*2.2.5 Prenylation*

Prenylflavonoids are useful natural compounds found in a wide range of plants. They frequently have diverse biological features, such as phytoestrogenic, antibacterial, antitumor, and antidiabetic qualities [28]. Prenyl groups are frequently found in phytoalexins and stress-induced isoflavonoids. They are occasionally cyclized. Pterocarpans having a 2′-oxy function and a phenyl group linked to the B-ring were the most active insect feeding deterrents [30]. Elicitor-challenged bean cell cultures

include a prenyltransferase in a microsomal fraction that adds a prenyl group at position 10 on the "B-ring" (also known as the D-ring) of 3,9-dihydroxypterocarpan to create phaseollidin, which is then cyclized to phaseollin. Dimethylallylpyrophosphate was the prenyl donor. The identical preparations were capable of introducing prenyl groups into medicarpin and coumestrol, but the products were not recognized. Previously, two distinct flavonoid-specific prenyltransferases that need Mn2+ for full activity were discovered in soybean cotyledons and cell suspension cultures [31].

#### *2.2.6 Sulfonation*

Nature mostly employs sulfation of endogenous and external substances to minimize possible harm. Sulfonated flavonoids have recently been found in substantial numbers. The majority of them are sulfate esters of common flavones and flavonols. Flavone sulfates are mostly composed of apigenin, luteolin, or its 6- and 8-hydroxy derivatives. Flavonol glycosides were sulfated through the sugar or a separate hydroxyl group. They are found in both dicots and monocots, primarily in herbaceous species or advanced morphological groupings. They are exclusively seen in ferns on rare occasions and have not been identified in bryophytes or gymnosperms [32].

#### **2.3 Stereochemistry**

Flavanones have a unique structural property known as chirality that separates them from all other groups of flavonoids (**Figure 3**). The chemical structure of all flavanones is based on a C6–C3–C6 configuration consisting of two aromatic rings connected by a three-carbon bond [33]. Almost all flavanones have one chiral carbon atom in position 2 (**Figure 3**). Except for a subgroup of flavanones known as 3-hydroxyflavanones or dihydroflavonols, which have two chiral carbon atoms in positions 2 and 3 (**Figure 4**). In the C7 position of ring A, certain flavanones include an extra d-configured mono or disaccharide sugar. These flavanone-7-O-glycosides occur as diastereoisomers or epimers with opposing configurations at just one of two or more tetrahedral stereogenic centers in the corresponding chemical entities [34].

Most natural flavonoids now only have one stereoisomer at C-2. The RS nomenclature identifies the R or S that changes at carbon 2 without any change in stereochemistry,

**Figure 3.** *Spatial disposition of the enantiomers of chiral flavanones.* *Application of Liquid Chromatography in the Analysis of Flavonoid Metabolism in Plant DOI: http://dx.doi.org/10.5772/intechopen.107182*

#### **Figure 4.** *Chemical structure of the chiral 3-hydroxyflavanones or dihydroflavonols.*

depending on the choice of the change adjacent groups should lead to confusion for flavonoid metabolism. It is not sufficient to utilize (+) - or 2,3-cis or -trans alone to define the four potential isoforms of dihydroquercetin or catechin; consequently, consideration should be given to alternate terminology. For mirror pictures, the ent-prefix is utilized. (+)-Catechin (2,3-trans isomer) with 2R, 3S absolute stereochemistry is simply known as catechin, whilst its mirror counterpart (−)-catechin (2,3-trans) with 2R, 3R stereochemistry is simply known as ent-catechin. Similarly, the (−)-epicatechin (2,3-cis) isomer (2R, 3R) and its mirror image (2S,3S) are known as epicatechin and ent-epicatechin [35].

There are other structures designated for hydroxylation patterns and inter-liquid bonding. To minimize ambiguity in the RS system of the configuration of the interflavanoid bond at C-4, Porter and Hemingway provided sugar chemistry terminology, particularly when defining proanthocyanidin isomers. The words are also used to characterize the stereochemistry of the added hydroxyl group at the C-3 position, which results in the 2,3-cis (a-OH) and more prevalent 2,3-trans (B-OH) forms. However, such language does not accurately describe the metabolic route [16].

#### **2.4 Overall pathways metabolism of flavonoid**

Flavonoids, which include chalcones, flavones, flavonols, anthocyanins, and proanthocyanidins, are abundant in plants and have been extensively researched using biochemical and molecular biology approaches. Until recently, liverworts and mosses were thought to be the earliest flavonoid-producing plants. Genes encoding enzymes in the phenylpropanoid biosynthetic pathway, including the first two enzymes for flavonoid biosynthesis (chalcone synthase and chalcone isomerase), have not been found in the algal genera Chlamydomonas, Micromonas, Ostreococcus, and Klebsormidium, whereas genes encoding enzymes in the shikimate pathway have been found in algae, liverwort [36].

The overall route to main flavonoid groups via 5,7-hydroxy A-rings. The key intermediates in the production of flavonoids are 4-coumaroyl-coA and 3-malonyl-coA. Synthesis of 4-coumaroyl-CoA and malonyl-CoA naringenin chalcone is synthesized by chalcone synthase, an enzyme involved in the phenylpropanoid pathway. Naringenin chalcone has the ability to spontaneously cyclize to naringenin. Furthermore, naringenin chalcone synthesizes a variety of chemicals such as chalcones, aurones, and biflavonoids (it can synthesize from flavanones). Naringenin has three routes for drug synthesis. The first is the direct synthesis of isoflavonoids, followed by the addition of

#### **Figure 5.**

*Overall pathway to major flavonoid groups with 5,7-hydroxyl a rings (DHK = dihydrokaempferol; DHQ = dihydroquercetin; DHM = dihydromyricetin).*

3' OH to produce eriodictyol, followed by the addition of 5'OH to produce 5'OH-erio flavones. Eriodictyol may be combined with 3-OH to get DHQ (dihydroquercetin), then with 5'OH to form DHM (dihydromyricetin) to form flavonols and flavan-3,4-diols (it can synthesize flavan-3-ols, proanthocyanidins, and anthocyanidins). Finally, adding 3-OH to naringenin results in DHK (dihydrokaempferol) (**Figure 5**) [16].
