Squalene cyclization

Through a series of cyclizations, squalene oxide (C30) affords lanosterol in animals and fungi and cycloartenol in plants (Fig. 19). In both instances, the intermediate is a protosteryl cation that can also undergo a series of Wagner-Meerwein rearrangements to afford the cytotoxic cucurbitacins of melons and cucumbers. Squalene oxide in a chair-chair-chair-boat conformation yields the dammarenyl cation, a parent of numerous triterpene skeleta (e.g., lupane, oleanane, ursane, and taraxerane) contained in the saponins found in many foodstuffs, in soaps, and in several... 

In 1997, Spinella et al. isolated testudinariol A (149, Figure 6.6) as a metabolite of the marine mollusc Pleurobrancus testudinarius. This compound is a structurally unique triterpene alcohol, and thought to be a defensive allomone of P. testudinarius, because 149 was ichthyotoxic against a fish Gambusia afjinis. The partially cyclized squalene skeleton present in 149 is unique and biosynthetically unusual as in the case of limatulone (148). The unique structure 149 of testudinariol A led us to achieve its synthesis in 2001.8,9... 

The mevalonate pathway in the cytosol is responsible for biosynthesis of sterols, sesquiterpenes, and triterpenoids. After conversion of mevalonic acid to isopentenyl pyrophosphate, three C5 units can be joined head to tail to produce a C15 compound, famesyl pyrophosphate. Two famesyl pyrophosphates are then united head to head to form squalene, the progenitor of the C30 isoprenoids from which sterols are derived. The plant squalene synthetase, like its mammalian homologue, is found in the ER and the reaction proceeds via a presqualene pyrophosphate intermediate (Chapter 14). In the last step prior to cyclization, squalene is converted to squalene 2,3-epoxide. 

Squalene Cyclization.—Squalene epoxidase from rats needs a supernatant protein fraction for full activity. However, this fraction does not seem to correspond to the sterol carrier proteins mentioned above. It is a heat-labile molecule with a molecular weight of about 44 000. A 2,3-dioxetan derivative of squalene has been suggested as an intermediate in this oxidation. " ... 

PhYtophthora and related "sterol-less Oomycetes possess an enzymic defect in Stage I. They synthesize squalene (9.19) but fail to produce squalene- oxide (9.10.18). VIhile some fungi cyclize squalene-oxide to pentacyclic triterpenoids it remains unknown Whether the pythiaceous fungi similarily perform this kind of cyclization. 

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-... 

Squalene (1) is converted to (35)-squalene 2,3-epoxide (2) by the action of microsomal squalene epoxidase in the presence of O2 and NADPH (EC 1.14.99.7) (Goad, 1991b Goodwin, 1985). Epoxidation can occur at either end of the squalene molecule, suggesting that an intermediate form is released from the synthetase enzyme complex prior to epoxidation and cyclization. Squalene epoxidase (squalene monooxygenase EC 1.14.99.7) is a two-component system. The nature of the terminal oxidase has not been examined in detail. The second component, a flavoprotein, has now been fully characterized as an NADPH-cytochrome c reductase. The monooxygenase from rat liver has a molecular weight of 47,000, and the protein requires NADPH, cyto-... 

Cycloartenol (3) is the major product of condensation of squalene 2,3-epoxide (2) by a chair-boat-chair-boat transition state in plants. This condensation is effected by squa-lene-2,3-oxide-cycloartenol cyclase (Fig. 23.6). Micro-somes from Rabdosia japonica (Acanthaceae) showed activity for cyclizing squalene 2,3-epoxide into cycloartenol, 3-amyrin, and a-amyrin. Purified cycloartenol cyclase had a molecular weight of 54,000 (Abe et al., 1989a, 1989b). 

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. 

Farnesol pyrophosphate is an immediate precursor of squalene, the key intermediate in steroid and triterpenoid biogenesis, which arises from the coupling of two farnesol pyrophosphate molecules or of C,s units derived therefrom. The numerous types of sesquiter-penoid carbon skeletons represent various modes of cyclization of farnesol (sometimes with rearrangement) and it is probable that farnesol pyrophosphate is also the source of these compounds. 

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). 

Recent syntheses of steroids apply efficient strategies in which open-chain or monocyclic educts with appropiate side-chains are stereoselectively cyclized in one step to a tri- or tetracyclic steroid precursor. These procedures mimic the biochemical synthesis scheme where acyclic, achiral squalene is first oxidized to a 2,3-epoxide containing one chiral carbon atom and then enzymatically cyclized to lanostetol with no less than seven asymmetric centres (W.S. Johnson, 1%8, 1976 E.E. van Tamden, 1968). 

FIGURE 26 10 The biosyn thetic conversion of squa lene to cholesterol proceeds through lanosterol Lano sterol IS formed by enzyme catalyzed cyclization of the 2 3 epoxide of squalene... 

Squalene is also an intermediate in the synthesis of cholesterol. StmcturaHy, chemically, and biogeneticaHy, many of the triterpenes have much in common with steroids (203). It has been verified experimentally that squalene is the precursor in the biosynthesis of all triterpenes through a series of cyclization and rearrangement reactions (203,204). Squalene is not used much in cosmetics and perfumery formulations because of its light, heat, and oxidative instabiUty however, its hydrogenated derivative, squalane, has a wide use as a fixative, a skin lubricant, and a carrier of Hpid-soluble dmgs. 

The most important oxirane, from an anthropocentric viewpoint, is probably squalene oxide (72), a precursor of lanosterol (73) and thus of the maligned but essential cholesterol (74 Scheme 87) 78MI50501). The cyclization of (72) to (73) represents nucleophilic tr-attack on oxirane carbon cf. Section 5.05.3.4.3(t)()), and the process has also been extensively investigated in vitro (68ACR1). Oxiranes are even more ubiquitous in steroid biosynthesis than had been thought, for a cholesterol epoxide has been shown to be a product of mammalian steroid biosynthesis <81JA6974). 

Pentacyclosqualene, the symmetrical hydropicene corresponding to squalene, has not been made by acid-induced cation-olefin cyclization of squalene, despite considerable experimental study. A simple, convergent synthesis of pentacyclosqualene using cation-olefin cyclization to generate ring C was developed. The Cjo-framework was constructed by radical coupling to a tetracyclic intermediate that was also used for the synthesis of onoceradiene. 

In 1953, Bloch, together with the eminent organic chemist R. B. Woodward, proposed a new scheme (see figure, part b) for the cyclization of squalene. (Together with Fyodor Lynen, Bloch received the Nobel Prize in medicine or physiology in 1964 for his work.) The picture was nearly complete, but one crucial question remained How could isoprene be the intermediate in the... 

The second part of lanosterol biosynthesis is catalyzed by oxidosqualene lanosterol cyclase and occurs as shown in Figure 27.14. Squalene is folded by the enzyme into a conformation that aligns the various double bonds for undergoing a cascade of successive intramolecular electrophilic additions, followed by a series of hydride and methyl migrations. Except for the initial epoxide protonation/cyclization, the process is probably stepwise and appears to involve discrete carbocation intermediates that are stabilized by electrostatic interactions with electron-rich aromatic amino acids in the enzyme. 

Steroids are plant and animal lipids with a characteristic tetracyclic carbon skeleton. Like the eicosanoids, steroids occur widely in body tissues and have a large variety of physiological activities. Steroids are closely related to terpenoids and arise biosynthetically from the triterpene lanosterol. Lanosterol, in turn, arises from cationic cyclization of the acyclic hydrocarbon squalene. 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

Scheme 1, Enzyme-induced cyclizations of squalene oxide (2) (a) and the Stork-Eschenmoser hypothesis (b).

Squalene epoxidase, a key enzyme in the biosynthesis of cholesterol (9), epoxidizes one face of one of the three different olefins in squalene (7) to give squalene epoxide (8), which then cyclizes eventually to give cholesterol (9) (Scheme 1). The AD of squalene (7)... 

Polyene cyclizations are of substantial value in the synthesis of polycyclic terpene natural products. These syntheses resemble the processes by which the polycyclic compounds are assembled in nature. The most dramatic example of biosynthesis of a polycyclic skeleton from a polyene intermediate is the conversion of squalene oxide to the steroid lanosterol. In the biological reaction, an enzyme not only to induces the cationic cyclization but also holds the substrate in a conformation corresponding to stereochemistry of the polycyclic product.17 In this case, the cyclization is terminated by a series of rearrangements. 

Synthesis of endogenic cholesterol is also controlled by exogenous cholesterol supplied in food the more dietary cholesterol is digested, the less endogenic cho-lesterol is produced in the liveV. Exogenous cholesterol inhibits the activity of hydroxymethylglutaryl-CoA reductase and the cyclization of squalene to lanosterol. 

As described earlier, the dichotomy at the point of squalene (26) cyclization has been used as a distinguishing feature of photosynthetic and non-photosynthetic organisms [1], Cycloartenol (32) is the primary cyclization product in the former group, while in the latter, lanosterol (33) is initially produced (Scheme 2). Recent investigations into the cyclization of squalene in marine organisms have provided interesting results. 

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