Stereoselective synthesis of trans-fused iridoid lactones and their identification in the parasitoid wasp Alloxysta victrix, Part II: Iridomyrmecins

Phosphonium Ion-Tethered Secondary Amines for Chemospecific 5-Enolexo Aldol Condensations of 6-Ketoaldehydes

A novel and highly selective 5-enolexo-exo-trig aldol condensation of 6-ketoaldehydes is presented using a proline-based alkylphosphonium ion catalyst. Bulky and oxophilic phosphonium ion plays a vital role in facilitating kinetic aldenamine formation and activating keto groups for aldol addition. This innovative approach exclusively targets five-membered carbo- and heterocyclic aldehydes, involving unusual aldehydes as donors and ketones as acceptors. Especially, enolizable aryl keto aldehydes and heteroatom-embedded ketoaldehydes exclusively produced cyclized products with our new catalyst, while other catalysts provided predominantly self-aldol or decomposed products. The scope and diversity of the method demonstrated by synthesizing different carboxaldehydes, including cyclopentene, indene, dihydrofuran, benzofuran, dihydropyrrole, indole, thiofuran, dihydrothiofuran, and benzothiofurans.

Morphology and ultrastructure of the allomone and sex-pheromone producing mandibular gland of the parasitoid wasp Leptopilina heterotoma (Hymenoptera: Figitidae)

Chemical communication by the parasitoid wasp Leptopilina heterotoma is based largely on (-)-iridomyrmecin. The female wasps use (-)-iridomyrmecin as a defensive allomone, a chemical cue to avoid competition with con- and heterospecific females, and as a major component of their sex pheromone to attract males. Males of L. heterotoma produce (+)-isoiridomyrmecin, which is also used for chemical defense. In this study we show that females and males of L. heterotoma produce the iridomyrmecins in a pair of mandibular glands. Each gland consists of a secretory part composed of class 3 gland cells and their accompanying duct cells, as well as a reservoir bordered by a thin intima. The gland discharges between the mandible base and the clypeus. Males have considerably smaller glands than females, which corresponds to the lower amount of iridomyrmecins produced by males. Chemical analyses of the mandibular gland contents showed that the gland of females contained mainly (-)-iridomyrmecin, as well as low amounts of the other previously described iridoid pheromone compounds, while the glands of males contained only (+)-isoiridomyrmecin. The morphology and sizes of the mandibular glands of males and females of L. heterotoma have evolved to the multi-functional use of iridomyrmecin.

Pharmacophagy in green lacewings (Neuroptera: Chrysopidae: Chrysopa spp.)?

Green lacewings (Neuroptera: Chrysopidae) are voracious predators of aphids and other small, soft-bodied insects and mites. Earlier, we identified (1R,2S,5R,8R)-iridodial from wild males of the goldeneyed lacewing, Chrysopa oculata Say, which is released from thousands of microscopic dermal glands on the abdominal sterna. Iridodial-baited traps attract C. oculata and other Chrysopa spp. males into traps, while females come to the vicinity of, but do not usually enter traps. Despite their healthy appearance and normal fertility, laboratory-reared C. oculata males do not produce iridodial. Surprisingly, goldeneyed lacewing males caught alive in iridodial-baited traps attempt to eat the lure and, in Asia, males of other Chrysopa species reportedly eat the native plant, Actinidia polygama (Siebold & Zucc.) Maxim. (Actinidiaceae) to obtain the monoterpenoid, neomatatabiol. These observations suggest that Chrysopa males must sequester exogenous natural iridoids in order to produce iridodial; we investigated this phenomenon in laboratory feeding studies. Lacewing adult males fed various monoterpenes reduced carbonyls to alcohols and saturated double bonds, but did not convert these compounds to iridodial. Only males fed the common aphid sex pheromone component, (1R,4aS,7S,7aR)-nepetalactol, produced (1R,2S,5R,8R)-iridodial. Furthermore, although C. oculata males fed the second common aphid sex pheromone component, (4aS,7S,7aR)-nepetalactone, did not produce iridodial, they did convert ∼75% of this compound to the corresponding dihydronepetalactone, and wild C. oculata males collected in early spring contained traces of this dihydronepetalactone. These findings are consistent with the hypothesis that Chrysopa males feed on oviparae (the late-season pheromone producing stage of aphids) to obtain nepetalactol as a precursor to iridodial. In the spring, however, wild C. oculata males produce less iridodial than do males collected later in the season. Therefore, we further hypothesize that Asian Chrysopa eat A. polygama to obtain iridoid precursors in order to make their pheromone, and that other iridoid-producing plants elsewhere in the world must be similarly usurped by male Chrysopa species to sequester pheromone precursors.

Chemical Ecology of Neuroptera

With 6,000 species, Neuroptera (lacewings, antlions, dustywings, and allies) is a relatively small order; however, most larval neuropterans are predacious, often in agricultural systems, lending added importance to this group. Advances in neuropteran phylogeny, most recently through genomic studies, stabilized the nomenclature of this ancestral order of Holometabola, facilitating basic and applied research on these important and interesting insects. The first pheromones for green lacewings (Chrysopidae) have been identified; this, and other research on antlions (Myrmeleontidae), suggests that male-produced long-range pheromones are the norm for the order. Characterizations of the myriad neuropteran exocrine gland systems, including prothoracic, metathoracic, abdominal, dermal, and anal glands, are revealing unforeseen trophic relationships with biological control implications. For examples, males of Chrysopa and other lacewing genera evidently must sequester specific chemical precursors from prey or plants to produce their attractant pheromones, and larval antlion venoms are potentially important genetic leads for insecticidal peptides.

The use of the lactone motif in chemical communication

A wide variety of organisms communicate via the chemical channel using small molecules. A structural feature quite often found is the lactone motif. In the present paper, the current knowledge on such lactones will be described, concentrating on the structure, chemistry, function, biosynthesis and synthesis of these compounds. Lactone semiochemicals from insects, vertebrates and bacteria, which this article will focus on, are particularly well investigated. In addition, some ideas on the advantageous use of lactones as volatile signals, which promoted their evolutionary development, will be discussed.

A nonspecific defensive compound evolves into a competition avoidance cue and a female sex pheromone

The evolution of chemical communication and the origin of pheromones are among the most challenging issues in chemical ecology. Current theory predicts that chemical communication can arise from compounds primarily evolved for non-communicative purposes but experimental evidence showing a gradual evolution of non-informative compounds into cues and true signals is scarce. Here we report that females of the parasitic wasp Leptopilina heterotoma use the defensive compound (−)-iridomyrmecin as a semiochemical cue to avoid interference with con- and heterospecific competitors and as the main component of a species-specific sex pheromone. Although competition avoidance is mediated by (−)-iridomyrmecin alone, several structurally related minor compounds are necessary for reliable mate attraction and recognition. Our findings provide insights into the evolution of insect pheromones by demonstrating that the increasing specificity of chemical information is accompanied by an increasing complexity of the chemical messengers involved and the evolution of the chemosensory adaptations for their exploitation.

Chemical communication can evolve from compounds used for other purposes, but experimental evidence is scarce. Here, Weiss et al. show a gradual evolution of a defensive compound into a competition avoidance mediator and a sex pheromone, which was accompanied by diversification of chemical messengers to obtain the required specificity.

Stereoselective synthesis of trans-fused iridoid lactones and their identification in the parasitoid wasp Alloxysta victrix, Part I: Dihydronepetalactones

Starting from the enantiomers of limonene, all eight stereoisomers of trans-fused dihydronepetalactones were synthesized. Key compounds were pure stereoisomers of 1-acetoxymethyl-2-methyl-5-(2-hydroxy-1-methylethyl)-1-cyclopentene. The stereogenic center of limonene was retained at position 4a of the target compounds and used to stereoselectively control the introduction of the other chiral centers during the synthesis. Basically, this approach could also be used for the synthesis of enantiomerically pure trans-fused iridomyrmecins. Using synthetic reference samples, the combination of enantioselective gas chromatography and mass spectrometry revealed that volatiles released by the endohyperparasitoid wasp Alloxysta victrix contain the enantiomerically pure trans-fused (4R,4aR,7R,7aS)-dihydronepetalactone as a minor component, showing an unusual (R)-configured stereogenic center at position 7.

Algicidal lactones from the marine Roseobacter clade bacterium Ruegeria pomeroyi

Volatiles released by the marine Roseobacter clade bacterium Rugeria pomeroyi were collected by use of a closed-loop stripping headspace apparatus (CLSA) and analysed by GC–MS. Several lactones were found for which structural proposals were derived from their mass spectra and unambiguously verified by the synthesis of reference compounds. An enantioselective synthesis of two exemplary lactones was performed to establish the enantiomeric compositions of the natural products by enantioselective GC–MS analyses. The lactones were subjected to biotests to investigate their activity against several bacteria, fungi, and algae. A specific algicidal activity was observed that may be important in the interaction between the bacteria and their algal hosts in fading algal blooms.