Many unsaturated compounds found in nature contain one or more acetylenic bonds, and these are predominantly produced by further desaturation of olefinic systems in fatty acid-derived molecules. They are surprisingly widespread in nature, and are found in many organisms, but are especially common in plants of the Compositae/Asteraceae, the Umbelliferae/Apiaceae, and fungi of the group
Basidiomycetes. These compounds tend to be highly unstable and some are even explosive if sufficient amounts are accumulated. Since only very small amounts are present in plants, this does not present any widespread hazard. Whilst fatty acids containing several double bonds usually have these in a non-conjugated array, molecules containing triple bonds tend to possess conjugated unsaturation. This gives the compounds intense and highly characteristic UV spectra which aids their detection and isolation.
The processes of desaturation are exemplified in Figure 3.9, in which oleic acid (probably as a thiol ester) features as a precursor of crepenynic acid and dehydrocrepenynic acid. The acetylenic bond is now indicated by a in the semi-systematic shorthand nomenclature. Chain shortening by P-oxidation (see page 18) is often a feature of these pathways, and formation of the C10 acetylenic acid dehydromatricaria acid proceeds through C18 intermediates, losing eight carbons, presumably via four P-oxidations. In the latter part of the pathway, the Z -double bond from oleic acid moves into conjugation with the polyacety-lene chain via an allylic isomerization, giving the more favoured E -configuration. Some noteworthy acetylenic structures (though they are no longer acids and components of fats) are given in Figure 3.10. Cicutoxin from the water hemlock (Cicuta virosa; Umbelliferae/Apiaceae) and oenanthotoxin from the hemlock water dropwort (Oenanthe crocata; Umbelliferae/Apiaceae) are extremely toxic to mammals, causing persistent vomiting and convulsions, leading to respiratory paralysis. Ingestion of the roots of these plants may frequently lead to fatal poisoning. Falcarinol is a constituent of Falcaria vulgaris (Umbelliferae/Apiaceae), Oenanthe crocata, Hedera helix (Araliaceae), and several other plants, and is known to cause contact dermatitis in certain oleic acid 18:1 (9c)
,co2h crepenynic acid 18:2 (9c,12a) t dehydrocrepenynic acid 18:3 (9c,12a,14c)
allylic isomerization dehydromatricaria acid 10:4 (2t,4a,6a,8a)
individuals when the plants are handled. Falcarinol (sometimes called panaxynol) and the structurally related panaxytriol are also characteristic poly-acetylene components of ginseng (Panax ginseng; Araliaceae) (see page 222). Wyerone from the broad bean (Vicia faba; Leguminosae/Fabaceae) has antifungal properties, and its structure exemplifies how the original straight chain may be cross-linked to produce a ring system. The furan ring may originate from a conjugated diyne.
The herbal preparation echinacea* is derived from the roots of Echinacea purpurea (Compositae/ Asteraceae) and is used for its immunostimu-lant properties, particularly as a prophylactic and treatment for the common cold. At least some of its activity arises from a series of alkylamides, amides of polyunsaturated acids with isobutylamine. These acids are predominantly C11 and C12 diene-diynes (Figure 3.11).
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