**3. Flavonoids**

Drug discovery is a pivotal field that has demonstrated the pharmacological properties of a broad library of natural products (NPs), such as avermectin, artemisinin, cyclosporine, irinotecan, lovastatin, morphine, penicillin, and paclitaxel [29]. NPs are compounds derived from animals, microorganisms, and plants. Their structural diversity is classified into alkaloids, cardiac glycosides, cyanogenic glycosides, tannins, terpenes, steroids, saponins, phenylpropanoids, polyketides, polysaccharides, and flavonoids.

*Shedding Light on Four Selected Flavonoids with Anti-non-small Cell Lung Cancer Properties DOI: http://dx.doi.org/10.5772/intechopen.105162*

#### **Figure 2.**

*Flavonoid's classification: anthocyanins, chalcones, flavones, flavanones, and isoflavonoids.*

Flavonoids are a large family of structurally and biosynthetically related polyphenolic compounds (~6000 members). Flavonoids are distributed in higher plants, and their general chemical structure is the flavan nucleus which consists of fifteen carbon atoms organized into three rings (C6-C3-C6). Two of those rings (A and B) are aromatic, whereas a three-carbon-atom heterocyclic ring connects them. The last ring, termed C, is an oxygen-containing pyran ring. According to their level of oxidation, conjugation, glycosidic residues, number of substituents, and enzymatic modification [30], flavonoids are classified into anthocyanins, chalcones, flavones, flavonols, flavanones, and isoflavonoids (**Figure 2**) [31]. Other classes of flavonoids include aurons and biflavones [32].

Although several factors influence the process of flavonoid biosynthesis (e.g., light, water availability, temperature, hormones, physical injuries, etc.) [33], the structural characteristics of these categories are determined by spectroscopy, preparation and extraction methods, chromatography, and spectrometry (e.g., mass spectrometry).
