Insect Alkaloids

In the latter years, few non-alkaloid insect defensive compounds have been synthesized. In this section, only enantioselective syntheses will be considered. 

Melia azadirachta L. (neem or margosa). The bark, according to Cornish, yields a minute amount of a bitter alkaloid, margosine. The fruit is reported to be toxic and to contain an alkaloid, azaridine. The leaves are stated to be insect-repellent and from them an alkaloid paraisine has been prepared. The flowers are alkaloid-free. ((1) Ind. Ann. Med. Sci., 1857, 4, 104 (2) Carratala, Rev. Asoc. med. Arg., 1939, 53, 338 (3) Volkonsky, Arch. Inst. Pasteur Alg rie, 1937, 427 (4) Subramanian and Rangaswamy, Curr. Sci., 1947, 16, 182.)... 

Pyrrolidine alkaloid transformations in insect organisms 98EJ013. 

This series in heterocychc chemistry is being introduced to collectively make available critically and comprehensively reviewed hterature scattered in various journals as papers and review articles. All sorts of heterocyclic compounds originating from synthesis, natural products, marine products, insects, etc. will be covered. Several heterocyclic compounds play a significant role in maintaining life. Blood constituents hemoglobin and purines, as well as pyrimidines, are constituents of nucleic acid (DNA and RNA). Several amino acids, carbohydrates, vitamins, alkaloids, antibiotics, etc. are also heterocyclic compounds that are essential for life. Heterocyclic compounds are widely used in clinical practice as drugs, but all applications of heterocyclic medicines can not be discussed in detail. In addition to such applications, heterocyclic compounds also find several applications in the plastics industry, in photography as sensitizers and developers, and the in dye industry as dyes, etc. 

Hartmann T, Ober D (2000) Biosynthesis and Metabolism of Pyrrolizidine Alkaloids in Plants and Specialized Insect Herbivores. 209. 207-243 Haseley SR, Kamerling JP, Vliegenthart JFG (2002) Unravelling Carbohydrate Interactions with Biosensors Using Surface Plasmon Resonance (SPR) Detection. 218 93-114... 

Caffeine is the alkaloid that made coffee fruit and seed so much desired. Caffeine is probably also part of the defense system of the coffee seed, since caffeine has been recognized as an antifungal,149 a selective phytotoxin,150 and a chemosterilant toward certain insects.151... 

Elder leaves contain the alkaloid sambucine, a precursor of hydrocyanic acid, which is somewhat toxic. Thus, the leaves are used only topically in ointments for sprains or bruises. Bruised leaves can be rubbed on the body or worn under a hat to prevent being pestered by insects. Cooled strained leaf tea can be applied to plants to discourage aphids. 

The number of natural products containing these tricyclic systems is relatively small, viz. a few alkaloids from marine (compound 293), fungal (compound 282), amphibian (compound 395), insect (compound 387), and plant sources (compound 288) and also some iridoid molecules (compounds 98-100). Some of those Myrmkaria alkaloids (from ants Section 12.16.6.5.2) which contain the 5 5 6 fused-ring system are perhaps the most extensively studied of these natural products, with several successful syntheses now recorded. 

The most well known and most widely used compounds mentioned in the chapter are the doubly protected monosaccharide derivatives (cf. Schemes 16-18 and 27-29), which serve as building blocks for the regio- and stereospecific synthesis of a host of more complex carbohydrates. A number of newly discovered alkaloids contain fused 5 6 6-triheterocyclic systems these compounds may come from not only traditional (plant) sources, for example, compounds 96, 292-294, 446, and 447, but also insects (compounds 531-533), marine animals of various kinds, for example, sponges (compounds 58 R = H or OH) and ascidians (compound 289), and amphibians (compound 535). Although the majority of these alkaloids are known to be toxic in sufficient quantity, any possible therapeutic applications have apparently not yet been disclosed. 

Alkaloids are compounds that contain nitrogen in a heterocyclic ring and are commonly found in about 15-20% of all vascular plants. Alkaloids are subclassified on the basis of the chemical type of their nitrogen-containing ring. They are formed as secondary metabolites from amino acids and usually present a bitter taste accompanied by toxicity that should help to repel insects and herbivores. Alkaloids are found in seeds, leaves, and roots of plants such as coffee beans, guarana seeds, cocoa beans, mate tea leaves, peppermint leaves, coca leaves, and many other plant sources. The most common alkaloids are caffeine, theophylline, nicotine, codeine, and indole... 

Aside from yielding the most complex insect alkaloids so far characterized, coccinellid beetles are sources of a wide array of structural types. 

In this discussion, we have restricted ourselves to the consideration of only a few examples of arthropod chemistry. From these alone, it is evident that insects synthesize defensive compounds by using all of the major biosynthetic pathways, producing acetogenins, simple aromatics and quinones, isoprenoids, and alkaloids. In addition, some of the millipedes, coccinellid beetles, and spiders we have studied utilize biosynthetic pathways that have yet to be characterized. 

When disturbed or molested, these insects release small droplets of hemo-lymph from the tibio-femoral joints of their legs, and it is now well established that the deterrency exhibited by many species of coccinellids towards potential predators results from the presence of repellent and bitter alkaloids in that fluid [ 12,13]. In ladybirds, this unpalatability is associated with a bright aposematic coloration and a characteristic smell due to 3-alkyl-2-methoxypyrazines [14, 15]. The beetles use these molecules not only to reinforce the visual alerting signal on an olfactory level, but also as aggregation pheromones [16]. 

Pyrrolizidine alkaloids (PAs) such as senecionine (159) (Fig. 29) are also taken up from plants by various butterflies and moths belonging to the Danainae, Ithomiinae, and Arctiidae. The chemical ecology of PAs and their role in the interaction between plants and adapted Lepidoptera were reviewed several times in details during the last decade [155-159]. These insects use them for defense and for the production of male pheromones. 

Many insect defensive alkaloids have already been synthesized and several of these syntheses have already been reviewed (e.g., [13,114,116]). Here we will limit ourselves to the presentation of recent syntheses of Myrmicaria and Tetra-ponera ant alkaloids. 

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