rapid hydrolysis of C27 and C24 bile acid conjugates using a commercial microwave oven. The advantages of this method include freedom from racemization, minimal activation, mild reaction conditions, and the highly stereocontrolled nature of the reaction, thus allowing for free bile acid recovery in high yield. For example, when (25R) 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oyl taurine, a major compound present in the bile of Alligator mississippiensis, was deconjugated with 4% NaOH/diethylene glycol or 1 M LiOH/propylene glycol in the microwave oven for 4-6 min, 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestan-26-oic acid (THCA) was obtained in 81% yield with retention of configuration at C-25. It is suggested that present studies will be helpful in delineating the absolute stereochemistry of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoyl-CoA oxidase, the peroxisomal enzyme that catalyzes the first step in the oxidation of THCA.