P-Amyrin Oleanolic acid
Figure 1-5. Examples of some structural types common in triterpenoid resin components in the large tropical families Burseraceae, Dipterocarpaceae, and Anacardiaceae (Chapters 8-10).
penes (Figure 1-6); (-)-ngaione is the best known because it is toxic to livestock, but freelingyne was the first acetylenic terpene isolated from natural sources. In contrast, species of the large genus Eremophila (Plate 21) accumulate quantities of diterpenes that are all structurally and stereochemically unique. These diterpenes exhibit configurational differences from those of conifers and angiosperms (particularly legumes), with labdane, abietane, pimarane, and kaurane skeletons (Figure 1-4) that arise along the pathway to the physiologically necessary gibberellins (Figure 1-2). Thus Richmond and Ghisalberti (1994) suggested the possibility that diterpenes in Eremophila are synthesized by processes different from those observed in most terrestrial plants. Among the novel diterpenes, Eremophila generates numerous structural types (e.g., bisabolane, serrulatane, cedrane, and eremane) that bear resemblance to sesquiterpenes (Figure 1-6). Eremophila resin is an example of the amazing diversity of structural types that can occur even within one genus; other such cases may become evident as more resins are analyzed in detail.
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