Isoflavonoids structures

Almost half of the isoflavonoids identified from the Leguminosae, in which prenylated isoflavones constitute >50% of the total (pterocarpanoids, rotenoids, isoflavanones, isoflavans, coumestans, 3-arylcoumarins, and coumaronochromones), exhibit a high degree of structural diversity and complexity of prenylation.8,86"148 More than half of the isoflavones are monoprenylated, whereas 40% are diprenylated, and very few are triprenyl derivatives, with prenylation at positions 6>8>3 5 >6,>2 and O-methylation at 4 >7=6=2 =3 >5 =5>8. A number of novel isoflavonoid structures with rare skeletons, as well as complex isoflavone O- and C-glycosides have recently been reported.8,148... 

The full characterization of some flavonoid and isoflavonoid structures found in the Leguminosae requires determination of absolute configuration, principally by CD spectroscopy (Slade et al, 2005). This information cannot be obtained from the hyphenated analyses involving MS and NMR described in the preceding sections, yet it is crucial to... 

Since the capacity to rearrange a flavonoid to an isoflavonoid structure is limited to essentially one phylogenetically related group of higher plants it is considered to be an isolated characteristic. It... 

Shikimates, which include phenylalanine, tyrosine, tryptophan, and their derivatives, are represented by many aromatic natural products, including hydroquinones found inbrownalgae such as Sargassum (Segawaand Shirahama 1987). Flavonoids are a structural class of shikimates found in plants, including isoflavonoids or neo-flavonoids, as is the y-pyrone (coumarin) core structure (Knaggs 2003). 

There are many branches to the flavonoid biosynthetic pathways, with the best characterized being those leading to the colored anthocyanins and proanthocyanidins (PAs) and the generally colorless flavones, flavonols, and isoflavonoids. Genes or cDNAs have now been identified for all the core steps leading to anthocyanin, flavone, and flavonol formation, as well as many steps of the isoflavonoid branch, allowing extensive analysis of the encoded enzymes (Table 3.1). In addition, several DNA sequences are available for the modification enzymes that produce the variety of structures known within each class of compound. 

FIGURE 7.2 Chemical structures of the main soy (genistein and daidzein) and red clover (biochanin A and formononetin) isoflavonoids (aglycones and glucosides). 

FIGURE 8.6 Structure of the phytoalexin isoflavonoid pterocarpans, maackianin, and pisatin from garden pea, and the isoflavones daidzein and genistein from soybean. 

Ito, C. et al., Chemical constituents of Millettia taiwaniana structure elucidation of five new isoflavonoids and their cancer chemopreventive activity. Journal of Natural Products, 67, 1125, 2004. 

Kinoshita, T., Tamura, Y., and Mizutani, K., Isolation and synthesis of two new 3-arylcoumarin derivatives from the root of Glycyrrhiza glabra (licorice), and structure revision of an antioxidant isoflavonoid glabrene, Natural Product Letters, 9, 289, 1997. 

Tahara, S. et al.. Structure revision of piscidone, a major isoflavonoid in the root bark of Piscidia erythrina. Phytochemistry, 31, 679, 1992. 

Structural types, in this section we will mainly focus our discussion on phenyl propanoids, coumarins, flavonoid and isoflavonoids, lignans and tannins. 

The phenolics include anthocyanins, anthraquinones, benzofurans, chromones, chromenes, coumarins, flavonoids, isoflavonoids, lignans, phenolic acids, phenylpropanoids, quinones, stilbenes and xanthones. Some phenolics can be very complex in structure through additional substitution or polymerization of simpler entities. Thus xanthones can be prenylated and flavonoids, lignans and other phenolics can be glycosylated. Condensed tannins involve the polymerization of procyaninidin or prodelphinidin monomers and hydrolysable tannins involve gallic acid residues esterified with monosaccharides. As detailed in this review, representatives of some major classes of plant-derived phenolics are potent protein kinase inhibitors. 

The parent nucleus of the flavonoids is flavone ((58), 2-phenylchromone or 2-phenylbenzopyran-4-one). Flavone and isoflavone ((59), 3-phenylchromone, the parent nucleus of the isoflavonoids) are the simplest oxygen-containing naturally occurring compounds that possess the 2-phenylnaphthalene -type structure. The chalcones, represented by the nucleus (60), may be regarded as open-chain flavonoids and are usually hydroxylated. The interconversion of chalcone and flavonone catalyzed by chalcone isomerase is well known [326, 327, 331], Chalcones can be precursors of both the flavonoids and the isoflavonoids [326-332]. 

Tamura, S., Chang, C., Suzuki, A., Kymai, S. Chemical studies on clover sickness. I Isolation and structural elucidation of two new isoflavonoids in red clover. Agri Biol Chem 1969 33 391-397. 

The flavonoid group is very diverse and contains several compounds including flavanones, flavones, flavonols, dihydroflavonols, isoflavonoids, anthocyanins, flavan-3,4-diols, flavan-4-ols, and flavan-3-ols. Flavan-3-ols are the structural units of the polymeric compounds termed condensed tannins abundant in plants. 

It is concluded that the sensitivities of the MDR- and MRP-mediated drug resistance mechanisms by the inhibitory flavonoids and isoflavonoids might differ because of selective activity based on their chemical structures this is in good agreement with other studies. 

Genistein (GEN Fig. 1.11a) is an isoflavonoid natural product that is an ER- selective partial agonist. The structure of the complex between human ER-y5 and genistein has been reported (PDB entry IGKM) [11]. GEN binds across the ligand-binding cavity in a manner similar to estradiol with ER-a. The A-ring phenol of GEN inter-... 

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