[Photos: Ginkgo(biflavones), Tea (flavonols) and Sesame (lignan)]
Many a times, the term “phenolics” are cursorily mentioned in constituents of a medicinal plant. But the renewed interest on antioxidants, after the realization of their active role in diseases such as atherosclerosis, stroke, cerebral thrombosis, diabetes, Alzheimer’s and Parkinson’s diseases and cancer (Tiwari, 2004), firmly put these compounds on a pedestal as pharmacologically active components. Since the phenolics are very effective antioxidants, besides their curative role, they play varied roles such as (1) protecting other active components in a medicinal preparation, (2) protect the membranes and tissues from oxidation at the site of medicinal action or (3) protect other antioxidants (needed elsewhere) from oxidation. The contribution of phenolics as antimicrobials providing a microbe-free environment, for the other drugs to act, also is significant. But for phenolics, no other group of compounds have a role in maintaining the system (from wear and tear).
Phenolics is a broad term including all compounds possessing an aromatic ring bearing a hydroxyl group or its substituents. Phenols are now found to have a universal occurrence, though sometimes in minute quantities. Most of the phenols are derived from 5-dehydroquinic acid. This is now probably the largest group of Natural products numbering more than 13,000. The principal groups of phenoilic compounds are the following.
1. Simple Phenols – C6/C7 These compounds, containing a single aromatic ring, in Aspidium (Male fern), Vanillin , Hops (Humulus lupulus .)
2. Phenolic acids – having an acidic group also C7, C8, C9 Eg. gallic acid in Amla
3. Acetophenones These are C6-C2 ketones (aromatic ketones), Acetophenone in Conifers
4. Phenyl propanes – C6-C3 having a propyl group also: Chicory, ginger, Turmeric, Kava Kava
5. Benzophenones ; C6-C1-C6 compounds in Cotoin in Aniba, Maclurin in Morus
6. Xanthones resemble benzophenones in having a C6-C1-C6 skeleton but differ in having a heterocylic oxygen ring in between the two benzene rings. Mangiferin, is reported from Mangifera,
7. Coumarins, lactones of o-hydroxy cinnamic acid. Aegle, Feronia, Psoralea, Eclipta, Bergenia
8. Chromones Similar to Coumarins but the kepto group is in para position to heterocyclic oxygen, Rotenone is a complex chromone derivative. Ammi visnaga
9. Stilbenes C6-C2-C6 compounds. Eg. Resveratrol from grapes
10. Lignans, dimers of phenyl propanes . Podophyllum, Phyllanthus amarus, Gmelina, Cubeb, Sesame and Linseed.
11. Flavonoids C6-C3-C6 compounds. 9 subgroups
a. Anthocyanins, Blue berry, Raspberry
b. Aurones: Snapdaogon
c. Biflavones: Gymnosperms
d. Chalcones; Butea
e. Flavon 3-ols, -3, 4-diols and proanthocyanidins : tannins
f. Flavones and flavonols, Tea, Ginkgo, Trigonella
g. Flavonones and flavononols, Lemon
h. Isoflavones, isoflavonones and homoisoflavones, Soyabean
i. Neoflavonoids, Calophyllum, Haematoxylem
12. Quinones: Aromatic diketones, 3 groups
a. Benzoquinones One ring : Embelia
b. Naphthoquinones: Henna, Plumbago
c. Anthraquinones, 3 rings . Rhamnus, Cassia, Aloe etc.
13. Tannins: Polyphenols 2 types
a. Hydrolysable tannins: esters of gallic/ellagic acids with sugars Eg. Hamamelis, Amla
b. Condensed tannins : Polymeric polyphenols, Catechu
Source:
Medicinal Plants: Chemistry and Properties. M.Daniel, (2006) Science Publishers, New Hampshire, USA. (Indian Edition published by and Oxford & IBH Publishers, Delhi)
Mammen Daniel
