Coleoptera

The Coleoptera (beetles) contains about 300,000 species and are very varied in form, diet and habitat. Equally they have very different kinds of pheromones and defensive secretions, many of them derived from fatty acids. The bean weevil Acanthoscelides obtectus uses an unusual allenic methyl ester (Figure 3.25). Its biosynthesis has not yet been investigated. Alienes are an unusual example of chirality. AnĂ³mala cuprea, a chafer grub produces two lactone sex attractants, derived from the unsaturated acids oleic and palmitoleic acids (Figure 3.25). Both are shortened by the loss of two acetic acid units, then oxidized (stereospecifically) at the allylic position and cyclized to the pheromone.

Figure 3.25 The unusual allenic ester pheromone of the bean weevil Acanthoscelides obtectus, and formation of the two-component sex pheromone of Anomala cuprea from a mixture of oleic and palmitoleic acids

Figure 3.25 The unusual allenic ester pheromone of the bean weevil Acanthoscelides obtectus, and formation of the two-component sex pheromone of Anomala cuprea from a mixture of oleic and palmitoleic acids

3.4.1 Coccinellines

Coccinellid beetles, commonly called ladybirds, which prey on aphids, make a range of defensive compounds. They are produced in the fat body and stored in the haemolymph. When a ladybird is disturbed, it exudes some of this toxic haemolymph (this is called reflex bleeding) at its leg joints. One group of such toxic compounds are the coccinellines, tricyclic amines and their N-oxides (Figure 3.26). The coccinellines were first shown to be made from acetate units using feeding experiments by radio-labelling with sodium [l-14C]acetate and [2-14C]acetate in 1975. Recently the experiments were repeated with in vitro studies using excised body parts of ladybirds that gave much higher incorporation of the radio-label (Laurent, Braekman, Daloze and Pasteels, Insect Biochemistry and Molecular Biology, 2002, 32, 1017). The fatty acid route was demonstrated because the formation of coccinelline was inhibited by 2-octynoic acid, a known inhibitor of fatty acid synthesis. If the compound is derived from palmitic or stearic acid, it would require loss of one or two acetate units by (3-oxidation, followed by further oxidations (Figure 3.26) at appropriate carbon atoms. This was supported by experiments that showed coccinelline formation was inhibited when the experiment was performed in a nitrogen atmosphere. The origin of the nitrogen atom in coccinelline was shown to be glutamine. This was demonstrated by

Jl ^ 11 ^ amidotransferase n n

glutamine glutamic acid o o

Jl ^ 11 ^ amidotransferase n n

glutamine glutamic acid

Figure 3.26 The probable transformation of a fatty acid by chain shortening, oxidation and cyclization to precoccinellines and coccinellines. The preferred source of the nitrogen atom is glutamine through the action of an amidotransferase. Ammonia is shown in brackets since it is unlikely to exist in the free state supplying various amino-acids to a tissue culture that could synthesize the coccinelline in the presence of oxygen. Precoccinelline (Figure 3.26) is basic, the final stage gives the weakly basic N-oxide. A number of isomers are possible by altering the stereochemistry at the ring junctions. The isomers most frequently met are coccinelline itself, found, for example in the common ladybird Coccinella septempunctata (Plate 3) which contains coccinelline and precoccinelline, and Hippodamia conver-gens (Plate 7) which contains hippodamine and convergine, the N-oxide of hippodamine. The true shape and difference between these isomers is shown more clearly in Figure 3.27.

Figure 3.27 Stereodiagrams of precoccinelline and hippodamine to show their different

Over 50 alkaloids (see Chapter 9) of several structural types have been isolated and identified from ladybirds. Two further compounds, now known to have very similar origin to the coccinellines are (-)-adaline and (-)-adalinine (Figure 3.28), from Adalia beetles. The intermediate piperi-deine (A in Figure 3.26) undergoes further oxidation and a Mannich reaction to give adaline (Figure 3.28). Feeding Adalia bipunctata with (-)-adaline deuterated in the side chain caused them to make deuterated adalinine, but when fed (+)-adaline, similarly deuterated, there was no labelled adalinine produced, only unlabelled compound. From this it is concluded that adalinine is formed from adaline by a reversal of the Mannich reaction on the other side of the ring. Addition of water and a further oxidation gives adalinine. In the Australian ladybird Cryptolae-mus montrouzieri is found two compounds, the first might be described as 6-methylpelleterine. Pelleterine is itself found in the root bark of the pomegranate tree and is long used as an anthelmintic. The second compound, clearly related to the first, can also be seen as a lower homologue of adaline (Figure 3.28).

3.4.2 Epilachnine

Epilachnine is an example of a quite different substance made by a coc-cinellid beetle and derived from a fatty acid, although that origin might not be immediately obvious. It is the defensive secretion of the pupae of the Mexican bean beetle Epilachna varivestis. The pupae carry the toxin

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