Volatile Pheromones

Social insects, living close together in colonies need for communication volatile substances that spread a message quickly and are then dispersed. Many of these substances are volatile acetogenins, made from a mixture of acetate and propionate units. Little work has been done to study their biosynthesis, but it can be easily predicted. A mixture of 2-heptanone (Figure 4.6) and 2-heptanol, or 3-octanone and 3-octanol are often present together as pheromone mixtures. To make 3-octanone requires three acetate units and one propionate unit (Figure 4.8). Alcohol dehydrogenase enzymes are usually present in the glands reducing the initial ketone to a mixture of alcohol and ketone.

3-octanone

Figure 4.8 Illustrating the formation of 3-ketones, typical of ant pheromones. The polyketide shown in square brackets does not have any real existence

3-octanone

Figure 4.8 Illustrating the formation of 3-ketones, typical of ant pheromones. The polyketide shown in square brackets does not have any real existence

The polyketide synthases of micro-organisms are rather unspecific for their substrate and supplying them with a slightly different substrate from the normal can induce them to make different products. If the same is true of insect PKS enzymes, it may explain why a mixture of simple volatile alcohols and ketones of different chain length are frequently found together. For example, the mandibular glands of the ant Myrmica scabrinodis contain 3-hexanol, 3-heptanol, 3-octanol, 6-methyl-3-octanol, 3-nonanol, 3-decanol and 3-undecanol and all their corresponding ketones.

The simple substance 4-methyl-3-heptanone (Figure 4.9) is found in secretory glands of a wide variety of insects, including the mandibular glands of several species of leaf-cutting ant. It can serve to illustrate how there are alternative biosynthetic routes possible. The molecule can be seen as composed of three propionate units, but these could be joined in two ways, as shown. The first is more likely, since the second breaks the rule that the keto-group is retained next to the carboxyl that is lost as carbon dioxide. Note that each branch methyl group usually introduces a new chiral centre, unless there is a double bond attached as in Figure 4.10. The final structure shown in Figure 4.9 is (S)-(+)-4-methyl-3-heptanone, first identified as the trail pheromone of two leaf-cutting ants, Atta texana and A. cephalotes. This natural enantiomer is 200 times more active in inducing trail-following than the unnatural

3-octanone

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