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The flavonoids are a group within the secondary natural products. They occur in plants normally as glycosides, but the aglycons can also occur in the plant. The aglycones derived from flavan with the exception of isoflavonoids.
Through the different glycosidation and the variation of the aglycones flavonoids comprise a large group of substances. However, these all have a common biosynthesis starting from phenylalanine, which comes from the Shikimic acid pathway. Flavonoids have been used for a long time as dyes.
The flavonoids are ubiquitously occurring substances in plants. This means that they are included in any higher plant. Therefore, they also occur in various herbal drugs and extracts, such as in chamomile flowers and lime blossom.
Centaurea cyanus, cornflower, owes its color to the flavonoid complex protocyanin (from Bilder ur Nordens Flora)
Through this diversity flavonoids can be used in many areas. Firstly, flavonoids are component of many medicinal plants, however, they don't have always a medical effect. A flavonoid, which can be used as a medicament, is rutin. Rutin can be found in many plants and is isolated from buckwheat herb. It can be used for hemostasis.
In addition to the medical aspects, flavonoids can be used as dyes. Substances such as luteoline and various anthocyanidins may be used for yellow, red and blue color. Malvidine for example is partly responsible for the red color of red wine.
The flavonoids, especially the anthocyanidins, are able to form complexes with ions, which lead to the formation of pigments. Such a complex is protocyanin, which is responsible for the blue color of the cornflower. 
With the exception of isoflavonoids the flavonoids have with flavan (2-Phenylchroman) the same skeleton. The isoflavones are structurally derived from a constitutional isomer of flavan, the 3-Phenylchroman. The flavonoids and isoflavonoids can be divided into further sub-classes, according to the substitution of the basic structure.
The Flavaone have a keto group at C-4 position and a double bond between C-2 and C-3. The C-3 atom is not further substituted in the flavones. Some flavones are the yellow dyes luteoline and apigenin as well as chrysin.
In contrast to the flavones the flavonols are substituated at the C-3 position with a hydroxy group. A well-known flavonol is kaempferol. The flavonols can be chelated with, for example, aluminum and form yellow complexes.
The flavanones have a single bond between C-2 and C-3. This creates stereocenters at the two atoms. Like the flavones and flavonols the flavanones have the keto group at C-4 in common. One example is the colorless naringenin.
Like the flavonols the flavanonoles are hydroxylated the C-3 position.
The chalcones are characterized by an open ring structure. The chalcones are isomeric structures from the flavanones and can be converted into this. In plants, they play a marginalized role.
The anthocyanidins are ionic compounds. Overall, only a few anthocyanidins as malvidine are known. They can play an important role, especially as dyes.
The isoflavonoids have the phenyl group at C-2 and not at C-3 position. Like the flavonoids the isoflavonoids can be subdivide in various subgroups such as isoflavanes and isoflavandioles. One example is genistein.
The biosynthesis of flavonoids starts with the amino acid phenylalanine. These amino acid is synthesized in the shikimic acid pathway. Phenylalanine is converted to p-coumaryl-CoA in several steps. It is now possible to synthesize the flavonoids from p-coumaryl-CoA in different ways. The shown pathway allows the biosynthesis of most of the described sub-classes.
Similar to the biosynthesis of the polyketides p-coumaryl-CoA is in the first step extended three times through malonyl-CoA. In this step CO2 is released. In the next step the formed polyketide is cyclized to tetrahydoxychalcone by an enzym called chalcon synthase.
Tetrahydroxychalcone belongs to the chalcones. The chalcones can be isomerized to the flavonones. In our pathway tetrahydroxychalcone is derived to the flavanone naringenin.
The biosynthesis of isoflavonoids branches off here. The enzyme is isoflavone synthase helps to form genistein from naringenin.
In another way naringenin can be oxidized to apigenin. In this case a C-C double bound between C-2 and C-3 is introduced by the enzym flavone synthase which forms the sub-class of flavones.
In a third way naringenin gets hydroxylated at the C-3 position and aromadendrin is formed.The enzym flavonone-3-hydroxylase catalyse the formation of the flavanonoles.
Similar to the formation of apigening, aromadendrin gets oxidized to kaempferol by the enzym flavonole synthase.
Finally pelargonidin-3-glycoside is synthesized in several steps from aromadendrin. It is possible to further glycosylate pelargonidin-3-glycoside.[3,4]
The biosynthesis of other substituated flavonoids is realized by derivatisation of the key compounds in the synthetic pathway. The formation of eriodyctiol for example can be managed by hydroxylation of naringenin at C-5 position or through modification of p-coumaryl-CoA to caffeoyl-CoA.
The flavonoids are often glycosidated. The aglycones are often glycolyzed at the hydroxyl groups at C-3 and C-7, but also at C-6 and C-8. The flavonoids can be glycolyzed by different sugars. For this there is a huge group of different flavonoids based on different glycosylation.
An example of such a glycoside is hesperidin. This is the major flavonoid of bowls of lemons and oranges, from which it was isolated for the first time. The aglycone of hesperidin, hesperitin is glycosidated with ritosin. Ritosin is a disaccharide consisting of β-D-glucose and α-L-rhamnose, which are 1 → 6 linked.
 Willstätter, Richard; Everest, Arthur E. Untersuchungen über die Anthocyane. I. Über den Farbstoff der Kornblume. Justus Liebigs Annalen der Chemie, 1913, 401, 2, 189-232. http://onlinelibrary.wiley.com/doi/10.1002/jlac.19134010205/abstract
 Reinprecht, Yarmilla, et al. In silico comparison of genomic regions containing genes coding for enzymes and transcription factors for the phenylpropanoid pathway in Phaseolus vulgaris L. and Glycine max L. Merr. Frontiers in plant science, 2013, 4 http://journal.frontiersin.org/Journal/10.3389/fpls.2013.00317/full
 Theo Dingermann, Karl Hiller, Georg Schneider, Ilse Zündorf; Arzneidrogen; Spektrum Akademischer Verlag, 5. Auflage; 2004
 Holton, Timothy A.; Cornish, Edwina C. Genetics and biochemistry of anthocyanin biosynthesis. The Plant Cell, 1995, 7, 7, 1071. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC160913/
 Martens, Stefan; Genetische, biochemische und molekularbiologische Untersuchungen der Flavonbiosynthese bei Gerbera Hybriden. 2000; Doktorarbeit; TU-München-Weihenstephan. https://mediatum.ub.tum.de/doc/603573/603573.pdf