**1. Introduction**

Secondary metabolites are the chemical compounds synthesized in plants having minor role in life processes and do not have direct role in normal metabolism and development of the plant but often have ecological roles e. g., attractants of pollinators and chemical defenses against microbes, insects and higher predators [1]. Several medicinal plants by virtue of their secondary metabolites acquire a number of biological and therapeutic activities. These phytochemicals influence the metabolic activities of human and animals and have been used as drugs since centuries. The secondary metabolites

are classified in three chemically distinct groups- terpenes, phenolic compounds, and nitrogen containing compounds.

Phenolic compounds are a vast group of plant secondary metabolites, present in almost every plant in varying quantities. These are chemically heterogeneous and the derivatives include some very important compounds such as flavonoids, tannins, lignins, anthocyanins etc. [2]. They show huge diversity in the structure ranging from simple structures, e.g. phenolic acids, to polyphenol compounds such as flavonoids, which consist of several functional groups (**Figure 1**). Phenolic compounds are very crucial for the diverse therapeutic actions in medicinal plants, color and flavor of fruits etc. During food processing and storage, plant phenolics are converted to a variety of derived compounds. The flavonoids are the largest group of phenolic compounds and play major role in defense and pigmentation. These are reported to be synthesized in the case of occurrence of infection. The activities of different groups of flavonoids are different as per their structural class. These are more commonly

**Figure 1.** *Some common phenolic acids.*

*Bioactive Phenolic Compounds from Indian Medicinal Plants for Pharmaceutical and Medical… DOI: http://dx.doi.org/10.5772/intechopen.99672*

known for their antioxidant potential by scavenging of free radicals mediated by the functional hydroxyl groups. Flavonoids are supposed to protect from infections, and also prevent from cardiovascular, cancer and age related problems [3]. This group of molecules are also responsible for the color, taste and fragrance to the flowers and fruits and in this way attract pollinators also.

It is a well-established fact that the secondary metabolites in plants are meant to enable the plants to sustain their lives in various kinds of environmental conditions. Phenolic and flavonoid compounds are reported to exhibit strong antioxidant potential by different mechanisms. The hydroxyl group present in the phenolic compounds are excellent hydrogen donors. The reactive oxygen and nitrogen species react rapidly with hydrogen donating moieties which lead to stoppage of generation of new free radicals [4]. Phenolic compounds have also been found to chelate the metal ions involved in the formation of free radicals and hence produce antioxidant effect. Apart from antioxidant potential, dietary supplement of phenolic compounds have also been found to act as anticancer agents [5], provide protection from cardiovascular and several autoimmune diseases [6].
