**4. Chemistry of flavonoids**

At the present scenario of scientific research, bioflavonoids are being considered as promising drug candidates, and extensive researches directed toward structural studies and biological efficacies of such class of compounds are in progress, which would eventually boost the on-going efforts leading to the discovery of new efficacious lead molecules. This remarkable class of natural compounds draws the attention of the scientists for their immense biological and pharmacological potentiality. Presently, over 10,000 individual flavonoid compounds are known, which are based on very few core structural skeletons (viz. flavone, flavanol, isoflavone, flavan, flavanone, chalcone, anthocyanin, coumarin etc. see in **Figure 1**) [9, 10, 30, 31].

Flavonoids are a group of natural compounds with low molecular weight polyphenolic substances based on the flavan nucleus are found mostly in plants. A novel chemical was extracted from oranges in 1930. It was given the name vitamin P at that time because it was thought to belong to a novel class of vitamins. Later, it was discovered that this material was a flavonoid (rutin), and as of now, more than 10,000 different flavonoid species have been found [48]. According to their chemical structure, flavonoids contain a 15-carbon skeleton. This skeleton consists of two benzene rings (A and B) linked by a heterocyclic pyrane ring (C) (**Figure 1**). These include flavones (such as luteolin apigenin and Itoside N), flavanols (such as kaempferol, myricetin and quercetin), flavanones (such as hesperetin, naringenin and abyssinoflavanone VI), and others. **Table 1** shows their general structures and other related information. Individual compounds within a class differ in the pattern of substitution of the A and B rings, whereas the many classes of flavonoids differ in the level of oxidation and pattern of substitution of the C ring [48]. Among the many forms of flavonoids, there are aglycones, glycosides, and methylated derivatives. Flavonoids contain aglycones as their basic structure (**Figure 1**). The α-pyrone (flavonols and flavanones) or its dihydro derivative (flavanols and flavanones) is a six-member ring that is condensed with the benzene ring (flavonols and flavanones) is a six-member ring that is condensed with the benzene ring. Based on the position of the benzenoid substituent, flavonoids are classified as flavonoids with a 2-position and isoflavonoids with a 3-position. Unlike flavanones, flavanols contain a hydroxyl group at the 3-position and a double bond

#### *Chemistry and Role of Flavonoids in Agriculture: A Recent Update DOI: http://dx.doi.org/10.5772/intechopen.106571*

#### **Table 1.** *Structure of some selective known flavonoids [9, 10, 49].*

between C2 and C3 [49–51]. The most common positions of hydroxylation for flavonoids are 3, 5, 7, 2, 3′, 4′, and 5′. There is evidence to suggest that alcohol group methyl ethers and acetyl esters occur in nature. It is normally found that glycosides form when the glycosidic linkage appears in positions 3 or 7, and the carbohydrate is usually L-rhamnose, D-glucose, glucorhamnose, galactose, or arabinose [49, 50, 52].
