Phenolics

In the phenols group, cresols and the parent compound itself are the most important compounds as well as thymol. Natural compounds such as pyrocatechol, guaiacol and their derivatives are not toxic. A well-known pyrocatechol derivative is adrenaline. Monohydric phenols provide numerous natural scents (e.g. vanillin, thymol, carvacrol, zingiverone (in ginger), and salicylaldehyde).

Phenolic compounds are widely distributed in plant flora. They constitute an important part of glycosides (phenolic glycosides), flavonoids, and tannins. Curcumins (Fig. 5.189) are phenolics compounds from Curcuma longa. They have antioxidant, anti-inflammatory, anti cancer, and hepatoprotective activities. The pharmacological activities of cucrcuminoids are due to unique molecular structure. In crude extracts of C. longa about 70-76% of curcumin is present alongwith 16% demethoxycurcumin and 8% bisdemethoxycurcumin. Tetracurcuminoids (Fig. 5.189) are derived from curcuminoids. This compound is, unlike the yellow curcuminoids.

Knema Furfuracea

HO y—CH2CH2COCH2COCH2CH2— -OH

Tetrahydrobisdemethoxycurcumin ho^ y—ch2ch2coch2coch2ch2 —oh

Tetrahydrocurcumin ho v y—ch2ch2coch2coch2ch2^ y—oh

Tetrahydromethoxycurcumin Fig. 5.190 Structure of Tetracurcuminoids.

Semimyrtucommulone from Myrtus communis has antioxidant activity. Kneracheline A and B, from Knema furfuracea which inhibits the proliferation of bacteria cultured in vitro. 3-undecylphenol and 3-(8Z-tridecenyl)-phenol from Knema hookeriana, inhibits the proliferation of Bursaphelechus xylophilus cultured in vitro with a maximum effective dose of 4.5mg/cotton ball and 20mg/cotton ball, respectively.

Meusone, a phenolic compound is found in Mesua ferrea. Habenariol from Habenaria repens has antioxidant activity. Bhilawanol and semecaprol are reported from Semecarpus anacardium. Oil of Pongamia glabra contains karanjin and pongamol. Bakuchiol, a phenolic is present in Psoralia corylifolia (bawachi) oil (Fig. 5.191).

Knema Furfuracea

Flavonoids and tannins (variety of phenolics) have been discussed under respective headings.

Bhilawanol
Hyperforin
r=ch3
Bhilawanol
OH

Margaspidin

Margaspidin

Bhilawanol

Gossypiol

Fig. 5.192 Structure of common phenolic compounds.

Gossypiol

Fig. 5.192 Structure of common phenolic compounds.

A variety known as polyphenols is found in fruits of some plants. They occur as natural color pigments and are responsible for the color of fruits. Phenols are classified in two groups:

A. Phenolic acids.

B. Flavonones, flavones, xanthones and catechins.

Caffeic acid (Fig. 5.193) is regarded as the most common of phenolic compounds distributed in plant flora. It is produced by hydroxylation of cinnamic acid. Caffeic acid is distributed in Coffea arabica, Echinacea purpurea and Cichorium intybus. Quinic acid is the degradation product of caffeic acid. Chlorogenic acid (Fig. 5.193) is a dark colored pigment and is the most abundant phenolic compound in plants next to caffeic acid. It is known to cause allergic dermatitis among humans. Other examples of caffeic acid derivatives are chicoric acid (Fig. 5.193), dicoffeayl tartaric acid and isocaffeic acid.

Caffeic acid

Caffeic acid

OH

Chlorogenic acid

Chlorogenic acid

Curcumin Chemical Structure
Rosmairinic acid Fig. 5.193 Structure of phenolic acids.

The administration of phenolic acids to experimental animals produces a wide range of biological manifestations. Caffeic acid is observed as an inhibitor of dopa receptor. Caffeic and oxidized rosmairinic acids inhibit the effects produced by human lutenizing hormone of chorionic origin and by an equal part of lutenizing hormone and follicle stimulating hormone from human hypophysis. An injection of ferulic acid induces the liberation of follicle stimulating hormone from the pituitary and inhibits the liberation of prolactin. Ferulic acid (Fig. 5.193) also antagonizes the effect of androgens on the prostate of castrated rats.

Table 5.9 Phenolics studied from medicinal plants.

S.No

Name of the phenolic

Source

1.

Adhyperforin

Hypericum perforatum

2.

Curcumin

Curcuma longa

3.

Filicin or filicic acid

Dryopteris filix-mas

4.

Gossypiol

Gossypium herbaceum

5.

Hypeforin

Hypericum perforatum

6.

Margaspidin

Dryopteris filix-mas

7.

Semimyrtucommulone

Myrtus communis

The oleoresin obtained from rhizome of ginger contains pungent phenylalkylketones or vanillyl ketones, including [6]-gingerol, [6]-paradol, shogaol, and zingerone (Fig. 5.194). They have been reported to possess a strong anti-inflammatory activity. Shogaols are artifacts formed during storage, probably created by a dehydration reaction of the gingerols. The ratio of shogaols to gingerols sometimes is taken as an indication of product quality.

[6]-gingerol ho

[6]-gingerol ho

oh

ho oh oh

[8]-gingerol oh ho oh oh oh

[8]-gingerol

O OH

O OH

Zingerone

Fig. 5.194 Structure of major constituents of ginger oleoresin.

Zingerone

Fig. 5.194 Structure of major constituents of ginger oleoresin.

Gingerol and shogaol (oral; 70-140 mg/kg), are shown to cause vagal stimulation and hence a decrease in both blood pressure and heart rate. 6-gingerol and 6-shogaol suppressed gastric contraction but increased gastrointestinal motility and spontaneous peristaltic activity in laboratory animals.

Gingerol has been found to have a similar structure to acetyl salicylic acid, and these two compounds have similar effects on prostaglandin production. Some of the beneficial medicinal qualities claimed for ginger may stem from zingerone's effectiveness as an antioxidant. Zingerone reacts with free radicals that can cause tissue damage and inflammation.

Topical application of [6]-gingerol or [6]-paradol 30 min prior to 12-O-tetradecanoyl-phorbol-13-acetate (TPA) attenuated the skin papillomagenesis initiated by 7,12-dimethylbenz[a]anthracene in female ICR mice. Galanals A and B, isolated from the flower buds of a Japanese ginger, Zingiber mioga, are potent apoptosis inducers in Human T lymphoma Jurkat cells.

Ginkgolic acid, alkylphenol present in Ginkgo biloba, has significant anxiolytic activity (Fig. 5.195).

Fig. 5.195 Structure of ginkgolic acid.

Anacardic acid (Fig. 5.196) from the bark of Ozoroa insignis inhibits Hep-G2 (human hepatocellular carcinoma), MDA-MB-231 (human mammary adenocarcinoma), and 5637 (human primary bladder carcinoma).

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