Phenolic Compounds - Natural Products Chemistry: Sources, Separations, and Structures - page 57

6 Phenolic Compounds
Phenolic compounds are produced in nature by a number of different pathways. Two
of the most important pathways to produce phenolic compounds are (1) the shikimic
acid pathway outlined in Chapter 4 and the metabolic route in plants, and (2) the
malonic acid pathway, of more importance in fungi.
Most of the phenolic compounds are derived from the amino acid phenylalanine.
Indeed, phenylalanine can be considered the branch point between primary and
secondary metabolites. In the steps of the shikimic acid pathway, the removal of
the amine function by the enzyme phenylalanine ammonia lyase (PAL) produces
the phenylpropanoid compound cinnamic acid. Thus, the two amino acids phenyl-
alanine and tyrosine appear as the precursors in the phenylpropanoid biosynthesis.
Furthermore, these phenylpropanoids are then used to elaborate the major phenolic
classes, including the flavonoids, coumarins, tannins, anthraquinones, anthocyanins,
lignin, and lignans.
6.1 FLAVONOIDS
Flavonoids are ubiquitous in nature and play various roles. Many of these phenolic
compounds have antimicrobial activity and are considered to be antioxidants. An
important flavonoid found in red wine is known as resveratrol (Figure 6.1), the com-
pound that may be responsible for explaining the “French paradox,” as it is believed
to lower the risk of heart attacks by inhibiting platelet aggregation and blood clots. In
the case of resveratrol, there is no middle oxygenated ring, which is a typical feature
of flavone structures. In fact, flavonoids are classified into different groups based
on the degree of oxidation of the 3-C bridge. This classification results in structures
belonging to the following: anthocyanins, flavones, flavonols, and isoflavones.
In their natural form, the flavonoids also may exist as their corresponding gly-
cosides. During the extraction process the intact glycoside may be isolated or the
glycosidic bond is ruptured due to solvent or hydrolysis conditions and only the intact
aglycone is recovered.
6.2 FLAVONES
Flavone compounds (Figure 6.2) are the pigmented compounds found in flowers.
They strongly absorb ultraviolet (UV) but not visible light, thus are not visible to the
human eye but visible to many insects. For this reason, their role in nature may be as
attractants and nectar guides. Flavones are present in the leaves, where they protect
against UV damage and they may be involved in nitrogen fixation mechanisms.
65