Flavonoids basic structure

Flavonoids are secondary metabolites generally occurring in various plants as glycosides. The chemical structure of flavonoids shows high variety. The basic structure of flavons and flavonols is the 2-phenylbenzo-gamma-pyrone. Flavonoids generally contain two phenol rings linked with a linear three-carbon chain (chalcones) or with three carbon... 

Species of the genus Vellozia have been extensively studied for their flavonoid complement in relation to chemosystematics. In addition to a series of C-methylflavonols and two C-prenylated flavonols, derivatives of vellokaempferol and velloquercetin are accumulated in whole plants, leaves, and leaf exudates. The basic structure of these compounds is characterized by 7,6-isopropenylfurano substitution, based upon kaempferol, quercetin, and their 0-methyl ethers. In addition, 8-C-methyl derivatives of these compounds were also identified from leaves of V. stipitata " So far, species of this genus are the only reported sources of these compounds, which in parts have been proved to be accumulated externally. ° Structures are exemplified by Figure 12.10. 

Flavonoids constitute a large class of polyphenols found in fruits and vegetables that share a common skeleton of phenylchromane. This basic structure allows a large number of substitution patterns leading to several subclasses of flavonoids, such as flavonols, flavones, flavanones, flavanols, anthocyanidins, isoflavones, dihydroflavonols, and chalcones. Among the diverse flavonoid subclasses, flavonols (especially quercetin) and flavanols (catechins) are the most abundant in our food. Flavonols are present in foods as diverse glycosides, whereas flavanols are usually found as aglycones. 

Fig. 2 Chemical structures of flavonoids (basic types of flavonoids and some selected examples)...

Figure 15.1 Basic structures of the six main classes of flavonoids. Ring nomenclature for all classes is indicated for flavonone. Caibon numbering and C-ring bonds for all classes except chalcones is indicated for flavone and flavonol, respectively.

There are two major classes of natural products primary and secondary metabolites. Primary metabolites are compounds that exist in all organisms and are involved in basal and vital metabolism (e.g., glucose, fatty acids, amino acids, etc.). Secondary metabolites, alkaloids, terpenoids, and flavonoids, are unique to a particular species and vary in their basic structures. Secondary metabolites are usually accumulated, as most of their end metabolites are in plants, but are excreted in animals and microorganisms and some of these are proven to have pharmacological and ecological significance. 

The basic structural model of flavanones is the 2-phenylbenzopiran-4-one skeleton [6], The flavanones are compounds of great interest due to the fact that they are a compulsory step in the metabolic pathway of the other flavonoids. Their metabolic precursors are the chalcones, and the flavones, the dihydroflavonols, and the isoflavones are biosynthesised from the flavanones. 

Anthocyanins are water soluble pigments responsible for the red to purple colour in plants. The common basic structure of all these compounds is the cation flavylium, which was proposed for the first time by Wilstaer in 1913 and later confirmed by Robinson in 1922. A special kind of flavonoids related to anthocyanins are the proanthocyanidins, also referred to as leucoanthocyanidins, in the case of monomeric proanthocyanidins, and condensed proanthocyanidins for the polymers of flavan-3-ols [32],... 

Products of the phenylpropanoid pathway. These compounds, commonly referred to as polyphenols, are derived from cinnamic acid and include the xanthones and flavonoids. The basic structural unit of the flavonoid family (Fig. 1) comprises two benzene rings linked through a heterocyclic pyran or pyrone ring (C ring) variations in the C ring and the extent of hydroxylation define the major classes [4]. 

FIGURE 8.4 Basic structures and examples of the main subclasses of dietary flavonoids. 

Biaryl structures are wide-spread in many of naturally occuring products including alkaloids, lignans, terpenes, flavonoids, tannins, as well as polyketides, coumarins, peptides, glycopeptides, etc. For example, vancomycin (1) is a basic structure of several related glycopeptide antibiotics [1] balhimycin, actinoidin A, ristocetin A, teicoplanin A2-2, complestatin, etc which are important in medicinal chemistry or as a HPLC chiral stationary phases (vancomycin) [2]. 

Many products synthesized by Claisen-Schmidt reactions find applications in the pharmaceutical industry. In particular, chalcones and fiavanones are intermediates in the synthesis of flavonoids. The basic structure of fiavonoids, that is, 2 -hydroxychalcone can be obtained by condensation of substituted 2-hydroxyacetophenone and substituted benzaldehyde at 323 K using basic zeolites as catalysts (46) or Mg-Al mixed oxides (47). 

Figure 1. Overview of the basic structure of flavonoids (R3 and R5 are either H or OH).

Flavones are an important class of flavonoids, having the basic structure of 2-phenylchromen-4-one (2-phenyl-l-benzopyran-4-one). Naturally occurring flavones include apigenin (4, 5,7-trihydroxyflavone), luteolin (3, 4, 5,7-tetrahydroxyflavone), tangeritin (4, 5,6,7,8-penta-methoxyflavone), 6-hydroxyflavone, baicalein (5,6,7-trihydroxyfla-vone), chrysin (5,7-OH), wogonin (5,7-OH, 8-OCH3), and scutellarein (5,6,7,4 -tetrahydroxyflavone). Flavoxate and diosmin are synthetic flavones that are yellow in color. 

FIGURE 19.2. Molecular structures of flavonoids. The basic structure consists of the fused A and C ring, with the phenyl ring B attached to through C-10 to C-2ndofthe C ring (numbered from the pyran oxygen). 

Depending on the degree of oxidation and C-3 chain substitution the following basic structures of flavonoids can be identified ... 

The basic structure of these flavonoid compounds is shown in formulae 9-72. Their degree of oxidation increases along the row from left to right, along with the colour intensity. The compounds listed in the columns below have the same degree of oxidation. A special case are anthocyanidins that contain a system of conjugated double bonds, therefore they are flavyhum or 2-phenylbenzopyrylium (2-phenylchromenyHum) cations. 

FIGURE 3.2 Basic structure and numbering system of flavonoids. 

FIGURE 4.1 Basic structure of flavonoid families present in grapevine. 

Hundreds of flavone-like pigments are widely distributed among plants. On the basis of their chemical structure, these pigments are grouped in several classes, the most important of which are listed in Table II. The basic structure of all these compounds comprises two benzene rings, A and B, connected by a heterocycle. The classification of flavonoids is based on the nature of the heterocycle (which is open in one class). 

FIGURE 3.23 The studied flavonoids (Putz et al., 2009b) (with basic structure of as no.O while the others are in the Table 3.29 characterized by associate QSAR data), covering the flavones, isoflavones, chalcones, flavonols and flavanones, as they assist the increase of mitoxantrone (MX) accumulation in BCRP-overexpressing MCF-7 MX 100 breast caner cells (Zhang et al., 2005)... 

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