Two types of chamomile (camomile) are commonly employed in herbal medicine, Roman chamomile Chamaemelum nobile (formerly Anthemis nobilis) (Compositae/Asteraceae), and German chamomile Matricaria chamomilla (Chamomilla recutica) (Compositae/Asteraceae). German chamomile, an annual plant, is the more important commercially, and is often called matricaria to distinguish it from the perennial Roman chamomile. Both plants are cultivated
in various European countries to produce the flowerheads, which are then dried for drug use. Volatile oils obtained by steam distillation or solvent extraction are also available.
Roman chamomile is usually taken as an aqueous infusion (chamomile tea) to aid digestion, curb flatulence, etc, but extracts also feature in mouthwashes, shampoos, and many pharmaceutical preparations. It has mild antiseptic and anti-inflammatory properties. The flowerheads yield 0.4-1.5% of volatile oil, which contains over 75% of aliphatic esters of angelic, tiglic, isovaleric, and isobutyric acids (Figure 5.29), products of isoleucine, leucine, and valine metabolism (see pages 100, 295, 306), with small amounts of monoterpenes and sesquiterpenes. Matricaria is also used as a digestive aid, but is mainly employed for its anti-inflammatory and spasmolytic properties. Extracts or the volatile oil find use in creams and ointments to treat inflammatory skin conditions, and as an antibacterial and antifungal agent. Taken internally, matricaria may help in the control of gastric ulcers. The flowers yield 0.5-1.5% volatile oil containing the sesquiterpenes a-bisabolol (10-25%), bisabolol oxides A and B (10-25%) (Figure 5.28), and chamazulene (0-15%) (Figure 5.32). Chamazulene is a thermal decomposition product from matricin, and is responsible for the dark blue coloration of the oil (Roman chamomile oil contains only trace amounts of chamazulene). a-Bisabolol has some anti-inflammatory, antibacterial, and ulcer-protective properties, but chamazulene is probably a major contributor to the anti-inflammatory activity of matricaria preparations. It has been found to block the cyclooxygenase enzyme in prostaglandin biosynthesis (see page 55) and the anti-inflammatory activity may result from the subsequent inhibition of leukotriene formation.
to produce the decalin ring system. In this case, a six-membered ring is most likely formed first giving the bisabolyl cation, and, again, a 1,3-hydride shift is implicated prior to forming the decalin system (Figure 5.27). Amorpha-4,11-diene is an intermediate in the pathway leading to artemisinin in Artemisia annua (Com-positae/Asteraceae) (Figure 5.34). This proceeds through artemisinic acid and dihydroartemisinic acid via modest oxidation and reduction processes. Dihydroartemisinic acid may be converted chemically into artemisinin by an oxygen-mediated photochemical oxidation under conditions that might normally be present in the plant, suggesting that all further transformations may in fact be non-enzymic. An intermediate in this process also found naturally in A. annua is the hydroperoxide of dihydroartemisinic acid. The further modifications postulated in Figure 5.34 include ring expansion by cleavage of this hydroperoxide and a second oxygen-mediated hydroperoxidation. The 1,2,4-trioxane system in artemisinin can be viewed more simply as a combination of hemiketal, hemiacetal, and lactone functions, and the later stages of the pathway merely reflect their construction. Artemisinin* is an important
cadinyl cation a-cadinene
one of the many terpenoids found in juniper berries (Juniperus communis; Cupressaceae) used in making gin, and this compound is derived from the ten-carbon ring-containing cis-germacryl cation. The double bonds in the cis-germacryl cation are unfavourably placed for a cyclization reaction as observed with the germacryl cation, and available evidence points to an initial 1,3-shift of hydride to the isopropyl side-chain generating a new cation, and thus allowing cyclization (Figure 5.27). Amorpha-4,11-diene (Figure 5.34) is structurally related to a-cadinene, but the different stereochemistry of ring fusion and site of the second double bond is a consequence of a different cyclization mechanism operating
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