Classification
Flavonoids can be classified according to their biosynthetic origins. Some
flavonoids are both intermediates in biosynthesis and end-products, e.g.
chalcones, flavanones, flavanon-3-ols and flavan-3,4-diols. Other classes are
only known as the end-products of biosynthesis, e.g. anthocyanins, flavones
and flavonols. Two further classes of flavonoids are those in which the 2phenyl side-chain of flavonoid isomerizes to the 3-position (giving rise to
isoflavones and related isoflavonoids) and then to the 4-position (giving rise
to the neoflavonoids). The major classes of flavonoids, with specific
examples, are summarized below.
Chalcone
Dihydrochalcone
O
OH
O
H
OH
Isoliquiritigenin
OH
O
O
H
OH
OH
Dihydronaringenin chalcone
Flavanone
Flavone
O
O
OH
O
H
OH
R
Naringenin R = H
Eriodictyol R = OH
O
O
OH
O
H
OH
R
Apigenin R = H
Luteolin R = OH
Flavanon-3-ol
Flavonol
O
O
OH
O
H
OH
R
OH
Dihydrokaempferol R = H
Dihydroquercetin R = OH
O
O
OH
O
H
OH
R
OH
Kaempferol R = H
Quercetin R = OH
Flavan-3,4-diol
Flavan-3-ol
O
OH
OH
OH
OH
O
H
R
Leucopelargonidin R = H
Leucocyanidin R = OH
2
3
4
4'
3'
5
7
O
OH
OH
OH
O
H
R
Afzalechin R = H
(+)-Catechin R = OH
2
3
4
4'
3'
5
7
6.7 PHENOLICS
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