Monoterpene Pheromones

The formation of Qxo-brevicomin (not a terpene). The retention of labelled oxygen from the ketone to brevicomin indicated the epoxide was not openedfirst. The stereochemistry is more complex than shown here 

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None of the monoterpene pheromones of bark beetles is represented by a specific compound per se however, species specificity of the signal is accomplished by qualitatively and quantitatively fine-tuned mixtures including enantiomeric proportions. 

Terpenes are known to play important ecological roles not only in the defense of the tree, but also in bark beetle chemical ecology. Two chapters in this volume (see chapters by Raffa, et al, and Tittiger, et al.) review aspects of terpenoids as chemicals in conifer-bark beetle interactions and isoprenoid formation in bark beetle chemical ecology. While some species of bark beetles may be able to produce monoterpene pheromones de novo, bark beetles can also use host-derived monoterpenes as precursors to their own sex and aggregation pheromones. Bark beetle monoterpene pheromones are often used in a stereospecific manner such that often only one enantiomer is able to serve as a sex or aggregation pheromone. Since both enantiomeric host-derived monoterpene precursors are accepted and chemically modified by the beetle, the exact monoterpene enantiomeric mixture of conifer resin is important to both the beetle s ability to produce the correct pheromone and to the trees ability to escape an attack. ... 

Gas chromatography (GC) has also been used for preparative purposes, but is restricted to relatively volatile racemates such as anesthetics, pheromones or monoterpenes and, therefore, very few applications are reported. Nevertheless, in the cases to which GC may be applied, it could be considered as an economical alternative to HPLC. Most of the resolutions of enantiomers were performed on cyclodex-trin-derived CSPs [109, 144-153], and only on very few occasions were other chiral selectors used [153]. 

Evidence for de novo synthesis of pheromone components was obtained by showing that labeled acetate and mevalonate were incorporated into ipsdienol by male Ips pini [103,104]. Similarly, labeled acetate and other labeled intermediates were shown to be incorporated into frontalin in a number of Dendroctonus species [105]. Possible precursors to frontalin include 6-methyl-6-hep-ten-2-one, which was incorporated into frontalin by D. ruffipennis [106]. The precursor 6-methyl-6-hepten-2-one also was shown to be converted to bre-vicomin in the bark beetle, Dendroctonus ponderosae [107]. In addition, the expression patterns of HMG-CoA reductase and HMG-CoA synthase are tightly correlated with frontalin production in Dendroctonus jeffreyi [108, 109]. A geranyl diphosphate synthase cDNA from I. pini was also isolated, functionally expressed, and modeled [110]. These data indicate that the de novo isoprenoid biosynthetic pathway is present in bark beetles. A variety of other monoterpene alcohols such as myrcenol, pityol, and sulcitol are probably synthesized through similar pathways [111]... 

Attractive Compounds. With the exception of (Z)-3-dodecenyl ( )-2-butenoate 213 (Scheme 24), the female produced sex pheromone of the sweetpotato weevil Cylas formicarius [389], the structures of weevil pheromones are represented by oxygenated monoterpenes, polyketides produced from propanoate units, and branched alcohols and ketones, probably originating from a mixed acetate-propanoate biosynthesis [5]. 

The cases where terpene metabolism has been studied In Insects are very few Indeed. Certain Ips and Dendroctonus bark beetles convert monoterpenes such as a-plnene, 6-plnene and myr-cene to oxidation products, some of which have pheromonal activities ( 5, 3A, 35). A Dendroc tonus bark beetle s cytochrome... 

Many of the monoterpenes found in essential oils of plants also occur as pheromonal substances in insects (45,57-60) and are often involved in plant-insect interactions. Some compounds found both in plants and insects are the monoterpenes citronellal, cit-ronellol, geraniol, myrcene, citral, 6-phellandrene, limonene, 2-terpinolene, a-pinene, 6-pinene, 1,8-cineole, and verbenone. 

Some volatile iridoid monoterpenes with biological activity are also found in essential oils and in insect pheromonal and defensive substances. Eisner ( ) found that 17 species of insects were repelled by the iridoid monoterpene nepetalactone. Lacewings (Chrysopa septempunctata) are attracted by the leaves and fruits of Actinidia polyqama (Actinidiaceae) which contain a series of volatile iridoid monoterpenes (67). 

Thompson A. C. and Mitlin N. (1979) Biosynthesis of the sex pheromone of the male boll weevil from monoterpene precursors. Insect Biochem. 9, 293-294. 

Thus far no sex pheromone has been described in the Phasmida. Some phasmids produce toxic monoterpenes in typical Class III integumentary glands located behind the head (e.g. Happ et al., 1966). The glands exhibit lipid reserves, carboxylic esterases, phosphatases, alcohol dehydrogenase, and a mevalonate kinase, all of which are suggested to be involved in the production of the toxic compounds (Happ et al., 1966). 

Figure 6.11 Biosyntheses of isoprenoid pheromone components by bark and ambrosia beetles from host conifer monoterpenes. (A) Conversion by the male California fivespined ips, Ips paraconfusus Lanier (Coleoptera Scolytidae), of myrcene from the xylem and phloem oleoresin of ponderosa pine, Pinus ponderosa Laws., to (4S)-(+)-ipsdienol and (4S)-(-)-ipsenol, components of the aggregation pheromone (Hendry et al., 1980). (B) Conversion by male and female I. paraconfusus of (1 S,5S)-(-)-a-pinene (2,6,6-trimethyl-bicyclo[3.1,1]hept-2-ene) from the xylem and phloem oleoresin of P. ponderosa to (1 S,2S,5S)-(+)-c/s-verbenol (c/s-4,6,6-trimethyl-bicyclo[3.1,1]hept-3-en-2-ol), an aggregation pheromone synergist and of (1 R,5R)-(+)-a-pinene to (1 fl,2S,5fl)-(+)-frans-verbenol (frans-4,6,6-trimethyl-bicyclo[3.1,1]hept-3-en-2-ol), a compound of unknown behavioral activity for /. paraconfusus. Male and female western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera Scolytidae), convert (1 S,5S)-(-)-a-pinene to (1S,2ft,5S)-(-)-frans-verbenol, an aggregation pheromone interruptant and (1R,5R)-(+)-a-pinene to (1 R,2S,5R)-(+)-frans-verbenol, a compound of...

Byers J. A., Birgersson G., Lofqvist J. and Bergstrom G. (1988) Synergistic pheromones and monoterpenes enable aggregation and host recognition by a bark beetle. 

Erbilgin N. and Raffa K. F. (2000) Opposing effects of host monoterpenes on responses by two sympatric species of bark beetles to their aggregation pheromones. J. Chem. Ecol. 26, 2527-2548. 

Hughes P. R. (1973a) Dendroctonus. Production of pheromones and related compounds in response to host monoterpenes. Z. angew. Entomol. 73, 294—312. 

Some early and obvious questions about bark beetle pheromone components concerned their origins are they synthesized de novo from acetate or derived from plant precursor molecules And are the biochemical reactions performed by insect tissues or symbiotic bacteria Ipsdienol and ipsenol are clearly monoterpenoid alcohols. Since de novo monoterpenoid biosynthesis was unprecedented in the Metazoa before 1995, and monoterpenes are produced by host trees, it seemed logical that monoterpenoid pheromone components were derived from ingested plant precursor molecules (reviewed in Vanderwel and... 

Sigmatropic rearrangements catalyzed by silver salts have been used in total synthesis. The synthesis of three monoterpene compounds that comprise the pheromone of male boll weevil Anthonomus grandis has been achieved using the silver-carbonate catalyzed rearrangement of an acetoxycyclohexyl acetylene as the key step (Scheme 3.35).56... 

However, whereas the silkworm female appears to attract males with a single sex pheromone, many other insects use blends of pheromones as chemical releasers of behavior. This phenomenon is strikingly illustrated in the case of males of the bark beetle Ips paraconfusus (=Ips confusus) which utilize three monoterpene alcohols as an aggregation pheromone (11). Maximum attraction of beetles in the field was exhibited in the presence of a mixture of all three compounds, whereas single or pairs of compounds were considerably less active (12). Similarly, in laboratory bioassays, mixtures of compounds were vastly superior to single constituents as attractants (13). 

Cephalic alarm pheromones are secreted by soldiers of termites in the genera Drepanotermes and Amitermes (29). These compounds have been identified as limonene and terpinolene, two of several monoterpene hydrocarbons forming part of the defensive battery of these insects. 

The frass of females of the western pine beetle Dendroctonus brevicomis is enriched with endo- (XIX) and exo-7-ethyl-5-methyl-6,8-dfoxabicyclot3.2.1]octane (XX), which are assigned the trivial epithets endo- and exo-brevicomin (61). Males of 2 brevicomis synthesize 1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane (frontalin) (XXI) in their hind guts (62), and this compound, in combination with the brevicomins and host-derived myrcene, constitutes a potent attractant for both sexes of J). brevicomis (63). Frontal in, also produced by females of . frontalis (62T7 is reported to function as a powerful aggregative pheromone when combined with host monoterpenes such as a-pinene (64). J). pseudotsugae... 

Stingless bees lay chemical trails with mandibular gland constituents which have been identified as normal aliphatic alcohols or monoterpene aldehydes. Trigona spinipes generates a trail with a mixture of 2-heptanol, 2-undecanol, and 2-tridecanol, and it has been possible to successfully lay artificial trails with these alcohols (136). Trail following in workers of Trigona subterranea is released by citral (130), the stereoisomers of which are also utilized as alarm pheromones and defensive compounds. Such pheromonal parsimony appears to be especially typical of eusocial bees and ants. 

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