Triterpenoids, synthesis

Now that a number of cloned CS, LuS and /lAS genes are available it will be possible to generate transgenic plants in which the levels of these enzymes have been manipulated. The effects of altering the levels of these different OSCs on sterol and triterpenoid synthesis can then be assessed. 

The compounds (160),114 (161),115 and (162)116 have been prepared as possible intermediates for pentacyclic triterpenoid synthesis. Treatment of oleana-12,15-diene-3p,28-diol 3-acetate with toluene-p-sulphonyl chloride in pyridine affords the D(16a)-homo compound (163). The mechanisms of this and related reactions are discussed.117 2a,3[3-Diacetoxy-28-noroleana-12,17-diene has been synthesized.118... 

Synthesis of acetylenes (2, 240). Ireland et al. applied the lithio-l-trimethyl-silylpropyne procedure of Corey and Kirst2 to a synthesis of the enediyne (3), desired as an intermediate in a triterpenoid synthesis. But application of this coupling procedure to the /ram -dibromide (1) afforded less than 5% of the desired disilyl derivative (2). They then explored the coupling of (1) with propargyl... 

The undoubted highlight of the year in triterpenoid synthesis is the paper by Kametani and his colleagues which describes the full details (see Vol. 8, p. 174) of a stereoselective synthesis of the pentacyclic intermediates (132) and (133), used by Ireland in his syntheses of alnusenone and friedelin. This paper also includes a short non-stereoselective synthesis of (132) and (133) (see the Scheme). The key step is an intramolecular cycloaddition of the o-quinodi-methane intermediate (130). 

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. 

V. K. Rajput and B. Mukhopadhyay, Concise synthesis of a pentasaccharide related to the anti-leishmanial triterpenoid saponin isolated from Maesa balansae, J. Org. Chem., 73 (2008) 6924—6927. 

A more detailed understanding of the biochemical pathways and enzymes involved in saponin biosynthesis will facilitate the development of plants with altered saponin content. In some cases, enhanced levels of saponins or the synthesis of novel saponins may be desirable (for example, for drug production 4 or improved disease resistance3,5,6), while for other plants, reduction in the content of undesirable saponins would be beneficial (for example, for legume saponins that are associated with antifeedant properties in animal feed7). This chapter is concerned with recent progress that has been made in the characterization of the enzymes and genes involved in the synthesis of these complex molecules and focuses on triterpenoid saponins. 

A related type of TLC limit test is carried out where the identities of impurities are not completely certain. This type of test is used, for instance, on compounds of natural origin or partly natural origin which may contain a range of compounds related in structure to the test substance which are eo-extracted with the raw starting material. For example, the range of synthetic steroids originate from triterpenoids extracted from plants, which are extensively modified by fermentation and chemical synthesis. 

The first committed step in the synthesis of triterpenoid saponins involves the cyclisation of 2,3-oxidosqualene to give one of a number of different potential products [9]. Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons, although lupanes are also common [9]. This cycHsation event forms a branchpoint with the sterol biosynthetic pathway, in which 2,3-oxidosqualene is cyclised to lanosterol (in animals and fungi) or to cycloartenol (in plants) (Fig. 2). Sterols are important membrane constituents and also serve as precursors for hormone biosynthesis. 

The synthesis of triterpenoid saponins from the skeletons shown in Fig. 2 involves a series of further modifications that may include a variety of different oxidation and substitution events [9]. Very little is known about the enzymes and genes involved in the elaboration of the triterpenoid skeleton, although genetic and biochemical analysis of saponin-deficient mutants of plants is likely to accelerate the dissection of these processes [16]. Progress has been made in the characterisation of saponin glucosyltransferases (primarily for steroidal and steroidal alkaloid saponins), and the first of these enzymes (StSGT from potato) has been cloned. Since glycosylation at the C-3 hydroxyl position confers am-... 

Terpenoids Sesquiterpenoids Diterpenoids Triterpenoids Carotenoids and Polyter-penoids Steroids Physical Methods Steroid Reactions and Partial Synthesis. 

The interest in sitosterol is related to its potential use in the synthesis of steroid hormones and related pharmaceutical products, such as cortisone derivatives. The bark of birches contains significant amounts of betulinol which is a triterpenoid alcohol. So far no commercial uses have been found for this bark constituent. 

The impressive work of van Tamelen s group in the area of biogenetically based synthesis of tetra- and penta-cyclic triterpenoids has been reviewed.1... 

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... 

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