Triterpenoids tetracyclic

Dammar resin was introduced into Europe in the nineteenth century, mainly as a paint varnish It is still used today since it has good optical properties and low acidity. It is derived from various species (the genus Hopea and Shorea of the Dipterocarpaceae family). It is characterised by tetracyclic triterpenoids of the dammarane series and contains minor amounts of pentacyclic triterpenoids of the series of olanane, ursane and hopane. It also contains a polymeric fraction named polycadinene or [S-resene. Dammar resin triterpenoids undergo oxidation with ageing, as described for the components of mastic resin. 

The tetracyclic alcohol 179 is produced by the action of boron trifluoride etherate or tin(IV) chloride on the oxirane 178 (equation 85)95. A similar cyclization of the oxirane 180 yields DL-<5-amyrin (181) (equation 86)96. In the SnCLt-catalysed ring-closure of the tetraene 182 to the all-fraws-tetracycle 183 (equation 87) seven asymmetric centres are created, yet only two of sixty-four possible racemates are formed97. It has been proposed that multiple ring-closures of this kind form the basis of the biosynthesis of steroids and tetra-and pentacyclic triterpenoids, the Stork-Eschenmoser hypothesis 98,99. Such biomimetic polyene cyclizations, e.g. the formation of lanosterol from squalene (equation 88), have been reviewed69,70. 

Triterpenoids (C30 compounds) are the most ubiquitous of the terpenoids and are found in both terrestrial and marine flora and fauna (Mahato et al., 1992). Diterpenoids and triterpenoids rarely occur together in the same tissue. In higher plants, triterpenoid resins are found in numerous genera of broad-leaved trees, predominantly but not exclusively tropical (Mills and White, 1994 105). They show considerable diversity in the carbon skeleton (both tetracyclic and pentacyclic structures are found) which occur in nature either in the free state or as glycosides, although many have either a keto or a hydroxyl group at C-3, with possible further functional groups and/or double bonds in the side-chains. 

Triterpenoids are a major group of natural products in higher plants. The tetracyclic triterpenoids, based on the lanostane, euphane, onocer-ane, and dammarane skeletons, are found mostly in vascular plants and... 

In an approach to the stereocontrolled creation of the acyclic side-chain of tetracyclic triterpenoids and other natural products, Trost and his colleagues have converted the acyclic starting compound (29) into the cyclopropanoid intermediate (31) via the diazo-ketone (30). The key step in the scheme is the cleavage of the cyclopropane with lithium dimethylcuprate to give (32). The stereochemistry at C-7 is determined by the configuration of the double bond in (29). The c.d. and u.v. spectra of a series of triterpenoid olefins have been measured. The Scott-Wrixon rules can be used to correlate the sign of the c.d. curves with molecular structure. A... 

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 steroids are modified triterpenoids containing the tetracyclic ring system of lanos-terol (Figure 5.55), but lacking the three methyl groups at C-4 and C-14. Cholesterol (Figure 5.76) typifies the fundamental structure, but further... 

In recent years, Ireland and his colleagues have made a major contribution to the total synthesis of tetra- and penta-cyclic triterpenoids. The details of some of this work have been published in a series of papers.55-58 The tetracyclic ketone (75) was conceived as a key intermediate in the synthesis of shionone (76). Three routes to (75) were undertaken.55 The most efficient involved a triethylaluminium-catalysed conjugate addition of cyanide to the enone (77) (see Vol. 5, p. 135). The second... 

Reviews have appeared on the palaeochemistry and biochemistry of hopanoids, the distribution of tetracyclic and pentacyclic triterpenoids, friedelin and associated triterpenoids, triterpenoid alcohols of seed-bearing plants (Sperma-tophyta), and the occurrence, biosynthesis, and metabolism of epoxides. ... 

Fasciculol G (13) is a further member of the series of plant growth inhibitors from Neamatoloma fasciculare (see Vol. 9, p. 188). Structure (14) has been proposed for fasciculatol from Veronia fasciculata Extraction of a fossil plant from the Ningyo-Toge uranium deposit afforded the nortriterpenoid ketones (15) and (16). The structure (17) of a new tetracyclic triterpenoid from Pertya robusta was confirmed by partial synthesis of its dihydro-derivative from 24-methyl-enecycloartenol. "... 

A new method for the degradation of the typical iso-octenyl side-chain of tetracyclic triterpenoids has been reported. The key step involves N-lithioethylene-diamine-induced double-bond isomerizations of the olefin (50), obtained by lead tetra-acetate decarboxylation of the trisnoracid (51), to a mixture of (52) and (53). The details of the n.m.r. spectra of 45 cycloartane derivatives have been described and assignments made for the methyl group signals. "... 

The saponin structure is either of the steroidal (commonly tetracyclic triterpenoids) or pentacyclic triterpenoid type. Triterpenoid saponins are found, for example in Quillaia bark and in liquorice root. Quillaia B.P. is defined as the dried inner part of the bark of Quillaja saponaria and other species of Quillaja and is used as an emulsifying agent. Liquorice, the root of which also contains tiiterpenoid saponins, has long been used in pharmacy as a flavoring agent, demulcent, and mild expectorant. 

Cycloastragenol 13, presents a tetrahydrofuranic ring at C-17, and this presence shifted to considerably weaker field the resonance of the 16a-H (6 4.4-5.0) respect to the chemical shift of the same proton, when a acyclic side chain is present (5 4.58-4.6) [219]. All the tetracyclic triterpenoids with a tetrahydrofuranic ring at C-17 show a base peak at m/z 143 in the mass spectrum (El). This peak results from the cleavage between C-17 and C-20 and suggests the presence of the partial structure A (a 25-hydroxy-20,24-epoxy). Fig.(l). 

Although cycloartenol (44) belongs to the tetracyclic triterpenoids as the parent compound, it is implicated as an important biosynthetic intermediate both in that class and in plant steroids and is thus a relevant introduction to the latter group which have been so significant in semi-synthetic applications. Following the enzyme-catalysed cyclisation of squalene epoxide, it is the first intermediary compound encountered and in the plant series it has a comparable position to lanosterol (45) in the animal... 

An improved route to 3a-acetoxy-4a,8 ,14j8-trimethyl-18-nor-androstan-17-one from fusidic acid has been reported. The details of the synthesis of the tetracyclic triterpenoid synthon (15) and the spectroscopic differentiation of the cis- and tran -isomers of (15) have appeared. 

Biogenetically quassinoids can be regarded as degraded triterpenoids and are almost certainly derived from tetracyclic triterpenes. The conversion of the tetracyclic triterpene into quassinoid has been experimentally verified by using labeled mevalonate precursors [3-5] and follows the route originally proposed by Arigoni et ah [6] but extended by further degradation to quassinoids as shown in Scheme 1. 

The tetracyclic triterpenoid quassin is an antifeedant to the aphid Myzus persicae [15], and the limonoids azadirachtins from Azadirachta indica are also potent insect antifeedants [13]. 

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