Triterpenes acyclic

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 cationic domino cyclization of an acyclic unsaturated substrate to give the tetracyclic scaffold of triterpenes and steroids is intensively described in the literature.1121 The concept has recently been used by Corey et al. to prepare enanti-opure (+)-dammarenediol II 18 in an exceptional short way.1131 The synthesis demonstrates the power of the combination of cation-olefin polyannulation with the aldol cyclization for tetraannulation (scheme 4). Successive treatment of the acylsilane 13 with 2-propenyllithium 14 and the iodoalkane 15 efficiently yidds the epoxytriene 16. The Lewis acid... 

Steroids are important lipids whose structures are based on a tetracyclic system. Most steroids function as hormone chemical messengers, and thus these molecules have been discussed in detail in chapter 5. Structurally, steroids are heavily modified triterpenes that are biosynthesized starting from the acyclic hydrocarbon squalene and progressing through cholesterol to the final steroid product Bloch and Cornforth, who were awarded Nobel Prizes in medicine (1964), contributed greatly to the elucidation of this remarkable biosynthetic transformation. 

While squalene, the parent of all triterpenoids, is a linear acyclic compound, the majority of triterpeneoids exist in cyclic forms, penta- and tetracyclic triterpenes being the major types. Within these cyclic triterpenoids distinct structural variations lead to several structural classes of triterpenoids. Some of the major structures types of triterpenoids are shown helow. 

Steroids arc heavily modified triterpenes that are hiosynthesized in living organisms from the acyclic hydrocarbon squalene (Section 27.6). The exact pathw ay by which this remarkable transformation is accomplished is lengthy and complex, but the key steps have now been worked out, with notable contributions made by Konrad Bloch and John Cornforth, who received Nobel Prizes for their efforts. 

The triterpenes, e.g. tetrahymanol, are larger in cross-section than cholesterol. A better intermolecular fit is restored with acyclic lipids which have a larger cross-section than do the n-acyl chains found in eucaryotes. Increased effective cross-sections can be achieved by the introduction of cis double bonds this is partly how Tetrahymena pyriformis adjusts the composition of its phosphoUpids in its sterol-free, tetrahymanol-containing state. Another way to obtain thicker lipids is to have branched chains. Whereas only n-acyl lipids have been reported in Tetrahymena and... 

Squalene [(a//- )-2,6,10,15,19,23-hexamethyl-2.6, 10,14,18,22-tetracosahexaene]. The most important aliphatic, acyclic triterpene, C30H50, Mr 410.73, mp. -4.8 to -5.2 C, bp. 284-285°C, formula, see steroids. S. was first isolated from fish liver oils and later detected in plant oils and human fat. It is composed of 6 isoprene units and is formed from activated acetate ( acetyl-CoA) via mevalonic acid. It is an intermediate in the biosynthesis of all cyclic triterpenoids and thus also of the steroids. Its enzymatic cyclization to IanosteroI or cycloartenoI requires molecular oxygen and proceeds through (35)-squalene 2,3-epoxide. lit. Annu. Rev. Biochem. 14,555-585 (1982)"Chem. Soc. Rev. 20,129-147 (1991) - Kaiier, No. 34 Nat. Prod. Rep. 2, 525 - 5W (1985) Phytochemisby 27,628 (1988) (biosy nthe-sis) Stryer 1995,692-695.-/HS290/29 CASHI-02-4]... 

The structures of terpenes or terpenoids varies widely and are classified according to various aspects, e.g. number of isoprene units (Cio monoterpenes, C15 sesquiterpenes, C20 diterpenes, C25 sesterterpenes, C30 triterpenes, C40 tetraterpenes) or division in acyclic, mono-, hi-, tri-, tetra-, pentacyclic terpenes. Often terpenes are named by their trivial names, e.g. d-limonene. 

The msyority of natural triterpenes are pentacyclic compounds. The next largest group are the tetracyclic triterpenes. There is also a smaller number of triterpenes with various other cyclic structures. The only important acyclic triterpene is squalene (and its 2,3-oxide as a metabolic intermediate). Most triterpenes are alcohols (3-OH). They are found free and as glycosides (saponins) (Karrer, 1958 Robinson, 1975) or esters. Free triterpenes are often components of resins, latex, or cuticle. Saponins are powerful surface-active agents and can cause lysis of red blood cells. Some of them have been... 

Steroids are tetracyclic lipids derived from the acyclic triterpene squalene. 

Steroids constitute a major class of lipids. They are related to terpenes in the sense that they are biosynthesized by a similar route. Through a truly remarkable reaction sequence, the acyclic triterpene squalene is converted stereospecifically to the tetracyclic steroid lanosterol, from which other steroids are subsequently synthesized... 

Terpene synthesis in nature is a complex process involving successive electrophilic additions followed by a variety of skeletal rearrangements, including those of the Wagner-Meerwein variety. These reactions are typically catalyzed by enzymes and are responsible for the wide array of structural diversity in these compounds, including 6-6-6-5 tetracycles, 6-6-6-6-5 pentacycles, 6-6-6-6-6 pentacycles, and the less abundant acyclic, monocyclic, bicyclic, tricyclic, and hexacyclic triterpenoids. Each of the more than 100 triterpene skeletons identified in nature are formed through the involvement of several multifunctional triterpene synthases. 

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