**3. Classification of phenolic compounds**

Polyphenols are classified according to the number of phenol rings they contain and the structural elements that connect these rings. The major classes of polyphenols are phenolic acids, flavonoids, stilbenes, and lignans. **Figure 1** depicts the various polyphenol groups and their chemical structures and **Figure 2** shows the structure of polyphenols.

### **3.1 Phenolic acids**

Phenolic acids, alternatively referred to as phenol carboxylic acids, are aromatic acids composed of a phenolic ring and a carboxyl functional group. As a result, these compounds contain an aromatic ring, a hydroxyl group, and a carboxyl group. Salicylic acid is one of the most basic phenolic acids. Additionally, hydroxycinnamic and hydroxybenzoic acids are important naturally occurring phenolic acids. Hydroxycinnamic acids are derived from molecules of non-phenolic cinnamic acid, whereas hydroxybenzoic acids are derived from molecules of non-phenolic benzoic acid.

**Figure 1.** *The different groups of polyphenols.*

#### **Figure 2.** *Structure of polyphenols.*

Naturally occurring phenolic acids are found in a variety of horse grams, dried fruits, the mushroom species Basidiomycetes, and human urine. Phenolic acids include protocatechuic acid (PCA), vanillic acid, p-hydroxybenzoic acid (PHBA), caffeic acid, ferulic acid, sinapinic acid, p-coumaric acid and syringic acid.

Phenolic compounds are phytochemicals found in cereals that are beneficial to health. Despite their antioxidant properties, phenolic compounds continue to garner considerable attention. The phenolic acids and flavonoids are the two most abundant types of phenolic compounds found in whole grains. In cereals, phenolic acids are the most abundant. The gut microbiota is widely accepted as a factor in the biotransformation of phytochemicals, including phenolic acids, resulting in the formation of food-derived metabolites that are excreted in the urine. Phenolic acids are easily absorbed through the intestinal tract's walls, which is beneficial for human health because they act as antioxidants, preventing cellular damage caused by free radical oxidation reactions. If humans consume them on a regular basis, they may also help to maintain anti-inflammatory conditions in the body [4].

#### **3.2 Lignans**

Lignans are bioactive, non-caloric, non-nutrient phenolic plant compounds found in abundance in flax and sesame seeds and lesser amounts in grains, other seeds, fruits, and vegetables. Enterolignans (occasionally referred to as mammalian lignans) are metabolites of food lignans produced by intestinal bacteria in humans. They have been identified in urine and plasma from humans. Their insignificant estrogenic and other biochemical properties suggest that they may have nutritional value in preventing cardiovascular and other chronic diseases [5].

Monolignols, which are derived from hydroxycinnamic acids (p-coumaric, sinapic and ferulic acids), are either dimerized to form lignans or polymerised to form larger lignin structures in the cell wall. These structurally diverse compounds play a role in plant defence (as antioxidants, phytoalexins, biocides and others), protecting plants from diseases and pests and possibly assisting in plant growth control. Lignans and lignins are two distinct compounds that should not be confused. Lignans are stereospecific dimers of these cinnamic alcohols (monolignols) bonded at carbon 8 (C8- C8) [6].

#### *Medicinal Plants and Phenolic Compounds DOI: http://dx.doi.org/10.5772/intechopen.99799*

Lignans (monolignol dimers) are found in plants either free or bound to sugars. There are numerous diglucosides of pinoresinol, secoisolariciresinol, and syringaresinol. 9–12 Sesame seeds contain sesaminol triglucoside and sesaminol diglucoside. Secoisolariciresinol occurs in flax as a diglucoside and is a component of an esterlinked complex or oligomer that also contains 3-hydroxyl-3-methylglutaric acid, a number of cinnamic acid glycosides (most commonly ferulic or p-coumaric acid), and the flavonoid herbacetin [7].

Lariciresinol, pinoresinol, matairesinol and secoisolariciresinol are the most abundant plant lignans found in foods. Numerous other lignans are found in a variety of foods, including medioresinol (found in sesame seeds, lemons and rye), syringaresinol (found in grains), sesamin, and sesamolin, a lignan precursor (in sesame seeds) [8]. Additionally, arctigenin, cyclolariciresinol (isolariciresinol), 7′-hydroxymatairesinol,b and 7-hydroxysecoisolariciresinol are found in foods but are rarely quantified. (Some cyclolariciresinol occurs naturally, while some are formed during the extraction and analysis of lariciresinol under acidic conditions). Lignans have no known nutritional value. While lignans are not classified as dietary fibres, they do share some chemical properties with lignin, an insoluble fibre [9].

Lignins are large plant polymers composed of the hydroxycinnamic alcohols, p-coumaryl, coniferyl, and sinapyl. They are racemic (non-stereospecific) polymers that contain monolignol units at C8 and four additional sites (C5-C5, C5-C8, C5-O-C4, C8-O-C4). Lignins are found in all higher plants' vessels and secondary tissues. They are found in a wide variety of foods, but are especially prevalent in cereal brans. Lignins are considered to be a type of insoluble dietary fibre from a nutritional standpoint. Lignins are necessary for plants because they strengthen the cell walls, aid in water transport, prevent the degradation of polysaccharides found in the cell walls, aid in the resistance of plants to pathogens and other threats, and provide texture in edible plants. Foods contain a small amount of lignan, typically less than 2 mg/100 g. The exceptions are flaxseed27 (335 mg/100 g) and sesame seeds (373 mg/100 g), which contain a hundred times the amount of lignan found in other foods. They are found in a wide variety of plant families, though the types and amounts vary considerably between them. Whole grains (particularly the bran layer) and seeds (in the seed coat) contain lignans. Several grains, including barley, flax, buckwheat, millet, rye, sesame seeds, oats, and wheat, contain a significant amount of lignans. Additionally, nuts and legumes are good sources. Although in smaller quantities than in grains, lignans are also found in fruits and vegetables such as asparagus, kiwi fruit, grapes, lemons, pineapple, oranges, and wine, as well as in coffee and tea [8].

In comparison to plants, animal foods contain almost no lignans. The enterolignans enterodiol and enterolactone are occasionally found in animal foods (milk products) as a byproduct of intestinal bacterial metabolism in the animals' guts, but these are exceptions. Little research has been conducted on the effects of storage and processing on lignans in the majority of foods, although it is known that the lignan content of flaxseed and sesame seed processing does not appear to change significantly [10].

#### **3.3 Stilbene**

Compounds of Stilbene in Plants While phenolic compounds are critical mediators of plants' adaptation and survival responses to acute and chronic stress, polyphenols also regulate cell growth, differentiation, pollen fertility, and nodulation, and thus appear to be essential for plant health. For instance, stilbenes are naturally occurring phenolic defence compounds found in a variety of plant species that

exhibit antimicrobial and antioxidant activity against phytopathogens and ozone or ultraviolet stress Stilbene compounds are found in a wide variety of plant species, including wine grapes, peanuts, sorghum, and a variety of tree species [11]. Additionally, commercial sources of stilbenes include a number of plants cultivated in Asia as folk medicines, including *Polygonum cuspidatum*, Rheum undulatum, *Rhodomyrtus tomentosa*, *Melaleuca leucadendron*, and *Euphorbia lagascae*, whereas pterostilbene is found primarily in bilberries, blueberries, and some other berries. Grape pomaces, winemaking residues, and other grape juice solids contain high levels of polyphenols and are also an excellent source of a variety of stilbene compounds, not just resveratrol. Conifer tree bark waste contains significant amounts of stilbene compounds such as piceatannol, pinosylvin, and trans-resveratrol (t-Res). As a result, this massive amount of industrial byproducts represents an extremely attractive and affordable source of stilbenes with commercial applications. Genetic tools are a very promising method for producing specific stilbenes such as pterostilbene in plants via coexpression of stilbene synthase and O-methyltransferase. These stilbenes may be particularly well suited for pharmacological applications. The enzyme stilbene synthase (STS) is required for the biosynthesis of stilbenic compounds. STS appears to have evolved independently from chalcone synthases (CHSs) in stilbene-producing plants. Interestingly, different STS genes express differently in different tissues and developmental stages. Thus, it has been reported that STS genes were expressed at a lower level in young grape leaves than in mature leaves, whereas the transcript levels of eight STS genes increased dramatically in the berry skins of Cabernet Sauvignon and Norton grape cultivars following veraison, peaking at harvest. Although pine trees' heartwood contains a high concentration of pinosylvin, young seedlings accumulate significant amounts of the compound in response to stress induction (fungal or UV light) [12].

#### **3.4 Flavonoids**

Flavonoids are a large class of polyphenolic compounds with a benzo—pyrone structure that is abundant in plants. They are produced through the phenylpropanoid pathway. According to available data, secondary phenolic metabolites, including flavonoids, are responsible for a variety of pharmacological activities [13].

In plants, animals, and bacteria, flavonoids perform a variety of biological functions. Flavonoids have long been known to be synthesised in specific locations in plants and are responsible for the colour and aroma of flowers, as well as the ability of fruits to attract pollinators and thus aid in seed and spore germination, as well as the growth and development of seedlings. Flavonoids protect plants from biotic and abiotic stresses and act as one-of-a-kind UV filters. They also act as signal molecules, allopathic compounds, phytoalexins, detoxifying agents, and antimicrobial defensive compounds. Flavonoids protect plants from frost and drought and may also play a role in heat acclimatisation and freezing tolerance.

#### *3.4.1 Family of flavonoids*

Flavonoids are phytonutrients that belong to the polyphenol class. According to the Global Healing Center, polyphenols have historically been used in Chinese and Ayurvedic medicine and are associated with skin protection, brain function, blood sugar and blood pressure regulation, as well as antioxidant and anti-inflammatory activity.

## *Medicinal Plants and Phenolic Compounds DOI: http://dx.doi.org/10.5772/intechopen.99799*

Flavonoids are classified into several major groups, including flavanols, anthocyanidins, flavones, flavonones, flavonols, and isoflavones. There are additional subgroups within the flavanol subgroup. Each of these subgroups and each flavonoid type has a unique set of actions, benefits, and source foods.

