APLICACIÓN DE RECUBRIMIENTOS

Secondary compounds such as those constituting resin differ from primary metabolites in having a restricted distribution in the plant kingdom. Usually, they occur only in particular groups of related plants. Terpenoid resin occurs in most conifer families but is widely scattered among the major evolutionary lineages of angiosperms (Chapter 2). Specific terpenoid skeletal types, how-ever, often characterize taxa such as particular families and genera; thus it has been assumed that the evolutionary history of various taxa can be significant to the understanding of the taxonomic distribution of some of these chemicals (Gershenzon and Mabry 1983).

I introduce a few skeletal structures in this chapter to exemplify compo-nents of resins in important conifer and angiosperm plant families, discussed

TERPENOID RESINS | 35

as of value either to the plants themselves or to humans in later chapters.

Conifers only produce internally secreted terpenoid resin whereas angio-sperms produce both terpenoid and phenolic resins, which may be secreted in-ternally or on the surface of the plant. This is discussed in detail in Chapter 3.

In addition to the skeletal structure of the compounds, the complexity of the mixture of compounds constituting a resin is important for ecological interactions and human use. In general, among the 20–50 or more com-pounds that constitute a resin, only a few occur in high concentration. The relative proportions of the compounds in the mixture are called its composi-tion, which may differ in constitutive and induced resins. Because this mix-ture involves volatile and nonvolatile fractions, the composition of either fraction (or just part of it) or both fractions may be analyzed and compared.

The volatile fraction, which has been most intensively studied, usually consists of mono- and/or sesquiterpene hydrocarbons with some oxygenated forms and, occasionally, diterpene hydrocarbons. The nonvolatile fraction of resin is primarily composed of di- or triterpene acids with some alcohols, aldehydes, and esters in addition to amorphous, neutral substances. The rela-tive proportion of volatile to nonvolatile compounds, which can vary even between species of the same genus, determines a resin’s fluidity, viscosity, and polymerization rate. These in turn influence its ecological properties (Chap-ter 5) as well as the methods used by humans to collect it (Chap(Chap-ters 7–10).

Conifer Resins

Conifer resins, such as those of the pine family (Pinaceae), are characterized by a large volatile fraction (20–50%) with monoterpenes predominating over sesquiterpenes. Both classes most commonly occur as hydrocarbons with a few oxidized forms, often as trace components. Under natural conditions, monoterpenes volatilize with varying degrees of rapidity, providing, for exam-ple, the fragrant aromas in conifer forests during warm weather and those from indoor Christmas trees. In fact, monoterpene hydrocarbons from these resins may reach significant proportions in our atmosphere and become trou-blesome as pollutants. In the soil, monoterpenes from resin may play a role in the nitrogen cycle in conifer forests by inhibiting nitrification. On the other hand, some may supply an energy source for forest soil microbes (Shukla et al.

1968), and others washed from conifer forest soils into estuaries may provide energy for marine microbes (Button 1984). These volatile components of

ter-36 | CHAPTER 1 What Plant Resins Are

penoid resin (both mono- and sesquiterpenes) play a major defensive role against insects and pathogens in amazingly intricate ways (Chapter 5). In com-mercial use in the naval stores industry, the volatile mono- and sesquiterpenes of pine resin produce turpentine, a product used worldwide in solvents and as a feedstock for the flavor and fragrance industries (Chapter 7). Sesquiterpenes (e.g., cedrene) are used as cedarwood oil, again particularly in the aroma industry. Structures of some of the most common volatile mono- and sesqui-terpenes in various conifer resins are shown in Figure 1-3. Note that the abun-dant monoterpenes are often the ones produced by multiproduct synthases.

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 com-monly 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).

Abietane Type Pimarane Type

Labdane Type

Kaurane Type COOH

Abietic acid Pimaric acid

COOH

Communic acid

COOH COOH

Ozoic acid

O

COOH

Hardwickiic acid Clerodane Type

COOH Trachylobanic acid

TERPENOID RESINS | 37

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 pre-served in fossilized resin. That is, certain terpenoids polymerize and, hence, are able to withstand degradation under certain depositional conditions, form-ing amber (Chapter 4). Extensive accumulations of fossilized resin are signif-icant 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 coni-fer 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).

Angiosperm Resins

Although monoterpenes predominate in the volatile fraction of the resin of the chemically best known conifers, such as Pinaceae, sesquiterpenes gener-ally 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

38 | CHAPTER 1 What Plant Resins Are

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 predomi-nates, resulting in a more viscous resin used for varnishes (Chapter 9).

On the other hand, the volatile fraction of resins in the large tropical fam-ily 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 frame-works (Figures 1-3 and 1-5). Aregullin et al. (2002) found a sesquiterpene lac-tone (8-β-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 (Chap-ter 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 α- and β-amyrins (Figure 1-5) occur in other plants, they are known to be components of resins only in the Burseraceae, where they are common. Inter-estingly, in Bursera, diterpenes occasionally occur along with triterpenes (Becerra et al. 2001).

TERPENOID RESINS | 39

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 pri-marily 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 numer-ous 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-Figure 1-5. Examples of some structural types common in triterpenoid resin compo-nents in the large tropical families Burseraceae, Dipterocarpaceae, and Anacardiaceae (Chapters 8–10). R = COOH Betulinic acid

R = CH₃ α-Amyrin R = COOH Ursolic acid

R = CH₃ β-Amyrin R = COOH Oleanolic acid

40 | CHAPTER 1 What Plant Resins Are

penes (Figure 1-6); (–)-ngaione is the best known because it is toxic to live-stock, but freelingyne was the first acetylenic terpene isolated from natural sources. In contrast, species of the large genus Eremophila (Plate 21) accumu-late 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 struc-tural 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.