Cholesterol-5a,6a-epoxide

Treatment of the chlorohydrin (328), obtained from cholesterol 5a,6a-epoxide (327), with potassium bisulphate followed by acetylation furnished the rearranged... 

Thus, product 4 may have a dual origin, arising from autoxidation as well as from en2ymatic monooxygenation [16] However, autoxidation always yields a mixture of 3 and 4 [16]. Also 20-hydroxycholesterol can be formed both by autoxidation and by enzymatic means. 20(S)- and 22(R)-hydroxycholesterol are intermediates in the synthesis of steroid hormones [16]. Cholesterol 5a,6a-epoxide 6 is mostly associated with autoxidation, but has also been shown to be enzymatically formed in different systems. It is also formed by the action of agents such as hydroxyl radical or hydrogen peroxide on cholesterol [9]. 

Much of our present knowledge about the stereochemistry of reactions was developed from steroid chemistry. In this experiment the double bond of cholesterol is stereospecifically converted to the 5a,6a epoxide. The a designation indicates the epoxide is on the backside of the molecule. A substituent on the topside is designated j8. Study of molecular models reveals that the angular methyl group prevents topside attack on the double bond by the perbenzoic acid hence the epoxide forms exclusively on the back, or a side of the molecule. 

Sevanian et al. (1994) applied GLC and LC/TS/MS for the analysis of plasma cholesterol-7-hydroperoxides and 7-ketocholesterol. Analysis of human and rabbit plasma identified the commonly occurring oxidation products, yet dramatic increases in 7-ketocholesterol and cholesterol-5p, 6P-epoxide were observed. The study failed to reveal the presence of choles-terol-7-hydroperoxides, which were either too unstable for isolation, metabolized or further decomposed. The principal ions of cholesterol oxides monitored by LC/TS/MS were m/z 438 (cholestane triol) m/z 401 (cholesterol-7-hydroperoxide) m/z 401 (7-ketocholesterol) m/z 367 (7a-hydroxycholesterol) m/z 399 (cholesta-3,5-dien-7-one) and m/z 385 (choles-terol-5a,6a-epoxide). The major ions were supported by minor ions consistent with the steroid structure. Kamido et al. (1992a, b) synthesized the cholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as stable secondary oxidation products of cholesteryl arachidonate and linoleate, respectively. These compounds were identified as the 3,5-dinitrophenylhydrazone (DNPH) derivatives by reversed-phase LC/NICI/MS. These standards were used to identify cholesteryl and 7-ketocholesteryl 5-oxovaleroyl and 9-oxononanoyl esters as major components of the cholesteryl ester core aldehydes generated by copper-catalysed peroxidation of low-density lipoprotein (LDL). In addition to 9-oxoalkanoate (major product), minor amounts of the 8, 9, 10, 11 and 12 oxo-alkanoates were also identified among the peroxidation products of cholesteryl linoleate. Peroxidation of cholesteryl arachidonate yielded the 4, 6, 7, 8, 9 and 10 oxo-alkanoates of cholesterol as minor products. The oxysterols resulting from the peroxidation of the steroid ring were mainly 7-keto, 7a-hydroxy and 7P-... 

Studies conducted by Barenghi eta.1. (1990) and Lodge etal. (1993) independently have demonstrated the facile, multicomponent analysis of a wide range of PUFA-derived peroxidation products (e.g. conjugated dienes, epoxides and oxysterols) in samples of oxidized LDL by high-field H-NMR spectroscopy. Figure 1.9 shows the applications of this technique to the detection of cholesterol oxidation products (7-ketocholesterol and the 5a, 6a and 5/3,60-epoxides) in isolated samples of plasma LDL pretreated with added coppcr(Il) or an admixture of this metal ion with H2O2, an experiment conducted in the authors laboratories. 

Dioxiranes react with cholesterol and its acetate to give ca 1 1 mixtures of 5a,6a- and 5/i,6/i-epoxides (in contrast to peroxy acids, that are known to produce ca 5 1 mixture)318. 

In 1963, cholesterol hydroperoxides were reported (36) in egg-containing foods irradiated by sunlight. Chicoye et al. (37) observed the following 5 photoxidation derivatives of cholesterol in spray-dried yolk exposed to either 40-watt fluorescent lamp (approx. 280 hours) or summer sunlight (5 hours) 33"hydroxy-cholest-5-en-7-one (7-keto) cholest-5-ene-33,7a-diol (7a-diol) cholest-5-ene-33,73-diol (73-diol) 5,63-epoxy-5a-cholestan-33-ol (3-epoxide) and 5a-cholestane-33, 5cx, 63-triol (triol). Subsequently, Tsai et al. (38) developed methodology to demonstrate the presence of 5,6a-epoxy-5a-cholestan-33-ol (a-epoxide) in dried egg products which were spiked with the epoxides. 

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