Cholesterol Cholesteryl acetate

Physical Barriers. Wu et al (46) observed that the inclusion of palmitic acid or cholesteryl acetate in linoleic acid monolayers on silica, exerted a protective effect against oxidation. They suggested that these compounds act as a spacer keeping the linoleic acid molecules farther apart while being only slowly oxidized themselves. Similarly, our recent work with cholesterol oxidation appears to indicate that carbohydrates do not change the pathway of cholesterol oxidation but rather act as a physical barrier against the migration of reactive species. 

The concept of a stable mesomeric cation cannot be inferred solely from kinetic results, but follows from analysis of reaction products, and studies of the reactions of "i-cholesterol and its derivatives. Most important was the demonstration by Winstein [35] that identical product mixtures are produced by the methanolysis of either cholesteryl tosylate or 6 trichloroacetoxy-3a,5a-cyclocholestane (15 X O-COCCls), from which it was argued that these steroids solvolyse through a common cation. Furthermore i-cholesteryl methyl ether (15 X = OMe) is converted by absolute ethanol into a mixture of i-cholesteryl ethyl ether (15 X — OEt) and cholesteryl ethyl ether [36]. i-Cholesteryl acetate (15 ... 

Fig. 7.2. Binding of structural analogues to cholesterol-imprinted polymer. Cholesterol (2), cholestane (3), cholesteryl acetate (4), epicholesterol (cholest-5-ene-3a-ol) (5) and cholest-5-ene-3-one (6), all 2 mM in hexane. Adapted from [9].

In the present experiment cholesterol is dissolved in acetic acid and allowed to react with acetic anhydride to form the ester, cholesteryl acetate. The reaction does not take place rapidly and consequently does not go to completion under the conditions of this experiment. Thus, when the reaction is over, both unreacted cholesterol and the product, cholesteryl acetate, are present. Separating these by fractional crystallization would be extremely difficult but because they differ in polarity (the hydroxyl group of the cholesterol is the more strongly adsorbed on alumina), they are easily separated by column chromatography. Both molecules are colorless and hence cannot be detected visually. Each fraction should be sampled for thin-layer chromatography. In that way not only the presence but also the purity of each fraction can be assessed. It is also possible to put a drop of each fraction on a watch glass and evaporate it to see if the fraction contains product. Solid will also appear on the tip of the column while a compound is being eluted. 

Cholesteryl acetate (mp 115°C) and cholesterol (mp 149°C) should appear, respectively, in early and late fractions with a few empty fractions... 

Relative retention time (RRt) is defined as the ratio of the analyte and the reference compound s retentions. The RRt of sterols relative to cholesterol and cholestane, stearyl acetates to cholesteryl acetate, estrogen derivatives to cholestane, estrogen TMS ether to cholestane on various packed columns was described by Heftman. The RRt of some estrane, androstane, and pregnane derivatives was also reported. ... 

Acetylation. An often used procedure is to dissolve a primary or secondary alcohol in pyridine, add excess acetic anhydride, and let the mixture stand overnight at room temperature. Another procedure is as follows. A mixture of 150 g. of cholesterol, 300 ml. of pyridine, and 150 ml. of acetic anhydride was heated on the steam bath for one-half hour and diluted extensively with water. The precipitated granular product was collected and washed well the yield of fully dry cholesteryl acetate was 166 g. (100%). 

Fig. 11.6.1. HPLC separation of cholesterol and cholesteryl ester standards. Chromatographic conditions column, Supelcosil LC-18 (250x4.6 mm I.D.) mobile phase, acetonitrile-methanol-chloroform (1 1 1, v/v/v) flow rate, 1.0 ml/min temperature, ambient detection, differential refractometer. Peaks 1, cholesterol, 2, acetate 3, propionate 4, butyrate 5, nonanoate 6, decanoate 7, arachidonate 8, laurate 9, linoleate 10, oleate 11, elaidate 12, palmitate 13, stearate. The average mass of lipid chromatographed was 20-40 ng. Reproduced from Perkins et al. (1981), with...

Katon et al. (1968) have extended their low-temperature work to other types of compounds and have given a description of the low-temperature cell they used. Other carbohydrates were examined (e.g., raffinose, sucrose, fructose, arabinose, xylose, lactose, mannose, maltose, galactose, rhamnose, cellobiose, and melibiose) as well as a noncrystalline trypsin (little or no change at the lower temperature), urea, diphenyl, stearoyl chloride, cholesterol (Fig. 3.20), cholesteryl acetate, serotonin creatinine sulfate, sodium creatinine phosphate hexahydrate, daunomycinone, carnosine, and... 

Cholestane Cholesteryl acetate 4 Cholest-5-ene-3-one O Epicholesterol Cholesterol... 

Bortolomeazzi et al. (1994) used GC/EI/MS with an ion trap to identify the thermal oxidation products of cholesteryl acetate as the 7P-hydroperoxy and 7a-hydroperoxy cholesteryl acetate, 7keto-cholesteryl acetate, the a and P isomers of 7-hydroxycholesteryl acetate, the a- and P-5,6-epoxy isomers and several derivatives arising from the loss of acetate and water. Dzeletovic et al. (1995b) have observed that saponification during sample preparation did not hydrolyse all of the oxysterol esters completely and that separation of oxysterols from cholesterol by HPLC was tedious and incomplete. They developed a stable isotope dilution GC/EI/MS SIM method for the determination of cholesterol oxidation products in human plasma. Nine oxysterols were determined by using deuterium-labelled internal standards. 

Today we know that the cholesterol esters crmsist of helical (chiral) molecules, and on cooling from the isotropic phase they rmdergo a transition into another phase called a cholesteric phase. This shows unique optical properties. In Fig. 1.3a we see a photo-image of a 20 pm thick polycrystalline layer of cholesteryl acetate viewed in a polarizing microscope. Upon heating the substance melts, that is it becomes... 

Fig. 125. Lengths of run of sterols and steroids of various polarities in 6 solvent systems of different eluotropic properties (Nos. 1—6 below). Substances 1 tetra-hydrocortisol 2 cortisol 3 cortisone 4 corticosterone 5 oestradioI-17/ 6 5/5-pregnane-3a,20a-diol 7 oestrone 8 testosterone 9 pregn-5-en-3j5-ol-20-one 10 androst-4"ene-3,17-dione ii deoxycorticosterone 12 progesterone 73 cholesterol 14 cholesteryl acetate 15 cholestane...

Fig. 3 Effect of cholesteryl ethers in liposomes prepared with DOPC on the relative amount of glucose released (A) (x) cholesterol (o) cholesteryl methyl ether ( ) cholesteryl ethyl ether (v) cholesteryl-n-propyl ether ( ) cholesteryl isopropyl ether (A) cholesteryl butyl ether (B) (x) cholesterol (o) cholesteryl (2 -hydroxy)-3-ethyl ether ( ) cholesteryl methoxy methyl ether ( ) cholesteryl acetate.

Havinga and Bots (1954) were the first to describe a method for the preparation of C -labeled cholecalciferol. Cholecalciferol-3-C was obtained by irradiation of 7-dehydrocholesterol synthesized from labeled cholesteryl acetate. The photochemical conversion from 7-dehydrocholesterol to cholecalciferol is accompanied by the formation of several other products such as lumisterols, tachysterols, and prechole-calciferol. A solution of NaNOs in water (0.4 ) effectively absorbs ultraviolet rays of wavelengths shorter than 250 mji which are supposed to promote die formation of side products. In order to minimize the formation of overirradiation products the ethereal solution of 7-dehy-drocholesterol-3-C is irradiated till 44.5% has been converted. The Havinga-Bots irradiation vessel, unique in itself, consists of three annular chambers made of quartz enclosing the irradiating lamp. 7-Dehydrocholesterol-3-C has been prepared recently from diolesterol-3-C (Kulkami et al., 1963) by the appKcation of an isocaproate adaptation (Nes et al., 1956) of the allylic bromination of cholesterol with 2V-bromosuccinimide followed by dehydrobromination. 

Tosyl cholesteryl acetate (V) (Akhtar and Barton, 1964) was first converted to the corresponding 19-iodide (VI) by treatment with sodium iodide in boiling ethyl methyl ketone. Reduction of the iodide with zinc and acid in the presenee of tritiated water provided cholesteryl acetate-19-H (VII) which was then converted to 7-dehydro-cholesterol-19-H (VIII), presxunably via the N-bromosuccinimide reaction. Ultraviolet irradiation and thermal rearrangement of the provitamin yielded cholecalciferol-9,19-H (IX). 

H -cholesteryl acetate has been prepared according to the procedure used for the preparation of H -cholesterol (Murray and Williams, 1958b) except that the hydrolysis in KOH was omitted. The acetate and butyrate esters of H -cholesterol were also synthesized on a microscale basis from 7a-H cholesterol, acetic anhydride, and butyiyl chloride, in pyridine (Deykin and Goodman, 1962). 

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