25-Hydroxycholesterols acetate

The known 25-hydroxycholesterol acetate was brominated and the resulting 25-bromocholesterol was reduced with lithium aluminum tritide to give cholesterol-25- H (Joly et al., 1969). 

A full account of the syntheses of the four isomeric 20,22-epoxycholesterols is accompanied by two similar reports. Pregnenolone has been converted by two independent routes into 25-oxo-27-norcholesteryl acetate. Both routes employed the highly stereoselective platinium-catalysed hydrogenation of the 20,22-double bond which in one case was shown to have the E-configuration. Three additional syntheses of 25-hydroxycholesterol have been reported. The 6/3-methoxy-3a,5a-cyclo-20-carboxaldehyde (261), derived from stig-masterol, was converted into the diene (262) " or the dichloro-olefin (263) and thence into 25-hydroxycholesterol as indicated in Scheme 10. Alternatively the side-chain of the bromo-derivative (264) was extended as indicated in Scheme 2 2 186... 

In contrast to these results, 25-hydroxycholesterol (and also, 20-hy-droxycholesterol, 7-ketocholesterol, and diosgenin) in aortic, smooth-muscle cells effectively blocks the incorporation of acetate into lipid-linked oligosaccharides (and, also, into cholesterol7). Thus, less of the lipid-linked oligosaccharides were available for glycosylation of proteins. In harmony with the presumed, inhibitory mechanism was the observation that incorporation of mevalonate into lipid-linked oligosaccharides was not inhibited, and that mevalonate itself (the product formed by HMG-CoA reductase from HMG-CoA and NADPH) could reverse the inhibition of glycosylation of protein (see Scheme 1). 

FIGURE 6-11. Demonstration of resolution improvement using recycle, (a) Separation of 26-hydroxycholesterol 3, 26-diacetate from human aorta. After seven recycles, resolution of the 25S-epimer (first peak) is achieved from the 25R-epimer (second peak). Column pPorasil (silica, 10 pm), 3.9 mm ID x 60 cm (2 columns). Mobile phase 2.5% (v/v) ethyl acetate in hexane. Flow rate 1 mL/min. Detector refractometer. (Reproduced from J. Redel, J. Chromatogr. 168, 273 (1979) with permission.)... 

A coupling appears to exist between the rate-limiting enzyme in the biosynthesis of cholesterol, HMG-CoA reductase, and cholesterol 7a-hydroxylase. The two enzymes seem to be located close to each other on the endoplasmic reticulum membrane [66], and the two activities covariate under most conditions. Results from both in vivo and in vitro experiments show that newly synthesized cholesterol is the preferred substrate for cholesterol 7a-hydroxylase. In an early study by Staple and Gurin, it was shown that the bile acids in bile had a higher radioactivity than cholesterol after administration of labelled acetate to rats [67]. Bjorkhem and Danielsson found that the specific radioactivity of 7a-hydroxycholesterol was higher than that of cholesterol after incubation of labelled mevalonate with the 10000 x g supernatant fluid of a rat liver homogenate [50]. Balasubramaniam et al. showed that 7a-hydroxycholesterol isolated from the livers of rats after intravenous administration of labelled cholesterol had a lower specific radioactivity than cholesterol [58]. Cronholm and collaborators measured the incorporation of isotope from [1- H2]-,... 

The possibility that the biosynthesis of bile acids is regulated by a negative feedback mechanism was supported by early experiments by Thompson and Vars [206] and Eriksson [207], who showed that the rate of bile acid synthesis in rats increased about 10-fold when a bile fistula is made. Bergstrom and Danielsson demonstrated that duodenal infusion of taurochenodeoxycholic acid in bile fistula rats restored the increased synthesis to a normal rate [208]. Danielsson et al. [44] showed that the cholesterol 7a-hydroxylase activity increased in parallel with the bile acid synthesis after cannulation of the bile duct in rats. In a subsequent work by Mosbach et al., it was reported that the incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol into bile acids was inhibited by duodenal infusion of taurocholate to bile fistula rats [209]. The incorporation of isotope from labelled acetate, mevalonate and cholesterol but not from labelled 7a-hydroxycholesterol was stimulated in perfused livers of cholestyramine-treated rabbits [210]. It was concluded that there are essentially no rate-limiting steps beyond 7a-hydroxycholesterol in the biosynthesis of bile acids from acetate. Since both cholesterol and bile acid biosynthesis was subjected to negative feedback inhibition by bile acids, it cannot be excluded that inhibition of cholesterol biosynthesis precedes inhibition of the bile acid biosynthesis, and that the latter inhibition is secondary to the former. 

The microbial conversion of 19-hydroxycholesterol, obtained chemically in three steps from cholestery acetate, to estrone by incubation with the microorganism CSD-10 afforded estrone in a single step in 72% yield (ref. 112) following... 

Since 19-hydroxycholesterol 3-acetate is readily available from cholesterol by the illustrated synthesis, it is possible to predict that this method or one closely allied to it will supplant presently used technology for estrone manufacture to some degree. Estrone, in addition to its uses as a female hormone is a key intermediate for the commercial synthesis of many widely used contraceptive agents. It has been selling for a price in the range of 0.50- 1.00/gram (1965). 

A shorter but less selective route to la-hydroxycholesterol has been described by Kaneko (90). Cholesta-l,4-dien-3-one (90) was deconjugated to its -isomer and then reduced with borohydride to give 3P-hydroxy-cholesta-1,5-diene (91). Selective hydroboration followed by alkaline peroxide treatment afforded a 30% yield of a separable 1 1 mixture of la- and 2a-hydroxycholesterol (89) and (93). If the acetate (92) was hydrated by an oxymercuration-demercuration procedure, a mixture of the 2 3-hydroxycholesterol (94) (14%) and la-hydroxycholesterol (89) (26%) was isolated after saponification (118). When starting with the readily available cholesta-l,4,6-trien-3-one, the deconjugation and reduction afforded 3P-hydroxycholesta-l,5,7-triene (95) (59). The 5,7-diene system was then protected as its 4-phenyl-l,2,4-triazoline-3,5-dione (PTD) adduct (96). Treatment of (96) with m-chloroperbenzoic acid gave a 5 3 mixture of the lp,2p- and la,2a-epoxides (98). More recently, Whalley and his co-workers have found that if the dimethyl-t-... 

A unique and surprisingly efficient synthesis of la-hydroxycholesterol has been described by Mihailovic and co-workers 114). The 5a-hydroxy-cholestanyl acetate (100), obtainable in high yield from cholesterol, was converted by irradiation with visible light in the presence of mercuric oxide and iodine into the 5,10-secosterol (101). Irradiation of (101) with a high pressure mercury lamp resulted in intramolecular cyclization to give the oxetane (102) as the major product. Treatment of this material with hydroiodic acid in acetic acid afforded la-hydroxycholesterol 3-acetate (103) in 20% overall yield from cholesterol. 

Dehydration of 20a-hydroxycholesterol with a catalytic amount of hydrochloric acid in methanol gave 20(22)-dehydrocholesterol. This product is a mixture of the 20(22)-double bond isomers in which the cis isomer preponderates. The identical product was obtained from the reaction of pregnenolone acetate with isohexylidenetriphenyl-phosphorane in dimethyl sulfoxide (Gut, 1970). 

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