*2.1.2.2 Hydroxycinnamic acids*

When the carboxylic acid functional group is separated from the phenol ring by a C=C bond, phenolic acids are described as hydroxycinnamic acids (**Figure 9**).

**Figure 9.** *General structures of hydroxyl-substituted cinnamic acids.*

*Phenolic Compounds: Classification, Chemistry, and Updated Techniques of Analysis… DOI: http://dx.doi.org/10.5772/intechopen.98958*

Examples of hydroxycinnamic acids are 2-, 3-, and 4-hydroxycinnmaic acid shown below (**Figure 10**).

Other common examples of cinnmaic acids are caffeic acid, ferulic acid, and sinapic acids shown below (**Figure 11**).

#### *2.1.2.3 Coumarins*

Hydroxycoumarins are hydroxyl-substituted coumarins (**Figure 12**). They are examples of phenolic compounds.

Examples of hydroxycoumarins are scopoletin and auraptene (**Figure 13**).

#### **2.2 Polyphenols**

Phenolic compounds that contain more than one phenol unit are considered "polyphenol". Polyphenolic compounds have C15 general skeleton representation.

#### **Figure 12.** *General structure of hydroxycoumarins.*

**Figure 13.** *Examples of hydroxycoumarins.*

#### *2.2.1 Flavonoids*

Flavonoids are polyphenolic compounds with the general structure shown below (**Figure 14**).

Generally, rings A and C are either mono, di, or trihydroxylated. The *O*-heterocycle B is usually a pyrone ring as in Luteolin but could also be a pyrlium ring as in delphinidin (**Figure 15**). If ring C is attached to C2 of ring B, the flavonoid is a flavone (as Luteolin), flavonol (as kaempferol), an anthocyanin (as delphindin) or a flavanone (as naringenin). If the ring C is attached to C3 of ring B, then the flavonoid is an isoflavone such as daidzein. Chalcones such as chalcone, are a class of flavonoids in which rings A and C are separated by 3-carbon linear chain rather than a ring. The bond between C2 and C3 of ring B is commonly double as in flavones, flavonols, chalcones and isoflavones. However, the C2-C3 bond could be single as in flavanones.

**Figure 14.**

*General structure of flavonoids.*

**Figure 15.** *Classification of falvonoids.*

*Phenolic Compounds: Classification, Chemistry, and Updated Techniques of Analysis… DOI: http://dx.doi.org/10.5772/intechopen.98958*

#### *2.2.2 Tannins*

Tannins are known to bind to and precipitate proteins and amino acids. They are subdivided into three types; hydrolyzable, condensed and complex. Hydrolyzable tannins can be gallotannins or ellagitannins. Gallotannins are polyols that are substituted with gallic acid units. The galloyl units in gallotannins are linked by depside (ester) linkages. Commonly the polyol core is a D-glucose that is substituted with gallic acid units. Tannic acid is an example of gallotannins (**Figure 16**).

Similar to gallotannins, ellagitannins are hydrolysable 1,2,3,4,6-pentagalloylglucose. However, unlike gallotannins characterized by depside linkages, adjacent galloyl groups in ellagitannins are linked by C-C bonds (**Figure 17**).

**Figure 17.** *Structure of ellagitannin.*

*Phenolic Compounds - Chemistry, Synthesis, Diversity, Non-Conventional Industrial...*

Condensed tannins (**Figure 18**) are polymeric phenolic compounds that consist of catechin units. When depolymerized, they give anthocyanidin. Thus condensed tannins are called proanthocyanidins.

Complex tannins are gallotannins or ellagitannins bonded to a catechin unit (**Figure 19**).

**Figure 18.** *General structure of condensed tannins.*

#### **Figure 19.**

*General structure of complex tannins.*

*Phenolic Compounds: Classification, Chemistry, and Updated Techniques of Analysis… DOI: http://dx.doi.org/10.5772/intechopen.98958*
