Flavanon-3-ols

Flavonoids can be classified according to their biosynthetic origins. Some flavonoids are both intermediates in biosynthesis and end-products, e.g. chalcones, flavanones, flavanon-3-ols and flavan-3,4-diols. Other classes are only known as the end-products of biosynthesis, e.g. anthocyanins, flavones and flavonols. Two further classes of flavonoids are those in which the 2-phenyl side-chain of flavonoid isomerizes to the 3-position (giving rise to isoflavones and related isoflavonoids) and then to the 4-position (giving rise to the neoflavonoids). The major classes of flavonoids, with specific examples, are summarized helow. 

Food and plant phenolics are commonly detected using DAD detectors (Tan and others 2008). Photodiode array detection allows collection of the entire UV spectrum during the elution of a chromatographic peak, which makes it possible to identify a phenolic compound by its spectra. Simple phenols, phenolic acids, flavanones, benzophenones, isoflavones, and flavan-3-ols have maximum absorbance at 280 nm, hydroxycinnamic acids at 320 nm, flavonols, flavones, and dihydroflavonols at 365 nm, and anthocyanins at 520 nm (Ibern-G6mez and others 2002 Merken Hand Beecher 2000). Hydrolyzable tannins show a characteristic shoulder at 300 nm, suitable for identifying them (Arapitsas and others 2007). For stilbenes, maximum absorbance of trans-forms are at 306 nm and at 285 nm for cA-forms (Lamuela-Raventos and others 1995). 

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. 

The probutinidins (see Section 11.2) represent a second class of proanthocyanidins with flavan chain-extension units. Only five members of this class of compounds have been identified (Table 11.14). Their structures and absolute configurations were also confirmed by synthesis via reduction of the flavanone, butin, followed by acid-catalyzed condensation with the appropriate flavan-3-ol. A notable feature of the synthetic studies was the apparent preference for (4 8) bond formation reported by both groups of authors. 

In the study on laryngeal cancer (Table 20.3), significant inverse relations were found for flavan-3-ols (OR,0.64), flavanones (OR,0.60), flavonols (OR,0.32), and total flavonoids (OR,0.60), although the overall trends in risk were significant only for flavanones and flavonols [Garavello et al., 2007], The estimates persisted after controlling for vegetable, fruit, and vitamin C intake. 

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. 

Dihydroflavonols. Widely distributed 2,3-dihydroflavon-3-ols include the antioxidant 2,3-dihydroflavonols aromadendrin (3,5,7,4 -tetrahydroxyflavanone 2,3-dihydrokaempferol), ampelopsin (3,5,7,3, 4, 5 -hexahydroxyflavanone 2,3-dihydromyricetin), fustin (3,7,3, 4 -tetrahydroxyflavanone 2,3-dihydrofisetin) and taxifolin (3,5,7,3, 4 -pentahydroxy-flavanone 2,3-dihydroquercetin). Some flavanols are sweet-tasting, notably 6-methoxy-aromadendrin 3-O-aceate, 6-methoxytaxifolin and taxifolin 3-0-acetate. Taxifolin and fustin inhibit NADH and succinate dehydrogenases and taxifolin inhibits 5-LOX. 

Flavonoids are a specific class of polyphenols. It is generally believed that flavonoids include a wide variety of phenolic compounds, such as flavones, flavonols, flavanones, flavanonols, anthocyanidins, flavan-3,4-diols, xanthones, flavan-3-ols, isoflavones, isoflavanones, chal-cones, dihydrochalcones, aurones, and homoisoflavones. Their separation poses special problems because there is often irreversible adsorption and even hydrolysis on solid supports. 

The word flavonoid comes from Latin flavus, which means yellow, and included at the beginning only a yellow-colored group of compounds with a flavone nucleus (Jovanovic et al 1998). Today, the term is used in a broader context and includes colorless (flavan-3-ol) to less colored (flavanone) compounds, as well as the red and blue anthocyanins, all commonly occurring in plants. For a period, flavonoids were referred to as vitamin P because of their ability to decrease capillary fragility and permeability in humans. The term vitamin C2 has also been used, as it was known... 

Flavonoids are one of the largest classes of phenylpropanoid-derived plant specialized metabolites, with 10,000 different members. They consist of two main groups, the 2-phenylchromans (flavonoids flavanones, flavones, flavonols, flavan-3-ols, anthocy-anidins) and the 3-phenylchromans (isoflavonoids isoflavones, iso-flavans, pterocarpans). Some flavonoids and their metabolites exhibit positive effects for disease therapy and chemoprevention [218,219],... 

A promelacacidin containing a unique (3 —> 4)-linkage between flavanone and flavan-3-ol moieties, that is, 7,8,3, 4 -tetrahydroxyflavanone-(3 —>4/3)-epimesquitol (209). The authors erroneously named the flavan-3-ol unit as an c r-epimesquitol moiety. 

---