COOH Trachylobanic acid
Figure 1-4. Some common diterpene resin acids. Those with abietane and pimarane structural types characterize conifer resins whereas those with labdane structural types occur commonly in both conifers and angiosperms. The conjugated diene in communic acid in conifers and ozoic acid in angiosperms enables polymerization and, hence, formation of amber (Chapter 4).
Nonvolatile terpenes in conifers are primarily diterpene acids. In pines, these diterpenes constitute what is known commercially as rosin, which has numerous uses but especially as a source of intermediate chemicals in various industries (Chapter 7). The nonvolatile fraction increases the viscosity of the resin, which can enhance the possibility of engulfing herbivores and other organisms visiting the tree. Such trapped organisms can be beautifully preserved in fossilized resin. That is, certain terpenoids polymerize and, hence, are able to withstand degradation under certain depositional conditions, forming amber (Chapter 4). Extensive accumulations of fossilized resin are significant components of some coals and even petroleum deposits (Chapter 9).
Diterpenes in conifer resins are characterized by three main skeletal types (abietane, pimarane, and labdane) that vary quantitatively in different conifer families (Chapter 2). Abietane- and pimarane-type diterpenic acids, for example, abietic and pimaric acids (Figure 1-4), are most abundant in resins of Pinaceae, remaining relatively soft and unpolymerized. However, resins with abietane-type compounds may sometimes become relatively solid with a hard surface, probably as a result of an abietadiene precursor that is prone to polymerization. On the other hand, labdane-type acids, such as communic and agathic acids, may contain conjugated diene compounds that readily polymerize. Labdane-type compounds are the primary diterpene constituents in the cedar family (Cupressaceae). All three skeletal types occur in resins of the araucarian family (Araucariaceae) although large quantities of labdanes in Agathis result in the production of very hard copals as well as amber (Chapters 4 and 9). In the Podocarpaceae and Cupressaceae s.l. (Chapter 2), an oxidation rearrangement leads to the formation of phenolic diterpenes such as ferruginol and totarol (Thomas 1990).
Although monoterpenes predominate in the volatile fraction of the resin of the chemically best known conifers, such as Pinaceae, sesquiterpenes generally dominate the volatile composition in most, but not all, flowering plants. For example, the volatile fraction in numerous genera of tropical trees in the legume family (Fabaceae, or Leguminosae, Chapter 2) consists of sesquiter-penes that most often occur as hydrocarbons (Figure 1-3). Caryophyllene is an example of a sesquiterpene that commonly occurs in angiosperm resins. The volatile fraction of resins from the large tropical family Dipterocarpaceae also is composed of sesquiterpenes, similar to those in leguminous resins. In both families there are genera in which the volatile fraction predominates, thus producing a more fluid resin that has been used medicinally and for fuel oil (Chapter 7), whereas in other genera the nonvolatile fraction predominates, resulting in a more viscous resin used for varnishes (Chapter 9).
On the other hand, the volatile fraction of resins in the large tropical family Burseraceae is much more diverse than that of resins of legumes and diptero-carps. It contains large proportions of both mono- and sesquiterpenes, giving it the characteristic high degree of fragrance when used for incense (Chapter 8). Monoterpenes that commonly occur in conifer resins are important in burser-aceous resins, along with numerous sesquiterpenes with diverse skeletal frameworks (Figures 1-3 and 1-5). Aregullinetal. (2002) found a sesquiterpene lactone (8-P-hydroxasterolide) in Trattinnickia resin. This is the first report of a sesquiterpene lactone, so common in the Asteraceae, in Burseraceae.
Diterpenes are the dominant components in the nonvolatile fraction of leguminous resins. They form the very hard copals used for varnishes (Chapter 9) because of the presence of labdadiene-type acids (or alcohols) such as ozoic acid (Figure 1-4) or zanzibaric acid, which are enantiomers of commu-nic acid. These components also can lead to fossilization of the resin in the legume Hymenaea, as they do in the conifer Agathis (Chapter 4). Leguminous resins also contain numerous other diterpenoids that do not polymerize, such as the clerodane-type hardwickiic acid.
In some angiosperm families, triterpenes rather than diterpenes dominate the nonvolatile composition of the resin. For example, triterpenes primarily with tetra- or pentacyclic skeletons (Figure 1-5) characterize resins from the large tropical families Burseraceae, Dipterocarpaceae, and Anacardiaceae. Resins from Burseraceae typically have tetracyclic euphane / tirucallane, and pentacyclic lupane, ursane, and oleanane triterpene skeletal types (Khalid 1985). Other structural types have been found in species of the chemically complex myrrh-producing genus Commiphora (Waterman and Ampoto 1985), however, emphasizing the great structural diversity of triterpenoids in Burseraceae. They have been much used medicinally (Chapter 8). Although a- and P-amyrins (Figure 1-5) occur in other plants, they are known to be components of resins only in the Burseraceae, where they are common. Interestingly, in Bursera, diterpenes occasionally occur along with triterpenes (Becerra et al. 2001).
Although the nonvolatile fraction of dammar resins from the Dipterocar-paceae also consists largely of triterpenes, the skeletal types are different from those of Burseraceae; the nonvolatile fraction of dipterocarps consists primarily of the tetracyclic dammarane series (Figure 1-5). The volatile fraction is composed of sesquiterpenes; cadinenes in some taxa may polymerize to form bicadinenes, structurally considered as triterpenoids (Chapter 4). Resins from certain genera of Anacardiaceae have some triterpene components in common with those of Dipterocarpaceae, but they are generally more numerous and have not been completely characterized (Mills and White 1994).
The structures of more than 200 terpene compounds elucidated by Ghis-alberti (1994) from the Australian resin-producing shrub family Myopor-aceae demonstrate the complexity that can occur in one family of only three genera. Myoporum, a small genus, is characterized by furanoid sesquiter-
Lupane Type Ursane Type
R = CH2OH Betulin R = COOH Ursolic acid
R = COOH Betulinic acid
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