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3-octanone

(S)-(+)-4-methyl-3-heptanone

Figure 4.9 The biosynthesis of 4-methyl-3-heptanone can be supposed to proceed through either of two routes. The imaginary polyketide intermediates are shown, although the former route is more likely because it allows decarboxylation of a [i-ke to-acid at the final stage. The final structure shown in this figure is fS)-(+)-4-methyl-3-heptanone, an ant trailpheromone

(i?)-(-)-enantiomer. Subsequently this same substance has been identified in other ants (frequently in mandibular glands), wasps, caddisflies (Trichoptera), and opilionids.

The opilionids (daddy-long-legs or harvestmen), which belong, with the spiders, to the class Arachnida, have defensive glands on the tops of their bodies. Eight species produce a mixture of short-chain branched alcohols and ketones (Figure 4.10), of the type often used as ant pheromones. The biosynthesis has not been studied, but they evidently are made largely from propionate with some acetate. Other opilionids secrete quinones.

Figure 4.10 Alcohols and ketones from opilionid defensive secretions, apparently made from acetate and propionate units

Figure 4.10 Alcohols and ketones from opilionid defensive secretions, apparently made from acetate and propionate units

The biosynthesis of the major component of the male-produced aggregation pheromone of Carpophilus freemani beetles has been studied in detail by deuterium labelling. It was clear that the molecule was made up of one acetate unit, one propionate and two butyrates. The labelling pattern indicated that decarboxylation was not the final step as would be expected, which left two possibilities, shown in Figure 4.11.

Some further examples of compounds from the aggregation pheromones of Carpophilus beetles are given in Figure 4.12. These are all

Figure 4.11 Two possibilities for the formation of a Carpophilus beetle aggregation compound suggested by the investigators knowing that the last step is not decarboxylation. The broad lines indicate the individual acid units for clarity. Reduction of car bony I groups and dehydration of the resulting alcohols are required from the intermediates to give the final products o o

Figure 4.11 Two possibilities for the formation of a Carpophilus beetle aggregation compound suggested by the investigators knowing that the last step is not decarboxylation. The broad lines indicate the individual acid units for clarity. Reduction of car bony I groups and dehydration of the resulting alcohols are required from the intermediates to give the final products

Figure 4.12 Structures of some other aggregation compounds from Carpophilus beetles.

All compounds except c start with acetate, c begins with propionate. They are then extended with propionate (to give methyl branches) or butyrate (for ethyl branches). Compounds a, b and d terminate with propionate and all the others with butyrate

Figure 4.12 Structures of some other aggregation compounds from Carpophilus beetles.

All compounds except c start with acetate, c begins with propionate. They are then extended with propionate (to give methyl branches) or butyrate (for ethyl branches). Compounds a, b and d terminate with propionate and all the others with butyrate biosynthesized from the same units of acetate, propionate and butyrate. The beetles are pest species and the pheromones have been field-tested.

Occasionally one might be led astray when looking at oxygenated alkyl compounds with branched chains. The two compounds shown in Figure 4.13 were found in the mandibular gland of the ant Dinoponera australis. They are both simply the two possible aldol condensation products of isobutyraldehyde. The clue to their origin was given by other compounds accompanying them which also contained isobutyraldehyde groups.

While myrmicine ants generally accumulate their trail pheromones in a part of the poison apparatus (the Dufour gland or the venom reservoir),

(£)-2-isopropyl-5-methyl-2-hexenal (Z)-2-isopropyl-5-methy!-2-hexenal

Figure 4.13 Two branched compounds from an ant which are not polyketides but aldol condensation products of isobutyraldehyde

(£)-2-isopropyl-5-methyl-2-hexenal (Z)-2-isopropyl-5-methy!-2-hexenal

Figure 4.13 Two branched compounds from an ant which are not polyketides but aldol condensation products of isobutyraldehyde formicine (non-stinging, formic acid-spraying) ants use the rectal bulb of the hindgut as the source. Two groups of lactone trail pheromones have been identified in formicine ants, one aromatic, of the isocoumarin type, including mellein (see Figure 4.3), and the other aliphatic, of the 5-lactone type (Figure 4.14). Since mellein is found in so many places, and it could be taken up from food, experiments were first made feeding deuterated mellein to three species, Camponotus rufipes, C. silvicola and Lasius niger. Deuterated mellein was isolated from the rectum, but no labelling was found in the other compounds. Similar experiments were made with deuterated methionine, to see if methylation of mellein was occurring, but there was no labelling observed. When deuterated acetic acid was fed as the sodium salt, the lactones were all labelled, but there was insufficient material to determine just where the deuterium was. With [3-2H3]propionic acid it was clear from the mass spectra of the products that CD3 groups were incorporated as indicated in Figure 4.14. These pheromones evidently are of polyketide origins. The compound known as invictolide (Figure 4.14 f) was first identified as part of the queen recognition pheromone of Solenopsis invicta ants and later shown also to be part of the trail pheromone of some Camponotus ants. It is not known if these compounds are made via type I or II polyketides.

Related in structure is 8-hydroxyisocoumarin, also called centipedin (Figure 4.15). It is an antibiotic substance isolated from the centipede Scolopendra subspinipes multilans (Plate 5). Although centipede bites can

Figure 4.14 Some lactone acetogenin trail pheromones made from acetate and propionate units. The CD3 groups indicate where the compounds were labelled when the ants were fed with sodium [3-2H}]propionate. Mellein, compound a, is the trail pheromone o/Lasius fuliginosus and Formic rufa. Compound b is the pheromone of Camponotus rufipes and c is the pheromone of C. silvicola, C. inequalis and Lasius niger. Three species of Camponotus use compound d, while C. herculeanus uses a mixture of d andf, and C. ligniperdus has all three ofd, e and f. Compounds g, h and i are all found in some of these species, but appear not to act as pheromone components

Figure 4.14 Some lactone acetogenin trail pheromones made from acetate and propionate units. The CD3 groups indicate where the compounds were labelled when the ants were fed with sodium [3-2H}]propionate. Mellein, compound a, is the trail pheromone o/Lasius fuliginosus and Formic rufa. Compound b is the pheromone of Camponotus rufipes and c is the pheromone of C. silvicola, C. inequalis and Lasius niger. Three species of Camponotus use compound d, while C. herculeanus uses a mixture of d andf, and C. ligniperdus has all three ofd, e and f. Compounds g, h and i are all found in some of these species, but appear not to act as pheromone components h

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