24-Methylene lanosterol

The isomerase does not act on cycloartenol or 24-methylene cycloartanol which explains the absence of lanosterol (1-A) and 24-methylene lanosterol (1-F) from photosynthetic tissues. 

Other, removable cation-stabilizing auxiliaries have been investigated for polyene cyclizations. For example, a sdyl-assisted carbocation cyclization has been used in an efficient total synthesis of lanosterol. The key step, treatment of (257) with methyl aluminum chloride in methylene chloride at —78° C, followed by acylation and chromatographic separation, affords (258) in 55% yield (two steps). When this cyclization was attempted on similar compounds that did not contain the C7P-silicon substituent, no tetracycHc products were observed. Steroid (258) is converted to lanosterol (77) in three additional chemical steps (225). 

The first pathogen-specific reaction is the S-adenosylmethio-nine-dependent side chain aklylation of lanosterol. This is pathogen specific since in cholesterol synthesis, a side chain alkylation does not take place. Secondly, the demethylation reactions at C - and C -positions of 24-methylene-dihydrolanosterol are pathogen-specific as well. In mammals demethylation reactions take place, but the substrate is not side chain alkylated, so the corresponding enzyme should possess different binding sites for the different substrates. 

The methyltransferase that catalyses the formation of 24-ethylidene sterols from 24-methylene precursors in bean rust uredospores (Uromyces phaseoli) can also alkylate lanosterol, 24-methylenecholesterol, zymosterol, and desmosterol to produce C29 sterols.[28- H]-24-Methylenecholesterol was efficiently (ca. 3% yield) incorporated into withaferin A (88) and withanolide D (89) by Withania somnif-era, whereas [28- H]-24-methylcholesterol did not act as a precursor in this system. However, since feeding experiments with [28- H]-24-methylcholesta-5,24-dien-3/8-ol gave ambiguous results it was not possible to rule out other 4-desmethyl sterols (with C28) as intermediates in withanolide biosynthesis. 

P. anthracophilus found on a Eucalyptus species were established at the same time. The work on eburicoic acid was carried out by Robertson in Liverpool. The elucidation of the structure of polyporenic acid A (5.161) benefited from experience gained in studies on the structure of lanosterol and on the penta-cyclic triterpenes. The identification of some structural features also showed the developing role of UV and IR spectroscopic methods in the 1950s. However, the purification of these compounds was impeded by the difficulty of separating the 8(9)-enes from the 7(8),9(ll)-dienes that often accompanied them. Initial studies on polyporenic acid A revealed the presence of a carboxylic acid, two hydroxyl groups and two double bonds, each of differing reactivity. One double bond was a terminal methylene since formaldehyde was obtained on ozonolysis whilst the other was more hindered. Typical of the tetracyclic triterpenes. 

An interesting feature of these triterpenes is the presence of the extra methylene at C-24 that is typical of ergosterol, indicating that this group may be introduced relatively early in the biosynthetic modification of lanosterol. 

Ergosterol also arises from lanosterol by a one-carbon addition to C-24 followed by a shift of the A24 double bond to the exo position, forming a A24 methylene function. There follows oxidative removal of the three methyls at C-14 and C-4 with concomitant shift of the A24 double bond to A22 position isomerization of A8 position to A7 and formation of an additional A5 double bond yielding ergosterol. 

Acyl-13-dithianes are available by the nucleophilic thioacetalisation of ketones bearing an enolisable methylene group with 1,3-propylene bis(p-tolue-nethiolsulfonate) (mp 73 C). The reaction was first applied in the Woodward-Barton synthesis of Lanosterol and has since been adapted to some useful synthetic procedures. For example, reaction of the enamine 94.1 [Scheme 2.94] with 13-propylene bis(p-toluenethiolsulfonate) afforded the 2-acyl-13-dithiane 94.2 after hydrolysis of the enamine. The ability of the dithiane to sta-... 

As per cent of total sterol 1, cholesterol 2, desmosterol 3, 24-methylene cholesterol 4, fucosterol 5, zymosterol 6, fecosterol 7, stigmasta-8, E-24(28)-dienol 8, lathosterol 9, ergosta-7, 24(28)-dlenol 10, stlgmasta-7, -24(28)-dienol 11, 7-dehydrofucosterol, 12, lanosterol. 

The second methylation step has recently been demonstrated in vitro in preparations from bramble cell cultures (Fonteneau et al., 1977). The enzyme, which is microsomal, is specific for 24-methylene lophenol (5-F) and will not act on substrates with an intact cyclopropane ring such as 24-methylene cycloartanol (2-F) the product of the reaction is 24-ethylidene lophenol (2-C). A similar enzyme which uses lanosterol (1-A) as substrate was reported in the uredospores of Uromyces phaseoli (Liu and Knoche, 1974), which is one of the few fungi which synthesize C29 sterols (Liu and Knoche, 1976). 

About 4% of the egg lipids consists of sterols. The main corrqtonent is cholesterol (96%), ca. 15% of which is esterified with fatty acids. The cholesterol content is 2.5%, based on the egg yolk solids. Disregarding mammalian brain, this level exceeds hy far that in all other foods (cf. 3.8.2.2.1) and, therefore, serves as an indicator of the addition of eggs. Cholestanol, 7-cholestenol, campesterol, P-sitosterol, 24-methylene cholesterol and lanosterol are other components of the sterol fraction. The quality of egg products is endangered hy autoxidation of cholesterol (cf. 3.8.2.2.1). 

Ch cholesterol. Erg Ergosterol, Obt Obtusifoliol 24-M 24 - methylene dihydro lanosterol. 

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