Flavones structures

Some flavone structures have been revised during the reporting period. The structure of 5,8,2 -triOH-6,7-diOMe flavone (compound 131 in Table 12.1) had been ascribed to a product isolated from Scutellaria baicalensis After synthesis, it needs to be revised to 5,7,2 -triOH-6,8-diOMe flavone (compound 132, Table 12.1). " Pedunculin, earlier isolated from Tithonia species and claimed as 5,8-diOH-6,7,4 -triOMe-flavone (compound 141, Table 12.1), needs to be revised, after synthesis, to 5,7-diOH-6,8,4 -triOMe flavone = nevadensin (compound 142 in Table 12.1). " In the previous review, the compound 5,6,7,4 -tetraOH-3, 5 -diOMe had erroneously been cited as a component of Artemisia assoana. Data have now been included for the correct structure, 5,7,4 -triOH-6,3, 5 -triOMe flavone (compound 251 in Table 12.1). A further flavone reported from Ageratum conyzoides (compound 263 in... 

The genus Tephrosia (Fabaceae) was selected to demonstrate the biosynthetic capacity of flavone substitution. In particular, there is a strong tendency towards formation of furano-residues, linked through C-bonds on position 8 of the flavone nucleus (e.g., apollinine, hookerianin Figure 12.5). The basic flavone structure is mostly 5- and 7-O-methylated. These compounds have been exclusively reported to occur in roots, leaf and stem as well as... 

Flavones. Flavone structural variation derives from hydroxylation, O-methylation and O-glycosylation. In addition, there can be C6- and C8-linked C-glycosides, isoprenyl (isopen-tenyl, Cj) substituents and G G or C-O—C links to form biflavones. Methylation of the phenolic OHs decreases polarity to permit an external location such as in the waxy leaf or fruit surface. 

NMR spectroscopy is a powerful analytical method for the determination of flavone structures. It has, however, some limitations as the sensitivity is rather low, and compounds have to be isolated. The assignments of the different proton and carbon signals in H and NMR can be based on chemical shifts (5) and coupling constants (J), and correlations observed in homo- and hetero-nuclear 2D NMR. NMR spectra of flavones have been extensively published previously (Markham and Chari, 1982 Agrawal, 1989 Markham and Geiger, 1993 Fossen and Andersen, 2006). 

Chari, V.M., S. Ahmad, and B.-G. Osterdahl C NMR Spectra of Chromeno-and Prenylated Flavones. Structure Revision of Mulberrin, Mulberrochromene, Cyc-lomulberrin and Cyclomulberrochromene. Z. Naturforsch. 33b, 1547 (1978). 

Flavonoids may be synthesized by using reactions similar to those used in the chalcone synthesis. For example, the basic flavone structure (II) can be simply derived firom a Claisen condensation between ethyl benzoate and 2-methoxyacetophenone, followed by treatment with HI ... 

Ceroplastol synthesis, 1, 428 Cetyl alcohol synthesis, 1, 478 Chaetoglobasins structures, 4, 376 Chalcone, o -azido-2 -oxy-synthesis, 3, 823 Chalcone, 2-hydroxy-reduction, 3, 751 Chalcone, 2 -hydroxy-mass spectra, 3, 618 Chalcone dibromides flavone synthesis from, 3, 823 Chalcones polymers, 1, 304 Chanoclavine synthesis, 6, 423 Charge density waves in stacks of ions, 1, 351-352 Chartreusin... 

The K-R reaction has also been useful for structural confirmation of natural products such as tambulin (71), a flavonoid isolated from the seeds of Xanthoxylum acanthopodium In the critical reaction (O-ethoxyphloroacetophenone (72) was allowed to react with anisic anhydride (38b) in the presence of sodium anisate (73) at 170 C to deliver flavone 74 in 65% yield. Flavone 74 was then converted after multiple steps to diethyl ether 75 which corresponded to the diethyl ether of tambulin (71). 

Many higher plants synthesize flavanes, flavanones, flavones, and isoflavones with a wide range of structural complexity. They make a significant contribution to the food intake of both herbivores and humans, and they have aroused particular interest on account of their degradation by mammals that are mediated by intestinal bacteria. Most of them exist naturally as glycosides and these are readily hydrolyzed to the aglycones. 

Havonoids are made up of a number of classes of very similar groups in which two phenyl rings are connected by a three-carbon unit [10], The open structure members are yellow in color and termed as chalcones, and simple cyclization to a furanoid structure deepens the color to the orange aurones. The most usual flavonoids are, however, the pale-yellow flavones and flavonols, or 3-hydroxyflavones, which will be treated here, and the red-blue anthocyanins, which will be treated in the next section. Figure 13.4 shows examples of these main classes and their structural relationships. The natural compounds of all classes often occur as glycosides and as methyl ethers. 

FIGURE 13.4 Typical structures for main classes of flavonoids naringin chalcone, 4,6,4 -trihydroxyaurone, apigenin (flavone), and pelargonidin (anthocyanidin). 

The basic structures of flavanones, flavones, and isoflavones together with coumestrol, an intermediate in the phenylpropane metabolism, are given in Fig. 2. The 3,5,7,3 -tetrahydroxy-4 -methoxyflavanone is a nod gene inducer in Rhizo-bium leguminosarum bv. viciae the 3, 4, 5,7-tetrahydroxyflavone, in Rhizobium ineliloti and 4,7-dihydroxyisoflavone, in Bradyrhizobium japonicum. Coumestrol, an intermediate in phenylpropane metabolism, is only a weak inducer (7). 

Figure 2 Structures of flavonoids present in root exudates of host plants and inducing nod gene expression in rhizobia (1) as 3,5,7,3 -tetrahydroxy-4 -methoxyflavanone, inducer in Rhizohium legiiminosarum bv. viciae (2) as 3, 4, 5, 7-tetrahydroxy-flavone, inducer in Rhizohium melilotr, (3) as 4, 7-dihydroxyisoflavone, inducer in Bradyrhizohium japonicum (4) as couinestrol, intermediate in phenylpropane metabolism, weak inducer. (From Ref. 64.)...

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