Table 4.

Important points about phenolic acids and their derivatives.

The Derivatives of Phenolic Acids	Key Points	References
Flavonoids	The largest group of natural phenolic compounds.	[54,114]
	Their structure is based on a 15-carbon phenyl benzopyran skeleton (C6-C3-C6, i.e., A-C-B rings).	[54,114]
	Based on differences in the pyran ring, flavonoids can be categorized into flavones, isoflavones, flavanonols, flavonols, flavanones, flavan-3-ols, and anthocyanidins.	[54,114]
	The majority occur as glycosides, except for flavan-3-ols, which are rarely glycosylated.	[54,114]
	Different patterns of hydroxylation and methylation of the A and B rings consequently result in a variety of compounds for each flavonoid category.	[54,114]
	Flavones have a double bond between C-2 and C-3, a keto function in C-4, and the B ring is attached at C-2.	[54,114]
	The most common flavonoes in medicinal and aromatic plants are luteolin, apigenin, and glycosides.	[54,114]
	In isoflavones, the B ring is attached at C-3 and the main components are daidzein, genistein, and glycitein.	[54,114]
	Flavonols are flavones bearing a hydroxyl group at C-3, such as kaempferol, quercetin, and myricetin.	[54,114]
	In flavanones, the C-ring has no double bond between C2 and C3, such as in naringenin, eriodictyol, and hesperetin.	[54,114]
	Flavanonols, also called dihydroflavonols, have the same saturated C-ring as flavanones but are hydroxylated at C-3.	[54,114]
	Flavan-3-ols, also referred to as flavanols, also contain a saturated C-ring, but lack the keto group at C-4, and are hydroxylated at C-3, such as catechin and gallocatechin, or as oligomers and polymers.	[54]
	In anthocyanidins, the C-ring lacks the keto group at C-4, is hydroxylated at C-3, and, uniquely, has two double bonds forming the flavylium cation, such as in cyanidin, petunidin, malvidin, pelargonidin, peonidin, and delphinidin.	[54]
Stilbenes	They are based on 1,2-diphenylethylene, which has a C6-C2-C6 skeleton.	[115]
	They can be found as aglycones, monomers, oligomers, or glycosylated derivatives.	[116]
Tannins	Tannins are high molecular weight polyphenolic compounds.	[117,118]
	They can be synthesized as a defensive mechanism in response to pathogen attack and abiotic stresses such as UV radiation.	[117,118]
	Based on their structures, tannins in plants can be classified into mainly hydrolysable tannins and condensed tannins, also known as proanthocyanidins.	[117,118]
	Hydrolysable tannins are built based on gallic acid and are divided into the gallotannins and ellagitannins.	[117,118]
Quinones	They contain a di-one or di-ketone group.	[119]
	They are distinguished into benzoquinones and naphthoquinones and are based on their derivative molecules.	[119]
	They may occur as monomers, dimers, trimers, glycosides, or in reduced forms.	[119]
Coumarins	They may occur in a free or glycosylated state.	[120]
	They are divided into six categories, namely simple coumarins, furanocoumarins, dihydrofuranocoumarins, pyranocoumarins, phenylcoumarins, and bicoumarins.	[120]
Curcuminoids	They widely occur in Curcuma spp., especially in the rhizomes of Curcuma longa (turmeric).	[121,122]
	There are three major curcuminoids, namely curcumin, demethoxycurcumin, and bis-demethoxycurcumin.	[121,122]
	The structure of curcumin consists of a keto-enol tautomeric unsaturated chain linking two aromatic rings bearing a hydroxyl and methoxy group.	[121,122]
Lignins	Lignans consist of two phenylpropane units joined together by a β-β′ bond.	[123]
	They are divided into eight categories, namely dibenzylbutyrolactols, dibenzocyclooctadienes, dibenzylbutanes, dibenzylbutyrolactones, arylnaphthalene, aryl-tetralins, furans, and furofurans.	[123]