Flavanones structures

FIGURE 16.1 Guide to the structures, ring labeling, and atom numbering of chalcones, dihydrochalcones, aurones, and auronols. The corresponding flavanone structure is shown for reference. 

The flavanone structure is highly reactive and has been reported to undergo hydroxylation, glycosylation, and O-methylation reactions. Flavanones are... 

This cyclization is followed by a conjugate addition of an ortho phenolic OH group on to the enone system. The product is a flavanone structure, which is always drawn a different way up to the molecules we have just been discussing. Redrawing the last product shows the cyclization. 

Within the plant secondary metabolites of flavonoids, flavanones define one of the minor subclasses. They may be called dihydroflavones. The basic chemical structure of flavanones involves two benzene rings (A and B), which are linked by a heterocyclic ring (C). The most characteristic point of flavanone structures is that the C-ring is saturated. Flavanones have an asymmetric carbon at C2-position. 

In addition to the bitter acids and essential oils, the flowers of hops offer a rich array of polyphenolic compounds, primarily chalcones and their accompanying flavanones, many of which are prenylated derivatives (Stevens et al., 1997,1999a, b). The most prominent flavonoid in all plants studied was xanthohumol [342] (3 -prenyl-6 -0-methylchalconaringenin chalconaringenin is 2, 4, 6, 4-tetrahydroxychalcone) (see Fig. 4.11 for structures 342-346). Several additional chalcones—variously adorned with 0-methyl and/or C-prenyl functions—were also encountered, along with their respective flavanones. Three new compounds were described in the Stevens et al. 

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. 

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). 

Flavanonols Flavanonols can be considered as flavanones with a hydroxyl group on position 3. They are sometimes referred to as dihydroflavonols. Similar to the situation of flavanones, flavanonols are no longer a minor subgroup of flavonoids, and they are a structurally highly diverse and multisubstituted subgroup (Grayer and Veitch 2006). A well-known flavanonol is taxifolin from citrus fruits (Kawaii and others 1999) (see Table 5.1 and Fig. 5.2). 

Flavonoids are a complex group of polyphenolic compounds with a basic C6-C3-C6 structure that can be divided in different groups flavonols, flavones, flavanols (or flavan-3-ols), flavanones, anthocyanidins, and isoflavones. More than 6,000 flavonoids are known the most widespread are flavonols, such as quercetin flavones, such as lu-teolin and flavanols (flavan-3-ols), such as catechin. Anthocyanidins are also bioactive flavonoids they are water-soluble vegetable pigments found especially in berries and other red-blue fruits and vegetables. 

Di Majo D, Giammanco M, La Guardia M, Tripoli E, Giammanco S and Finotti E. 2005. Flavanones in citrus fruit structure-antioxidant activity relationships. Food Res Int 38(10) 1161-1166. 

HRAZDINA, G., KREUZALER, F., HAHLBROCK, K GRISEBACH, H., Substrate specificity of flavanone synthase from cell suspension cultures of parsley and structure of release products in vitro, Arch. Biochem. Biophys., 1976,175, 392-399. 

Shimokoriyama M (1957) Interconversion of chalcones and flavanones of phloroglucinol-type structure. J Am Chem Soc 79(15) 4199-4202... 

Tanner DC (2000) Structural elucidation of flavanone 7-O-glucosyltransferase in grapefruit (Citrusparadisi) seedlings. M.A. Thesis, East Tennessee State University... 

Fig. 13. Structure of methoxylated flavanone derivatives from Beihchmiedia zenkeri.

Reports on the antiviral activity of flavanones isolated from medicinal plants have also been found in the literature. Two new antiviral flavanone derivatives were isolated from a methanol extract of leaves of Pithecellobium clypearia (Jack.) Benth (Fabaceae), guided by antiviral assays.The structures were characterized as 7-0-galloyltricetifavan (Fig. 14) and 7,4 -di-0-galloyltricetifavan. CPE reduction assay showed that both compounds possess antiviral activity against respiratory syncytial virus, influenza A HlNl virus, CVB3 and HSV-1. Examples of other... 

This chapter, therefore, aims to present a brief unified summary of general techniques, with reference to the different categories of structure flavones and flavonols (and their glycosides), isoflavones, flavanones, chalcones, anthocyanins, and proanthocyanidins. 

---