Squalene 2,3-oxide, and

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). 

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... 

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). 

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. 

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... 

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). 

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-... 

Optically active epoxy-terpenes have aroused considerable interest in recent years because of their intermediacy in biosynthetic pathways. Several reports on the syntheses of chiral terpenes have appeared, " among which are the syntheses of / -( + )- and S-(-)-squalene-2,3-oxide from L-glutamic acid (Scheme 1). 

Caras, I.W. and Bloch, K., Effects of a supernatant protein activator on microsomal squalene-2,3-oxide-lanosterol cyclase, J. Biol. Chem. 254 (23), 11816-11821, 1979. 

A few simple eukaryotic organisms can cyclize squalene but do not produce sterols. For example, Tetrahymena pyri-formis (a protozoan) produces tetrahymanol (14), a pentacyclic analog of lanosterol (15) (Fig. 23.1), from squalene. The mechanism of condensation of squalene (1) to tetrahymanol in Tetrahymena pyriformis does not involve squalene 2,3-oxide (2) and includes a protonation step, cyclization, and, finally, addition of hydroxyl group from the medium. The stereochemistry of the proton predicted to be incorporated at the 21p-position of the compound has been estab-... 

The common precursor in the biosynthesis of limonoids and cucurbitacins is squalene-2,3-oxide (IV). Based on some identified intermediary compounds, the biosynthetic pathway is probably as postulated in Reaction 18.7. 

E. E. van Tamelen and J. H. Freed (1970), Biochemical conversion of partially cyclized squalene 2,3-oxide types to the lanosterol system. Views on the normal enzymic cyclization process. J. Amer. Chem. Soc. 92, 7206-7207. 

Further studies have been reported on the rat liver mierosomal enzyme squalene epoxidase, which catalyses epoxidation of squalene (12) to squalene 2,3-oxide (25). For epoxidase activity there is a requirement for oxygen, NADPH, microsomes, a heat-labile cytoplasmic protein, and a phospholipid. The system is also slightly stimulated by FAD, which might be a component of the electron-transport chain for oxygen activation. The heat-labile supernatant protein has a molecular weight of 44000, but does not bind either squalene, 10,11-dihydrosqualene (which can also act as substrate for the enzyme), or squalene 2,3-oxide, which suggests that it does not have carrier activity analogous to the reported sterol carrier proteins. There is also no evidenee at present that this supernatant acts catalytically. 

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