**2. Classification**

In general, all flavonoids have a general structural compound with a chromane‐type skel‐ eton composed of three phenolic rings referred to as A, B and C rings (**Figure 1**) with phenyl

**Figure 1.** Flavonoid basic structure [13].

substituent in the C2 or C3 position in B ring. Their functions depend on their structural class, degree of hydroxylation and conjugation and degree of polymerization. They vary in the structure around the heterocyclic oxygen ring, but all have the characteristic C6‐C3‐C6 carbon skeleton [3, 10–12].

They can be grouped into different classes with respect to the basic structure, which allows a wide substitution patron and variations in C ring leading to their classification [11].

#### **2.1. Flavones**

In the classification, we can find that in the main structure of *flavones* a double bond between carbons 2 and 3 of the central ring is present (**Table 1**). Also, the structure of flavones has a car‐ bonyl group in position 4 in C ring. These types of flavonoids are pale yellow in colour, and their representative forms are apigenin, chrysin, myricetin, rutin, sibelin, luteolin, diosmetin and quercetin [5, 14–17]

#### **2.2. Flavonols**

**Figure 1.** Flavonoid basic structure [13].

**2. Classification**

its environment, such as pigments or defensive compounds. Secondary metabolites included

Flavonoids are a large group of natural substances with variable structures present almost in all growing parts of the plants, being reported as the most abundant plant pigment along with chlorophyll and carotenoids, also providing fragrance and taste to fruits, flowers and seeds, which makes them attractants for other organisms [1, 2]. These compounds are also one of the largest groups of secondary metabolites [3]. Besides their relevance in plants, flavonoids are important for human health because of their high pharmacological activities. Recent interest in these substances has been stimulated by the potential health benefits arising from the anti‐

In this chapter, we focused in the antioxidant, antibacterial, antiviral and anti‐inflammatory activities of flavonoids, which according to many studies are conferred mainly by the content of hydroxyl groups attached to base structures of these compounds. Biochemical actions of these compounds depend primarily on the presence and position of their substituent groups, which can affect the metabolism of each one [4]. One of the most important characteristics of flavonoids is that they often occur in the glycosidic form, which possibly let them take place

However, they attract attention due to their antioxidant activity and reduce free radical for‐ mation and also scavenge free radicals [6]. Flavonoids have other important biological activi‐ ties such as protect skin from UV light exposure, protect DNA from damage, strengthening of capillaries, anti‐inflammatory effect and protective action against radiation, moistening, softening, soothing, antiseptic and other. Due these properties, flavonoids can be used as

Nowadays, flavonoids are major bioactive compounds known for their potential health ben‐ efits, which have been used against many chronic diseases such as cancer, antiviral, inflam‐ mation, cardiovascular and neurodegenerative disorders; it is widely assumed that active

In general, all flavonoids have a general structural compound with a chromane‐type skel‐ eton composed of three phenolic rings referred to as A, B and C rings (**Figure 1**) with phenyl

ingredients in the production of cosmetics and pharmaceutical products [7, 8].

dietary constituents are antioxidant nutrients present in fruits and vegetables [9].

a group of compounds known as phenolic; in this group, we can find the flavonoids.

oxidant activities of these polyphenolic compounds.

in the gastrointestinal tract [5].

354 Flavonoids - From Biosynthesis to Human Health

As flavones, *flavonols'* structure exhibits a double bond between carbons 2 and 3, and a car‐ bonyl group in carbon 4, also a hydroxyl group in carbon 3 of C ring, for example, quercetin, rhamnetin and kaempferol.


**Table 1.** Flavonoid subgroup structures and their different substitution patterns [13, 23].

#### **2.3. Flavanones**

The basic structural model of flavanones has 2‐phenylbenzopiran‐4‐one‐skeleton. These com‐ pounds are of great interest due to the fact that they play an important role in the metabolic pathway of the other flavonoids. Their metabolic precursors are chalcones, flavones, dihydro‐ flavonols; isoflavones are biosynthesized from the flavanones [18]. Their chemical structures have a hydroxyl group in 3 position of C ring [19]. Physically, some flavanones are colourless (Hesperidin), pale (naringenin, eriodictyol) and others yellow (neohesperidin) [15, 16].

#### **2.4. Flavanols**

Flavanols, well known as flavan‐3‐ols or catechin, have a hydroxyl group in C3 of C ring, with no carbonyl group. These flavonoids are colourless (catechins, gallocatechin, epicatechin, epi‐ gallocatechin, gallate) and some of them show yellow colouration (procyanidin, theaflavines).

#### **2.5. Anthocyanidines**

This is the other subgroup, which structurally has a hydroxyl group in 3 position and also a double bond between carbons 3 and 4 of C ring. These molecules are water soluble, and are the pigments responsible for the red to purple color in plants.

#### **2.6. Isoflavones**

This is a distinctive subclass of flavonoids. They structurally have a 3‐phenylchromone skel‐ eton with same three phenolic rings referred to as A, B and C rings. But also, in some of their derivative compounds, they can form an additional heterocyclic ring (D ring) (rotenoid, coumestane, pterocarpan) [20].
