Table 2.

Classification of flavonoids according to structural arrangement.

Classification	Structural characteristic	Examples	References
Flavones	They have a double bond between position 2 and 3 and a ketone in position 4 of the C ring	Apigenin, luteolin, acacetin, chrysin	Panche et al., 2016
Flavonols	They have a –OH group in position 3 of the C ring, which may also be glycosylated	Quercetin, kaempferol, rutin, and myricetin	Panche et al., 2016
Flavanones	Flavanones also called dihydroflavones, have the C ring saturated	Hesperetin, naringenin	Grayer and Veitch, 2006; Kumar and Pandey, 2013
Flavan-3-ols (Flavanols)	Has an -OH group in position 3 of the C ring	(+)-cathechin, (–)-epicatechin, (–)-epigallocatechin, (–)-epicatechingallate, (–)-epigallocathechingallate, and (+)-gallocatechin	Naczk and Shahidi, 2004; Panche et al., 2016
Anthocyanidins (Anthocyanins)	Has the -OH group in position 3, but also have a double bond between carbons 3 and 4 of C ring	Delphinidin, cyanidin, pelargonidin, petunidin, peonidin, and malvidin	Middleton et al., 2000; Nijveldt et al., 2001
Chalcones	Chalcones form a wide range of dimers and oligomers, they are characterized by the absence of C ring of the basic skeleton structure	Butein, okanin, phloridzin, arbutin, phloretin, and chalconaringenin	Naczk and Shahidi, 2004
Isoflavonoids	Often referred as phytoestrogens, have the B ring in position 3 of the pyran	Genistein, genistin, daidzein, daidzin, biochanin A, formononetin	Pietta, 2000