Chemistry Reference
In-Depth Information
aglycone moiety (33). In addition to their soap-like behavior in aqueous solution
because of this combination of polar (carbohydrate) and nonpolar (aglycone)
structural elements, saponins exhibit a diverse range of pharmacological and
medicinal properties. Within the plant kingdom saponins are present in two major
taxonomic classes, Magneliopsida (dicot) and Liliopsida (monocot) (33). Some
important examples of saponins include glycyrrhizic acid (
K10
) from licorice
and digitoxin (
K11
) from foxglove (
Digitalis purpurea
).
1.5 PRODUCTS OF SHIKIMATE PATHWAY
The shikimate pathway links the metabolism of carbohydrates to the biosynthesis
of aromatic natural products via aromatic amino acids. This pathway, which is
found only in plants and microorganisms, provides a major route to aromatic and
phenolic natural products in plants. To date, over 8,000 phenolic natural products
are known, which accounts for about 40% of organic carbon circulating in the
biosphere. Although the bulk of plant phenolics are components of cell wall
structures, many phenolic natural products are known to play functional roles
that are essential for the survival of plants.
It has been noted that the chemical diversity of plant phenolics is as vast as the
plant diversity itself. Most plant phenolics are derived directly from the shikimic
acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a com-
bination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products
of each of these pathways undergo additional structural elaborations that result
in a vast array of plant phenolics such as simple benzoic acid and cinnamic
acid derivatives, monolignols, lignans and lignin, phenylpropenes, coumarins,
stilbenes, flavonoids, anthocyanidins, and isoflavonoids.
1.5.1 Benzoic Acid Derivatives
As apparent from their structures, many benzoic acid derivatives are directly
formed from shikimic acid by dehydration, dehydrogenation, and enolization
reactions. Gallic acid is a component of gallotannins common in some plants
that are used in the tanning of animal hides to make leather. Astringency of
some foods and beverages, especially coffee, tea, and wines, is because of their
constituent tannins. Other benzoic acid derivatives that occur in plants include
protocatechuic acid, 4-hydroxybenzoic acid, and salicylic acid.
1.5.2 Cinnamic Acid Derivatives
Cinnamic acid and its derivatives found in plants originate from the aromatic
amino acids L-phenylalanine and L-tyrosine by the elimination of ammonia.
Some common natural cinnamic acid derivatives include
p
-coumaric acid, caffeic
acid, ferulic acid, and sinapic acid.
