Flavonoid Glycosides

2.4 Flavonoid Glycosides

Flavonoid constitute one of the largest class of naturally occuring plant products mostly phenols either in the free state or as their respective glycosides. As the very name suggests they are usually yellow-coloured compounds (flavous is a latin word yellow colour). Interestingly, more than 2000 different chemical compounds have been isolated, identified and reported from plant sources. In fact, their chemical structures are solely based upon a C6—C3—C6 carbon skeleton having a pyran or chroman ring bearing a second benzene (aromatic) ring strategically positioned at C—2, C—3 or C—4 as shown below:

In nature they are invariably available as: flavones, flavanones, flavonols, isoflavones, and anthocyanidins*. In certain specific instances either the 6-membered heterocyclic ring (pyrones) is replaced by a 5 membered heterocyclic ring (aurones) or exists in an open-chain isomeric form (chalcones). Besides, the normally existing glycosylated derivatives found in nature, other types of derivatives, such as methylated, acetylated, prenylated, or sulphated ones also exist. Nevertheless, it has been established that a large variety of flavonoids exert a wide range of activities in nature, namely: antimicrobial agents, signaling molecules, or stress metabolites. The structures of a few typical flavonoids are represented here as follows:
* Anthocyanidins are the colored aglycones found as a large number of pigments from flowerd and fruits (Gr. Antho flower + Gr. Kyanos, blue). Investigations of these pigments were initiated by Willstatter in 1914 and later on extended by Karner R Robinson, GM Robinson and others.

The structure (a) is more stable, because it has a naphthalenoid system of linkages, whereas (b) contains a quinonoid system.
The structures of only the key portions of pyrones, aurones and chalones are as follows:

The flavonoid glycosides mostly occur as O-glycosides or C-glycosides contained in the cell sap of relatively younger tissues of higher plants belonging to several families, such as: Compositae, Leguminosae, Polygonaceae, Rutaceae and Umbelliferae . It has been observed that a host of natural plant products containing flavonoid glycosides exert a variety of therapeutic effects, namely: antiasthmatic, antispasmodic, diuretic, fungicidal and oestrogenic activities. A few typical flavonoid glycosides shall be discussed in the sections that follows, namely:
(a) Flavone Glycosides,
(b) Flavonol Glycosides,
(c) Flavanone Glycosides,
(d) Chalcone Glycosides,
(e) Isoflavonoid Glycosides, and
(f ) Anthocyanidin Glycosides.

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