Compounds Of Mixed Biosynthetic Origin

Some compounds of mixed biosynthetic origin have already been met in earlier chapters. Epilachnine (Figure 3.29) is made from a fatty acid and an amino-acid, loganin is composed of a sugar bonded to a monoter-pene (Figure 6.11), the blatellostanosides have a sugar bonded to sterols (Figure 7.9) and ecdysone palmitate is a sterol with a fatty acid attached (Figure 7.15). Philanthotoxin-433 in Figure 9.14 has a phenyl-C3 portion, a simple aliphatic acid and a polyamine part. It is useful to look at some further examples of compounds of mixed origin so that they present less difficulty in the practical elucidation of structure of insect substances.

Plant substances, as might be expected, give the most frequent examples of mixed biosynthetic type. Humulone, one of the bitter substances of the hop vine (Humulus lupulus) that gives beer its characteristic taste has a polyketide core, with isoprene units attached to it (Figure 9.17). These isoprene units are now known to be made by the methylerythritol phosphate pathway (Chapter 6). The important groups of plant substances, the anthocyanins (flower pigments), the flavonoids and isoflavonoids have a phenyl-C3 unit extended with a polyketide (Figure 9.17). The polyketide added here is called a type III polyketide. Both flavones and anthocyanins are found in insects. The Common Blue butterfly, Polyommatus icarus, contains three flavones and an isoflavone, 3-methoxykaempferol and quercetin-3',4'-diglucoside (Figure 9.17). All are found in the larval plant food and are therefore not really insect products. The Marbled White butterfly, Melanargia galanthea, has twelve flavones. In all cases they are probably obtained from their food plants.

A very remarkable example of a pheromone is the oviposition deterrent of the cherry fruit fly Rhagoletis cerasi. This substance is placed on the cherry fruit by the female fly after she has laid her egg in it (Ernst and Wagner, Helvetica Chimica Acta, 1989, 72, 165). Its purpose is to stop other females of that species from laying eggs in the same fruit. The pheromone must be stable and non-volatile and remain intact in sun and rain. It consists of sugar, fatty acid and amino-acid portions, which together ensure a non-volatile, UV transparent and insoluble substance (Figure 9.18).

Adult chrysomeline beetles, when disturbed, release from their elytra (the hardened fore-wings of beetles) and from their thorax curious

Figure 9.17 Humulone, an example of mixed biosynthetic origin, is made of a triketide with three isoprene units grafted onto it (shown with thick black lines). The phenyl-C3 unit of p-coumaric acid is extended with a triketide to give at least four series of important plant compounds. Flavones, isoflavones and anthocyanins have been found in insects. 3-Methoxykaempferol and quercetin 3',4'-diglucoside are from the Common Blue butterfly.

Figure 9.17 Humulone, an example of mixed biosynthetic origin, is made of a triketide with three isoprene units grafted onto it (shown with thick black lines). The phenyl-C3 unit of p-coumaric acid is extended with a triketide to give at least four series of important plant compounds. Flavones, isoflavones and anthocyanins have been found in insects. 3-Methoxykaempferol and quercetin 3',4'-diglucoside are from the Common Blue butterfly.

Figure 9.18 The oviposition deterrent pheromone of the cherry fruit fly. It consists of a dihydroxystearic acid with a glucose molecule attached as an ether and an amide of the amino-acid sarcosine

Figure 9.18 The oviposition deterrent pheromone of the cherry fruit fly. It consists of a dihydroxystearic acid with a glucose molecule attached as an ether and an amide of the amino-acid sarcosine isoxazole glycosides esterified to nitropropionic acid (Figure 9.19). By feeding adults of Chrysomela tremulae with l-[U-14C]aspartic acid {i.e. uniformly labelled aspartic acid) it was shown that the beetles convert this into both the isoxazole and nitropropionic acid parts.

An equally unusual structure has been found in the venom of a funnel-web spider Hololena curta. The funnel-web spiders are noted for their particularly deadly venom. The compound, designated HF-7 is a disul-phate of guanosine, linked to an acetylated fucose sugar (Figure 9.20). It acts as many neurotoxins do by blocking calcium channels in cell membranes. This compound and the oxalic acid amide of agmatine, or decarboxylated arginine (Figure 9.20) are further examples of spiders hooc^Y

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