Lignan chemical structure

Fig. 2.86. Chemical structures of the iosflavones daidzein, genistein, O-desmethylangolensin, equol, and the lignans enterolactone and enterodiol. Reprinted with permission from Z. Kuklenyik el al. [209].

The biosynthesis of extractives is controlled genetically and hence each wood species tends to produce specific substances. As a result of secondary changes, heartwood contains a large variety of phenolic substances. From the chemotaxonomical point of view, chemical structures of various flavonoids, lignans, stilbenes, and tropolones are of interest. For example, species within genera such as Pinus, Acacia, and Eucalyptus can be classified according to their characteristic composition of phenolic substances. 

Figure 11.1 Chemical structures of two major lignans present in sesame seed and oil.

The principal skeletons of lignans are listed in Table 1 and the chemical structures of the most relevant pharmacologically active lignans are listed in Tables 2-10. 

The mechanism of actions involves tubulin binding, reverse transcriptase inhibition, integrase inhibition and topoisomerase inhibition. Podophyllo-toxins bind to tubulin and are able to disrupt the cellular cytoskeleton and interfere with some vital virus processes. There is no relationship between the inhibition of reverse transcriptase (RT) and chemical structure in the case of lignans, because all the chemical antiviral structures are able to bind to the enzyme. As to the rest of the mechanism, there is not much information available. The effects of rabdosiin may be due to its topoisomerase inhibitory effects. Charlton concluded that the antiviral activity of lignans is not strong and that except for podophyllotoxin, which is used topically to treat various warts caused by HPV, none of them are of interest for commercial application. 

Lignans are widespread secondary metabolites in plant kingdom. They occur in many parts of plants especially wood, resin and bark trees [11]. They are also found in many roots, leaves, flowers, fruits and seeds [61]. There is an evidence that lignans play a major role in plant-plant, plant-insect and plant-fungus interactions [11]. The chemical structures of... 

Figure 2 Chemical structures of selected plant polyphenols. Structures include a flavonol (quercetin), isoflavone (daidzein), cinnamic acid (chlorogenic acid), flavan-3-ol (catechin), a lignan microbial metabolite (enterodiol), and a stilbene (resveratrol).

The stereochemistry at the chiral centres of the three new lignans, erythro-strebluslignanol, threo-T-methoxyl strebluslignanol and erythro-T-meth-oxyl strebluslignanol has been established by Li et al by analysis of vicinal proton-proton couplings as well as CD spectra and optical rotation data. The determined chemical structures of these compounds were (7 R,8 5)-5-allyl-5 -(l,2-dihydroxypropyl)[l,l -biphenyl]-2,2 -diol, (7 R,8 R)-5-allyl-5 -(2-hydroxy-l-methoxypropyl)[l,l-biphenyl]-2,2 -diol and (7 R,8 5)-5-allyl-5 -(2-hydroxy-l-methoxypropyl)[l,l-biphenyl]-2,2 -diol. 

Moreover, according to chemical structure, polyphenols can be divided in classes as simple phenols, benzoquinones, phenolic acids, acetophenones, phenylacetic acids, hrodroxycinnamic acids, phenylpropenes, coumarins, isocoumarins, chromones, naphthoquinones, xanthones, stilbenes, anthraquinones, lignans, neolignans, lignins, and flavonoids. The latter allow to consider 13 subclasses with more than 5,000 compounds (Fig. 74.2). 

The chemical structures of main lignans separated from Cornus spp. fruits are presented in Figure 5. 

Fig. 18.3. Chemical structures of polyphenols Hydroxybenzoic acids (Hba), Hydroxy cinammic acids (Hca), Flavonoids (F), Chalcones (C), Stilbenes (S, cf. 20.2.6.6), Lignans (L). R H, OH or OCH3...

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

The first chemical constituent was isolated from podophyllin in 1880 and named podophyllotoxin (97). A structure was proposed in 1932 and after some fine-tuning (98) was shown to be the lignan (60). As might be expected, the crude resin contains a variety of chemical types, including the flavonols quercetin and kaempferol (99). Although these other constituents undoubtedly have biological activity, it is the lignans that have received most attention and to which we shall devote the remainder of this section. 

The structure (121) of thomasic acid was established by extensive spectroscopic and chemical degradation studies (116,119,120) and presents several interesting features. It is one of the few lignans occurring naturally in racemic form and in possessing a free carboxyl group. In addition, the trans vicinal substituents (at C-1,2) adopt a diaxial conformation. The synthesis of this product (Scheme 25) was... 

Wallis, A. R A. (1998) Structural diversity in lignans and neolignans. In Lewis, N. G., and Sarkanen, S. (eds). Lignin and Lignan Biosynthesis, American Chemical Society, Washington, DC. 

The Leech Book of Bald (ca. AD 950), a herbalist who lived in the time of Alfred the Great, contains a wealth of plant lore that includes ointments to protect against the elfin race and nocturnal visitors , but also mentions an extract of the wild chervil (probably Myrrhis odorata) as a salve for the treatment of tumours. This plant produces a number of chemicals known as lig-nans, which are related in structure to podophyllotoxin, although much better sources of these cytotoxic lignans are the Himalayan plant Podophyllum... 

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