| About Herbal Medicines |
 Chemical Constituents of Plants Used as Herbal Medicines |
Chemical Constituents of Plants Used as Herbal Medicines
Alkaloids
A typical alkaloid is chemically basic (alkaline) and contains a secondary or tertiary amine function within a heterocyclic ring (e.g. codeine). Alkaloids may be classified by their chemical skeleta (Figure 1) into the following major types: pyrrolidine (e.g. betonicine from white horehound); pyridine (e.g. gentianine from gentian); piperidine (e.g. lobeline from lobelia); pyrrolizidine (e.g. symphytine from comfrey); quinolizidine (e.g. sparteine from broom); quinoline (e.g. quinine from cinchona); isoquinoline (e.g. boldine from boldo); indole (e.g. harman from passionflower); tropane (e.g. hyoscine from belladonna); imidazole (e.g. pilocarpine from jaborandi); and xanthine (e.g. caffeine from maté). Biosynthetically related compounds that do not follow the above definition of an alkaloid may also be referred to as alkaloids, for example phenylalkylamines that do not contain an N-heterocyclic ring (e.g. ephedrine from ephedra), or that are not basic (e.g. colchicine from colchicum). For further information on alkaloids the reader is referred to other texts (e.g. references 1, 3 and 6).
Glycosides
A glycoside consists of two components, an aglycone (non-sugar) part and a sugar part. The aglycone portion may be of several different types of secondary metabolite (Figure 2), including coumarin (e.g. scopolin from horse-chestnut), flavonoid (e.g. rutin from buchu, or hydroxyanthracene (e.g. cascaroside A from cascara). The sugar moiety is linked to the aglycone by a direct carbon-to-carbon bond (C-glycoside), or through an oxygen-to-carbon bond (O-glycoside). Cyanide glycosides, (e.g. amygdalin from apricot) release toxic hydrogen cyanide when cells are damaged and act as a defence mechanism. Glucosinolates (e.g. sinigrin from horseradish) contain nitrogen and sulphur and are pungent. Hydroxyanthracene glycosides are the active principles of the laxative herbs cascara and senna.
Phenolics
Many of the aromatic constituents of plants contain hydroxy substituents and are phenolic. There is a wide variety of phenolics in medicinal plants and they range in chemical structure from simple phenolic acids (Figure 3), e.g. caffeic acid from artichoke, to complex tannins.
Where chemical structures are included in a monograph they may be the active principles or they may be compounds that can be used as chemical markers for that plant, i.e they are present in significant quantities or are otherwise characteristic for a particular plant. As some compounds are of common occurrence in medicinal plants, their chemical structures are not necessarily included within a monograph. Some of the commonly encountered natural products, including mono-, sesqui-, di- and tri-terpenes, flavonoids and tannins, are briefly summarised below.
Terpenes
Terpenes are derived from two C5 units (isopentane), dimethylallylpyrophosphate and isopentenylpyrophosphate. The monoterpenes contain two isopentane units (C10) and are constituents of many volatile oils. Some of the more common monoterpenes are shown in Figure 4. Sesquiterpenes contain three isopentane units (C15) and occur as different skeletal types, e.g. eudesmane, germacrene, guaiane (Figure 5). A large number of sesquiterpenes contain a ?-lactone ring and these are known as sesquiterpene lactones. Some sesquiterpene lactones are allergenic. Examples of different skeletal types of molecule, including eudesmolide, germacranolide and guaianolide (e.g. constituents of comfrey) and pseudoguaianolide (e.g. matricine, chamomile), are illustrated in Figure 5. Diterpenes are derived from four isopentane units (C20) and examples of abietane (e.g. carnosic acid from sage), daphnane, kavane, labdane (e.g. rotundifarine from agnus castus), taxane and tigliane are given in Figure 6. The ginkgolides from ginkgo are examples of complex diterpenes. Triterpenes are derived from six isopentane units (C30), and some commonly occurring compounds are illustrated in Figure 7, e.g. campesterol, ?-sitosterol, stigmasterol, ?- and ?-amyrin, oleanoic and ursolic acids. Cardiac glycosides (cardenolides) and saponins are examples of triterpenes that are less widely distributed in plants than the triterpenes illustrated in Figure 7.
Flavonoids
Flavonoids are biosynthesised from a phenylpropane unit (C6–C3), derived via shikimic acid and phenylalanine, and a C6 unit from three molecules of malonyl–CoA. They are widely distributed in the plant kingdom and occur in many medicinal plants. There are five major types: chalcones, flavanones, flavones, flavonols and anthocyanins (Figure 8). The flavones and their 3-hydroxy analogues (flavonols) are the most widespread. The five aglycones kaempferol, quercetin, myricetin, apigenin and luteolin, as well as the quercetin glycosides quercitrin and rutin are among the most commonly present in medicinal plants (Figure 9).
Tannins
Tannins are also common constituents of many medicinal plants and they occur as two major types – the hydrolysable tannins and the non-hydrolysable (condensed) tannins. The hydrolysable tannins are esters of sugars with phenolic acids and they are either gallotannins (galloyl esters of glucose), e.g. pentagalloyl glucose, or ellagitannins (hexahydrodiphenic acid, derived from two units of gallic acid, esters with glucose), e.g. agrimoniin from agrimony. Non-hydrolysable tannins, also known as condensed tannins or proanthocyanidins, are polymers of catechin or gallocatechin linked by C-C bonds (e.g. cola tannins). Examples of some chemical structures of hydrolysable and non-hydrolysable tannins are given in Figure 10.
