Squalene-oxide

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

Epoxyfarnesol was first prepared by van Tamelen, Stomi, Hessler, and Schwartz 4 using essentially this procedure. It is based on the findings of van Tamelen and Curphey5 that N-bromosuccinimide in a polar solvent was a considerably more selective oxidant than others they tried. This method has been applied to produce terminally epoxidized mono-, sesqui-, di-, and triterpene systems for biosynthetic studies and bioorganic synthesis.6 It has also been applied successfully in a simple synthesis of tritium-labeled squalene [2,6,10,14,18,22-Tetracosahexaene, 2,6,10,15,19,23-hexamethyl-, (all-E)-] and squalene-2,3-oxide [Oxirane, 2,2-dimethyl-3-(3,7,12,16,20-pentamethyl-3,7,ll,-15,19-heneicosapentaenyl)-, (all-E)-],7 and in the synthesis of Cecropia juvenile hormone.8... 

R. Nadeau and R. Hanzlik, Synthesis of Labeled Squalene and Squalene-2,3-Oxide, ... 

Processes of this kind are important in the biosynthesis of steroids and tetra- and pentacyclic terpenes. For example, squalene 2,3-oxide is converted by enzymatic catalysis to dammaradienol. 

FIGURE 1.4 Proposed biosynthetic route for the biosynthesis of (A) squalene oxide (squalene-2,3-oxide) via the isoprenoid pathway and (B) triterpene saponins of the dammarane-type and oleanane-type from squalene oxide. PP, diphosphate group GPS, geranyl phosphate synthase FPS, farnesyl phosphate synthase NADPH, nicotinamide adenine dinucleotide phosphate. 

NT325 Reid, W. W. Accumulation of squalens-2,3-oxide during inhibition of phytosterol biosynthesis in Nicotiana tabacum. Phytochemistry 1968 7(3) NT338... 

The all-tra 5 -squalene (C30H50), discovered in shark liver oil in the 1920s, is a triterpene, but one in which the isoprene rule at violated in one point. Rather than a head-to-tail arrangement of six units of isoprene, there appear to be farnesyl units that have been connected tail to tail. Almost aU steroids are biosynthesized from cholesterol. Cholesterol is biosynthesized from squalene, which is first converted to lanosterol. The conversion of squalene to the steroid skeleton is an oxirane, squalene-2,3-oxide, which is transformed by enzymes into lanosterol, a steroid alcohol naturally found in wool fat. The whole process is highly stereoselective. 

Most animal steroids arise from cholesterol, which in turn is derived from squalene. This C30 triterpene, whose biosynthesis is described in Section B, is named after the dogfish Squalus in whose liver it accumulates as a result of blockage in oxidation to cholesterol. Squalene is also a prominent constituent of human skin lipids. Its conversion to cholesterol, which takes place in most animal tissues,117/154-156 is initiated by a microsomal enzyme system that utilized 02 and NAD-PH to form squalene 2,3-oxide (Fig. 22-6, step a). 

The two remaining reactions in the biosynthesis of lanosterol are shown in figure 20.9. In the first of these reactions, squalene-2,3-oxide is formed from squalene. As can be seen in figure 20.8, squalene is a symmetrical molecule, hence the formation of squalene oxide can be initiated from either end of the molecule. The oxide is converted into lanosterol. The reaction can be formulated as proceeding by means of a protonated intermediate that undergoes a concerted series of trans-1,2 shifts of methyl groups and hydride ions to produce lanosterol (see fig. 20.9). 

Cyclization of squalene is via the intermediate squalene-2,3-oxide (Figure 5.55), produced in a reaction catalysed by a flavoprotein requiring O2 and NADPH cofactors. If squalene oxide is suitably positioned and folded on the enzyme surface, the polycyclic triterpene structures formed can be rationalized in terms of a series of cycliza-tions, followed by a sequence of concerted Wag-ner-Meerwein migrations of methyls and hydrides... 

As shown in the Section II, A, daphniphylline (1) and daphmacrine (18), whose stereostructures have been unambiguously determined, have the same amine moiety but differ in the oxygen heterocyclic skeleton. Thus, it is quite reasonable to suppose that two different moieties must be constructed from such a common precursor as A which can be derived from squalene via squalene-2,3-oxide, and from a monocyclic olefin (Scheme X). 

A convenient synthesis of optically active squalene 2,3-oxide from L-glutamic acid has been reported.The (5)-acetonide (1), derived from glutamic acid, was converted by standard methods into the C30 compound (2). The corresponding diol (3) was transformed," via the mesylate (4), into (3i )-squalene 2,3-oxide (5). Hydrolysis of (5), mesylation, and displacement afforded the enantiomeric (35)-oxide (6). 

A cell-free system from the prokaryotic bacterium Acetobacter rancens converts squalene into hop-22(29)-ene (8) and hopan-22-ol (9). Incubation of (3/ ,5)-[12,13- H2]squalene 2,3-oxide under the same conditons afforded the corresponding 3a- and 3/3-hydroxyhopane derivatives (10)—(13) (see p. 151). The failure to detect ketonic intermediates in the incubation leads to the conclusion that the 3a-hydroxy-compounds are formed directly from (3i )-squalene 2,3-oxide (see Vol. 1, p. 162 and Vol. 5, p. 124). 

A major breakthrough was achieved in the elucidation of the biosynthesis of sterols. Strong evidence has been obtained showing that squalene 2,3-oxide is an intermediate in the conversion of squalene to cholesterol. ... 

Previous investigations from several laboratories have demonstrated that both microsomal membranes and the cytosolic fraction from rat hver are required for the biological synthesis of cholesterol [1-4]. Specifically, the following conversions have been reported to require both microsomes and cytosol acetate to cholesterol [4] squalene to cholesterol [1] squalene-2,3-oxide to lanosterol [3] lanosterol to cholesterol [1,5] A -cholestenol to cholesterol [6] lanosterol to dihydrolanosterol [7] various 4,4-dimethyl sterols to cholesterol [8] and 7-dehydrocholesterol to cholesterol [9,10]. 

There is little doubt that the pathway from acetyl-CoA to squalene 2,3-oxide in plants is the same as that in animals and as it is detailed in the chapter on cholesterol biosynthesis it will not be reiterated here. 

In animals (Chapter 1) squalene 2,3-oxide is first converted into lanosterol (TA) and this reaction also occurs in yeasts. However, in higher plants and algae the first cyclic product is cycloartenol (2-A). 

The enzyme involved is squalene 2,3-oxide cycloartenol cyclase and the substrate is the S enantiomer. The reaction is initiated by H" " attack on the oxygen of the epoxy group of the substrate held in the chair, boat, chair, boat unfolded conforma-... 

It is important to note that in the methylation reaction in yeasts the best substrate for the methylase enzyme is zymosterol (7-A), a compound almost at the end of the line in the biosynthetic sequence. In contrast to the plant enzyme it is not active on the first product of cyclization of squalene 2,3-oxide, in this case lanosterol (1-A). 

The resolution of synthetic presqualene and prephytoene alcohols via their etienic acid derivatives has been reported. This work confirmed that the active (-f-)-enantiomers in both series have the same absolute configuration [(li , 2/ , 3/ )]. It has been established, by use of Hn.m.r., that the proton (deuteron) introduced at C-3 during the cyclization of squalene to tetrahymanol by Tetrahymena pyriformis has the 3/8 configuration. Both antipodes of the trimethyldecalol (13) have been shown to be effective inhibitors of cholesterol biosynthesis in rat liver enzyme preparations and cultured mammalian cells. The accumulation of squalene 2,3-oxide and squalene 2,3 22,23-dioxide in the treated systems indicates that inhibition occurs at the cyclization stage. The inhibitor is metabolized to the diol (14). The results of other sterol inhibition... 

More direct methods for the degradation of the side-chain of cyclolaudenol have been published/ It has been shown, using the squalene 2,3-oxide (38) with a chiral methyl group, that the 1,3-proton loss in the formation of the cyclopropane ring of cycloartenol occurs with retention of configuration at the C-10 methyl group/ ... 

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