Sterol biosynthesis: Establishment of the structure of 3β-p-bromobenzoyloxy-5α-cholest-8(14)-en-15β-ol

Synthesis and Biological Activity of Sterol 14α-Demethylase and Sterol C24-Methyltransferase Inhibitors

Sterol 14α-demethylase (SDM) is essential for sterol biosynthesis and is the primary molecular target for clinical and agricultural antifungals. SDM has been demonstrated to be a valid drug target for antiprotozoal therapies, and much research has been focused on using SDM inhibitors to treat neglected tropical diseases such as human African trypanosomiasis (HAT), Chagas disease, and leishmaniasis. Sterol C24-methyltransferase (24-SMT) introduces the C24-methyl group of ergosterol and is an enzyme found in pathogenic fungi and protozoa but is absent from animals. This difference in sterol metabolism has the potential to be exploited in the development of selective drugs that specifically target 24-SMT of invasive fungi or protozoa without adversely affecting the human or animal host. The synthesis and biological activity of SDM and 24-SMT inhibitors are reviewed herein.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Inhibitors of sterol synthesis. Chemical synthesis of 7 alpha-ethyl and 16 alpha-ethyl derivatives of delta 8(14)-15-oxygenated sterols and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase in CHO-K1 cells

The enolate of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one (II), formed upon treatment of II with potassium tert-butoxide in tert-butanol, was alkylated with ethyl iodide. In addition to the major products, 3 beta-hydroxy-14 alpha-ethyl-5 alpha-cholest-7-en-15-one and its 3 beta-ethyl ether, small amounts of 3 beta-hydroxy-7 alpha-ethyl-5 alpha-cholest-8(14)-en-15-one (V), 3 beta-hydroxy-16 alpha-ethyl-5 alpha-cholest-8(14)-en-15-one (VI) and the 3 beta-ethyl ether of VI were isolated. When the enolate of II was formed by treatment with lithium diisopropylamide in tetrahydrofuran, the same alkylation furnished VI as the major product. Reduction of VI with lithium aluminum hydride gave 16 alpha-ethyl-5 alpha-cholest-8(14)-ene-3 beta, 15 alpha-diol (IX) and its 15 beta epimer X, which were separated by column chromatography. Full 1H and 13C nuclear magnetic resonance (NMR) assignments, augmented by nuclear Overhauser effect difference spectra for VI, established the stereochemistry of these diols at C-15 and C-16. The NMR results indicate that the 16 alpha-ethyl group affects the side-chain conformation. The effects of II, V, VI, IX and X on the levels of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were studied in CHO-K1 cells. With the exception of IX, each of the compounds reduced the levels of HMG-CoA reductase activity. The order of potency with respect to suppression of the elevated levels of HMG-CoA reductase activity induced by transfer of the cells to lipid-deficient medium, was II greater than V greater than VI greater than X.

Inhibitors of sterol synthesis. Chemical synthesis and spectral properties of (25R)-5 alpha-cholest-8(14)-ene-3 beta,15 beta,26-triol, a potential metabolite of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one and its effects on 3-hydroxy-3-methylglutaryl-coenzyme A reductase in CHO-K1 cells

(25R)-5 alpha-Cholest-8(14)-ene-3 beta,15 beta,26-triol (III) was prepared by reduction of (25R)-3 beta,26-diacetoxy-5 alpha-cholest-8(14)-en-15-one with sodium borohydride followed by treatment of the crude product with lithium aluminium hydride. The trihydroxysterol III, a potential metabolite of 3 beta-hydroxy-5 alpha-cholest-8(14)-en-15-one, was characterized by the results of mass spectral studies and by nuclear magnetic resonance (NMR) spectroscopy. Full 1H and 13C NMR assignments for III and 5 alpha-cholest-8(14)-ene-3 beta,15 beta-diol are given and used to establish the structure of III. The triol was found to be very potent in lowering the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in Chinese hamster ovary cells.

Stereospecific 1,4-addition to an alpha,beta-unsaturated steroidal epoxide: syntheses of new 15-oxygenated sterols

3 beta-Benzoyloxy-14 alpha,15 alpha-epoxy-5 alpha-cholest-7-ene (1) is a key intermediate in the synthesis of C-7 and C-15 oxygenated sterols. Treatment of 1 with benzoyl chloride resulted in the formation of 3 beta,15 alpha-bis-benzoyloxy-7 alpha-chloro-5 alpha-cholest-8(14)-ene (2). Reaction of 2 with LiAlH4 or LiAlD4 resulted in the formation of 5 alpha-cholest-7-ene-3 beta,15 alpha-diol (3a) or [14 alpha-2H]5 alpha-cholest-7-ene-3 beta,15 alpha-diol (3b). Diol 3b was selectively oxidized by Ag2CO3/celite to [14 alpha-2H]5 alpha-cholest-7-en-15 alpha-ol-3-one (4). Treatment of 1 with MeMgI/CuI gave 7 alpha-methyl-5 alpha-cholest-8(14)-ene-3 beta,15 alpha-diol (5). Selective oxidation of 5 with pyridinium chlorochromate (PCC)/pyridine or oxidation with PCC resulted in the formation of 7 alpha-methyl-5 alpha-cholest-8(14)-en-3 beta-ol-15-one (6) and 7 alpha-methyl-5 alpha-cholest-8(14)-ene-3,15-dione, respectively. Reduction of 6 with LiAlH4 yielded 5 and 7 alpha-methyl-5 alpha-cholest-8(14)-ene-3 beta,15 beta-diol (6). Reaction of 1 with benzoic acid/pyridine gave 3 beta,7 alpha-bis-benzoyloxy-5 alpha-cholest-8(14)-en-15 alpha-ol (9). Treatment of 9 with LiAlH4 or ethanolic KOH resulted in the formation of 5 alpha-cholest-8(14)-ene-3 beta,7 alpha,15 alpha-triol (10). Dibenzoate 9, upon brief treatment with mineral acid, gave 3 beta-benzoyloxy-5 alpha-cholest-8(14)-ene-15-one (11). Oxidation of 9 with PCC yielded 3 beta,7 alpha-bis-benzoyloxy-5 alpha-cholest-8(14)-ene-15-one (12). Ketone 12 was also prepared by the selective hydride reduction of 5 alpha-cholest-8(14)-en-7 alpha-ol-3,15-dione (13) to give 5 alpha-cholest-8(14)-ene-3 beta,7 alpha-diol-15-one (14), which was then treated with benzoyl chloride to produce 12.

Conformational analysis of sterols: comparison of X-ray crystallographic observations with data from other sources

Cystallographic data on over 400 steroids collected in the Atlas of Steroid Structure provide information concerning preferred conformations, relative stabilities and substituent influence of the interactive potential of steroid hormones. Analysis of these data indicates that observed conformational details are intramolecularly controlled and that the influence of crystal packing forces is negligible. Crystallographic data on the orientation of the progesterone side chain contradict published force-field calculations. In 84 of 88 structures having a 20-one substituent, the C(16)-C(17)-C(20)-O(20) torsion angle is between 0 degrees and -46 degrees. The 4 torsion angles that lie outside this range do so because of a 16 beta-substituent and not because of crystal packing forces. Not one of the 88 structures is found to have a conformation in which the C(16)-C(17)-C(20)-O(20) torsion angle is within +/- 15 degrees of the most commonly calculated minimum energy value. The narrow range of side chain conformations seen in very different crystalline environments in the 88 crystal structure determinations and the predictable substituent influence apparent in the data strongly suggest that crystallographically observed conformers seldom deviate from minimum energy positions, regardless of hypothetical broad energy minima, metastable states and small barriers to rotation. The 96 crystallographically independent determinations of the cholestane 17-side chain show that the chain has 4 principal conformations (A:B:C:D), occurring in the ratio 69:8:8:11. Although the fully extended side chain is clearly the energetically most favored one, in 16 observations of cholesterol itself only 6 are in the extended conformation. Some of the correlated conformational changes in the chains can be rationalized on the basis of model studies, but others apparently result from subtle intramolecular forces. The unsaturated B ring provides another element of flexibility in the structure of cholesterol. The 5-ene B ring is normally observed in an 8 beta, 9 alpha-half-chair conformation. However, in structures containing more than one molecular in the crystallographic asymmetric unit, at least one of the 2 molecules is found to differ significantly from this form. It may be that this inherent flexibility is responsible for the presence of conformationally distnct molecules in the same crystal. The intermolecular interaction observed in the crystal structure of cholesterol and its fatty acid derivatives illustrate the type of interaction between the steroid ring system and hydrocarbon chains that can be expected in membrane bilayers.

Further studies on the inhibition of sterol biosynthesis in animal cells by 15-oxygenated sterols

The chemical syntheses of a number of C27 15-oxygenated sterols and their derivatives have been pursued to permit evaluation of their activity in the inhibition of sterol biosynthesis in animal cells in culture. Described herein are chemical syntheses of 3 alpha-benzoyloxy-5 alpha-cholest-8(14)-en-15-one, 5 alpha-cholest-8(14)-en-3 alpha-ol-15-one, 5 alpha-cholest-8(14)-en-15-one-3 beta-yl pyridinium sulfate, 5 alpha-cholest-8(14)-en-15-one-3 beta-yl potassium sulfate (monohydrate), 5 alpha-cholest-8(14)-en-15-one-3 alpha-yl pyridinium sulfate, 5 alpha-cholest-8(14)-en-3 alpha-yl potassium sulfate (monohydrate), 5 alpha-cholest-8(14)-en3,7,15-trione, 5 alpha-cholest-8(14)-en-15 alpha-ol-3-one, 5 alpha, 14 alpha-cholestan-3 beta, 15 beta-diol diacetate, 5 alpha, 14 beta-cholestan-3 beta, 15 beta-diol diacetate, 5 alpha, 14 alpha-cholestan-3 beta, 15 alpha-diol, 5 alpha, 14 alpha-cholestan-15 alpha-ol-3-one, 5 alpha, 14 beta-cholestan-3 beta, 15 beta-diol, 5 alpha, 14 alpha-cholestan-3,15-dione, and 5 alpha, 14 beta-cholestan-3,5-dione. The effects of 8 of the above compounds and of 5 alpha-cholesta-6,8(14)-dien-3 beta-ol-15-one, 3 beta-he misuccinoyloxy-5 alpha-cholest-8(14)-en-15 one, 3 beta-hexadecanoyloxy-5 alpha-cholest-8(14)-en-15-one, 5 alpha-cholest-8(14)-en-3,15-dione, 5 alpha-cholesta-6,8(14)-dien-3,15-dione, 5 alpha-cholest-8-en-3 beta, 15 alpha-diol, 5 alpha-cholest-7-en-3 beta, 15 alpha-diol, 5 alpha-cholest-8(14)-en-15 alpha-ol-3-one, 5 alpha-cholest-8-en-15 alpha-ol-3-one, and 5 alpha-cholest-7-en-15 alpha-ol-3-one on the synthesis of digitonin-precipitable sterols and on levels of HMG-CoA reductase activity have been investigated and compared with previously published data on 7 other C27 15-oxygenated sterols.

Sterol synthesis. Syntheses of 15-oxygenated 5alpha, 14beta-cholest-7-en-3beta-ol derivatives

Treatment of 3beta-benzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene with boron trifluoride-etherate gave, in 43% yield, 3alpha-benzoyloxy-5alpha, 14beta-cholest-7-en-15-one with the unnatural C-D ring juncture. Reduction of the latter compound with lithium aluminum hydride gave 15alpha, 14beta-cholest-7-en-3beta, 15alpha-diol and 5alpha, 14beta-cholest-7-en-3beta, 15beta-diol in 9% and 81% yields, respectively.

Sterol synthesis. A novel reductive rearrangement of an alpha,beta-unsaturated steroidal epoxide; a new chemical synthesis of 5alpha-cholest-8(14)-en-3beta, 15alpha-diol

Reduction of 3beta-benzoyloxy-14alpha,15alpha-epoxy-5alpha-cholest-7-ene with either lithium triethylboro-hydride or lithium aluminum hydride (4 molar excess) gave 5-alpha-cholest-8(14)-en-3beta,15alpha-diol in high yield. Reduction of the epoxy ester with lithium triethylborodeuteride or lithium aluminum deuteride (4 molar excess) gave [7alpha-2-H]-5alpha-cholest-8(14)-en-3beta,15alpha-diol. Reduction of 2beta-benzoyloxy-14alpha,15alpha-epoxy-5alpha-cholest-7-ene with a large excess (24 molar excess) of lithium aluminum hydride gave, in addition to the expected 5alpha-cholest-8(14)-en-3beta,15alpha-diol, a significant yield (33%) of 5alpha-cholest-8(14)-en-3beta-o1. Reduction of the epoxy ester with a large excess (24 molar excess) of lithium aluminum deuteride gave [7alpha-2H]-5alpha-cholest-8(14)-en-3beta,15alpha-diol and 5alpha-cholest-8(14)-en-3beta-o1 which contained two atoms of stably bound deuterium.

Sterol synthesis. Chemical synthesis of 3beta-benzoyloxy -14alpha, 15alpha-epoxy-5alpha-cholest-7-ene, a key intermediate in the synthesis of 15-oxygenated sterols

3BETA-Benzoyloxy-14alpha, 15alpha-cholest-7-ene was obtained in 96% yield upon treatment of 3beta-benzoyloxy-5alpha-cholesta-7, 14-diene with m-chloroperbenzoic acid. The delta7-14alpha, 15alpha-epoxy-steryl ester provides a useful intermediate for the syntheses of sterols with an oxygen function at carbon atom 15. For example, treatment of 3beta-benzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene with methanolic hydrochloric acid gave 3beta-benzoyloxy-5alpha-cholest-8(14)-en-15-one in 82% yield.

Synthesis an crystal structure of 3beta-p-bromobenzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene

The chemical synthesis of 3beta-bromobenzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene is described. Single crystal structral analysis was employed to unambiguously determine the location and absolute configuration of the epoxide moiety in the 3beta-p-bromobenzoyloxy-14alpha, 15alpha-epoxy-5alpha-cholest-7-ene. The space group of the crystal was P1, with unit cell parameters: a=10.873 A, b=13.841 A, c=11.037 A, alpha=75.19 degrees, beta=78.79 degrees, gamma=101.57 degrees, and two molecules per unit cell. Intitial phases were derived from the two bromine atoms. Least squares refinements on all non-hydrogen atoms were carried out to a final unweighted R value of 0.10 and weighted R value of 0.04. The epoxide ring was located at the 14, 15 position and was found to extend to the alpha side of the molecule. Molecular measurements for asymmetry parameters of the sterol nuceus indicate that ring A has a symmetrical chair conformation and ring B has a half chair conformation due to the double bond at C(7). Ring C has a fairly distorted chair conformation due to the trigonal C(8) on one sie and the almost planar 5-membered ring on the other. Ring D has the 17alpha-envelope conformation.