Simple Phenols

Most of the simple phenols are monomeric components of the polymeric polyphenols and acids which make up plant tissues, including lignin, melanin, flavolan, and tannins. These individual components are obtained by acid hydrolysis of plant tissues. The components include p-hydroxybenzoic acid, protocatechuic acid, vanillic, syringic, and salicylic and gallic acids. Free phenols which do not require degradation of cell-wall polymers are relatively rare in plants. Hydro-quinone, catechol, orcinol, and other simple phenols are found in relatively low concentrations (Figure 1.18).

FIGURE 1.18 Some phenolic plant natural products.

FIGURE 1.18 Some phenolic plant natural products.

Thymol, which comprises 30 to 80% of the essential oil of thyme (Thymus vulgaris), has been used as an expectorant.

1.3.2.2 Phenol Ethers

Many of the phenols also exist as their methyl ethers. For illustration, a few are shown in Figure 1.19. Khellin and visnagin are the active coumarin derivatives of the ammi visnaga fruit (Ammi visnaga). Trans Anethole is chiefly responsible for the taste and smell of anise seeds (Pimpinella anisum). Apiol is a major constituent of the essential oil of parsley seed and is a powerful diuretic.

FIGURE 1.19 Phenol ethers.

1.3.2.3 Phenylpropanoids

As the name implies, the phenylpropanoids contain a three-carbon side chain attached to a phenol. Common examples include the hydroxycoumarins, phe-nylpropenes, and the lignans. Also common are various types of hydroxycin-namic acids, including the caffeic and coumaric acids. Coumarin is common to numerous plants and is the sweet-smelling volatile material which is released from newly mowed hay. The phenylpropenes are not phenols at all since they lack the hydroxyl functionality. Thus, they are not water soluble but rather are essential oils and include eugenol, the major principle of oil of cloves. Anethole and myristicin, the principles of nutmeg, are also representative of this class of compounds (Figure 1.20).

Caffeic and p-coumaric acid are hydroxycinnamic acids present in green and roasted coffee beans. Umbelliferone and scopoletin are coumarin phenyl-propanoids that have been known since 1884 and are isolated from the roots of Scopolia japonica. The phenylpropene, eugenol, has been isolated from several plant sources and has been used as a dental analgesic.

1.3.2.4 Flavonoids

The flavonoids have two benzene rings separated by a propane unit and are derived from flavone. They are generally water soluble compounds. The more conjugated compounds often are brightly colored. They are generally found in plants as their glycosides which can complicate structure determinations. Several classes are shown in Figure 1.21.

The different classes within the group are distinguished by additional oxygen-containing heterocyclic rings and hydroxyl groups. These include the catechins, leucoanthocyanidins, flavanones, flavanonols, flavones, anthocyanidins, flavonols, chalcones, aurones, and isoflavones whose general structures are shown in Figure

^ ■ '.-JIIUIH. hi I k Ml i'ii iK *.<<] <H I -1 :

^ ■ '.-JIIUIH. hi I k Ml i'ii iK *.<<] <H I -1 :

FIGURE 1.20 Examples of plant phenylpropanoids.

FIGURE 1.20 Examples of plant phenylpropanoids.

1.21. Other common groups include the xanthones and the condensed tannins. The catechins and leucoanthocyanidins are structurally very similar and only rarely exist as their glycosides. They polymerize to form condensed tannins which help give tea its color. They also are sufficiently prevalent to darken the color of streams and rivers in some woody areas.

The flavanones and flavanonols are fairly rare and normally exist as their glycosides. The flavones and flavonols are the most widely distributed of all the phenolics. The anthocyanins are the common red and rare blue pigments of flower petals and can make up as much as 30% of the dry weight of some flowers. They exist typically as glycosides. The chalcones, such as butein, lack the pyran ring found in flavonoids, although this is often subject to pH-controlled equilibria. The chalcone is more fully conjugated and normally brighly colored. Phlorizin is a strong inhibitor of apple seedling growth. The aurones are golden yellow pigments common in certain flowers.

Was this article helpful?

0 0
Healthy Chemistry For Optimal Health

Healthy Chemistry For Optimal Health

Thousands Have Used Chemicals To Improve Their Medical Condition. This Book Is one Of The Most Valuable Resources In The World When It Comes To Chemicals. Not All Chemicals Are Harmful For Your Body – Find Out Those That Helps To Maintain Your Health.

Get My Free Ebook


Post a comment