Linear Triterpenes

About 5 000 naturally abundant triterpenes are documented in the literature Most of these are derived from squalane and squalene, with two famesane units linked in the tail-to-tail manner. 

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Protonated 2,3-epoxysqualene is the biosynthetic precursor of polycyclic triterpenes such as protostanes and dammaranes. The geneses of the cations of proto-stane and dammarane start from two different conformers of the carbenium ion, which arise intermediately from opening of the protonated 2,3-epoxysqualene. 

Squalene occurs in cod-liver oil and in several vegetable oils, for example those from rape and cotton seed. Red algae Laurencia okamurai produce (+)-l,10-epoxy-squalene. Botryococcane is the parent hydrocarbon of some branched and alkylated triterpenoid polyenes found in the algae Botryococcus braunii and referred to as C3o-C37-botryococcenes, depending on the number of their carbon atoms. 

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Quassinoids, and Linear Triterpene from Eurycoma longifolia... 

Cytotoxic Quassinoids, Linear Triterpenes and Canthin Alkaloids from Eurycoma longifolia [38,39]... 

All edible oils contain hydrocarbons with an even or an odd C-number (Cn to C35). Olive, rice and fish oils are particularly rich in this class of compounds. The main hydrocarbon constituent of olive oil (1-7 g/kg) and rice oil (3.3 g/kg) is a linear triterpene (C30), squalene ... 

In addition to the above mentioned hydrocarbons, oils also contain alkenes and monoterpenic and sesquiterpenic hydrocarbons. For example, the amount of alkenes in olive oils range from 0.5 to 2 mg/kg and includes a series of alk-9-enes from C22 to C27, heptadec-8-ene and 6,10-dimethyl-undec-l-ene. An important compound is the linear triterpenic (C39) hydrocarbon squalene (3-81), aIl-tra s-2,6,10,15,19,23-hexamethyltetracosa-2, 6,10,14,18,22-hexaene, which is the universal precursor of all triterpenoids and steroids. It was given this name because it was discovered in the Hver of sharks (Squalidae). Shark Hver oil contains about 30% squalene, about 7% pristane (2,6,10,14-tetramethylpentadecane) and smaller amounts of phytane (2,6,10,14-tetramethylhexadecane). Squalene occurs in small amounts in edible oils and especially in oHve oil, where its content is in the tenths of a percent (1-7 g/kg). 

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. 

The importance of a linear tetrasaccharide fragment and an 18(20)-lactone for biological activity, the absence of a significant contribution of 4-0-sulfate at first xylose to activities of tetraosides and, in contrast, its important role for activity of biosides were shown for hemolytic action of triterpene glycosides from sea cucumbers [33]. It confirmed a common mechanism of their activity against membranes of both erythrocytes and fungal cells. 

Most of sea cucumber triterpene glycosides are tetra- or pentaglycosides. The majority of tetrasaccharides show a linear chain with the most common 3-0-Me-Glc-(l—>3)-Glc-(1 4)-Qui-(l—>2)-Xyl structure. The few disaccharides that have been isolated show a Qui-(1 2)-4-0S03Na-Xyl chain attached to C-3 of the triterpenoid aglycone. Some hexasaccharides have been isolated from sea cucumbers of the order Aspidochirota Stichopus japonica, Stichopus chloronotus, Parastichopus californius and Bohadschia bivittata [55]. They are non-sulfated glycosides with a linear 3-0-Me-Glc-(1 3)-Glc-(l- 4)-Xyl chain and a branching of a linear trisaccharide at C-2 of the xylose unit. 

Derbesy and Richert reported that HPLC of the unsaponifiables (sterol and triterpene alcohol fraction) revealed less than 1% of shea butter in cocoa butter. With measurement at 280nm, one specific peak area was linearly related to the percentage of shea butter. Homberg and Bielefeld suggested that sterols, methyl sterols, and triterpene alcohols be determined to identify cocoabutter adulteration. 

In contrast to the previously discussed mono-, sesqui-, and diterpenes that are biosynthesized by linear condensation of five carbon units, triterpenes arise via dimerization of two famesyl pyrophosphate units to produce an intermediate compound, squalene (1) (Fig. 23.2). Squalene (usually as the 2,3-epoxide) may then be cyclized by several mechanisms from different conformations on an enzyme surface to produce the parent skeletal types of many different kinds of triterpenes that undergo subsequent modification (Banth-orpe and Chari wood, 1980 Harrison, 1985). The structures of many triterpenes and steroids may be explained by the biogenetic isoprene rule see Chapter 18) (Connolly and Hill, 1991). 

Cyclization of the linear diterpene precursor usually proceeds along two alternative parallel routes (Figure 2 [p. 392]), one leading to the bicyclic normal labdane (normal-labdane) absolute stereochemistry and the other resulting in its antipodal, enantiomeric (e/iMabdane) absolute stereochemistry. As a consequence of these two parallel biosynthetic routes, diterpenes lack the homogeneous absolute stereochemistry of the triterpenes. 

Figure 9.8 Examples of sesterterpenes, triterpenes and polyunsaturated linear alkenes isolated from Rhizosolenia setigera (Sinninghe Damste...

Almost all publications concerning the terpenes of brown algae refer to the above-mentioned orders Dictyo-tales and Fucales, in which numerous diterpenes and some sesquiterpenes, a single linear tri-nor-triterpene and a linear tetraterpene have been identified, but no monoterpene. Because, with very few exceptions, the terpenes of brown algae do not contain any halogens, it is necessary to distinguish clearly between the terpenes of Phaeophyceae and those of Chlorophyceae and Rhodo-phyceae. However, similar to what has been observed in... 

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