Total Synthesis of Himachalene Sesquiterpenes ofAphthonaandPhyllotretaFlea Beetles

Trapping of Crucifer-Feeding Flea Beetles (Phyllotreta spp.) (Coleoptera: Chrysomelidae) With Pheromones and Plant Kairomones

Flea beetles (Coleoptera: Chrysomelidae) of the genus Phyllotreta are major pests of cole crops, canola, and related crops in the mustard family (Brassicaceae). Adults may damage seedlings or larger crop plants, impairing crop growth, rendering crops unmarketable, or killing seedlings outright. The two major North American crucifer pest species, Phyllotreta striolata (F.) and Phyllotreta cruciferae (Goeze), have male-produced pheromones attractive to both female and male adults. We tested the racemic synthetic pheromones, himachaladiene and hydroxyhimachalanone, as well as the host-plant-produced allyl isothiocyanate, alone and in combination, with experimental trapping in Maryland, Virginia, and North Dakota, using clear and yellow sticky traps and the ground-based 'rocket' trap (modified from boll weevil trap). Phyllotreta striolata was consistently attracted to the hydroxyketone, and captures were often enhanced by allyl isothiocyanate (AITC), but its response to pheromones, AITC, and trap color were variable from state to state. Phyllotreta cruciferae was strongly attracted to AITC, but its response to pheromone components varied by state, and this species was found rarely at the Maryland site. Phyllotreta bipustulata (F.) was attracted to the diene component, a new finding for this species. Several other genera of flea beetles were captured, some showing response to the semiochemicals and/or color. Results will be helpful in monitoring and possibly population suppression; however, further research is necessary to develop more efficient syntheses, optimal lure loadings, combinations, and controlled release methods.

Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

Upon irradiation at λ = 350 nm, cyclohept-2-enone undergoes an isomerization to the strained (E)-isomer. The process was studied by XMS-CASPT2 calculations and found to proceed by two competitive reaction channels on either the singlet or the triplet hypersurface. (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions with various dienes. Ten different dienes were probed, most of which─except for 1,3-cyclohexadiene─underwent a clean Diels-Alder reaction and gave the respective trans-fused six-membered rings in good yields (68-98%). The reactions with furan were studied in detail, both experimentally and by DLPNO-CCSD(T) calculations. Two diastereoisomers were formed in a ratio of 63/35 with the exo-product prevailing, and the configuration of both diastereoisomers was corroborated by single crystal X-ray crystallography. The outcome of the photoinduced Diels-Alder reaction matched both qualitatively and quantitatively the calculated reaction pathway. Apart from cyclohept-2-enone, five additional cyclic hept-2-enones and cyclooct-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclopentadiene (80-98% yield). The method was eventually applied to a concise total synthesis of racemic trans-α-himachalene (four steps, 14% overall yield).

Conformational, configurational, and supramolecular studies of podocephalol acetate from Lasianthaea aurea

Although podocephalol (1) and its derived acetate 2 were found in Lasianthaea podocephala four decades ago, and 1 was later detected in the essential oils of several vegetal species, its absolute configuration (AC) and conformational preferences remained to be established. The structures of ar-himachalene 1, now isolated from Lasianthaea aurea, and its derived acetate 2, were herein confirmed by extensive 1D and 2D nuclear magnetic resonance (NMR) studies, while the conformational preferences of the cycloheptene was established by density functional theory (DFT) calculations, which in combination with vibrational circular dichroism measurements provided the (R) absolute configuration of the molecules. The structure and AC were further verified through the Flack and Hooft parameters calculations derived from single crystal X-ray diffraction data of 2. In addition, careful evaluation of the crystal data allowed observing supramolecular layers cell package, an uncommon property in natural terpenes that might have potential applications. A transmission electron microscopy analysis of crystal of 2 was also possible, providing its physical characteristics at the micrometric scale.

Stereochemical studies on pheromonal communications

Pheromonal communications are heavily dependent on the stereochemistry of pheromones. Their enantioselective syntheses could establish the absolute configuration of the naturally occurring pheromones, and clarified the unique relationships between absolute configuration and bioactivity. For example, neither the (R)- nor (S)-enantiomer of sulcatol, the aggregation pheromone of an ambrosia beetle, is behaviorally active, while their mixture is bioactive. Recent results as summarized in the present review further illustrate the unique and diverse relationships between stereochemistry and bioactivity of pheromones.

Male-specific sesquiterpenes from Phyllotreta flea beetles

Flea beetles in several genera are known to possess male-specific sesquiterpenes, at least some of which serve as aggregation pheromones that attract both sexes. In continuing research on the chemical ecology of Phyllotreta flea beetles, six new male-specific sesquiterpenes were identified, one from P. striolata (hydroxyketone 9) and five from P. pusilla (aldehydes 10-12 and 14 and alcohol 13); both species are crop pests. The minute amounts from beetles provided mass spectra and chromatographic data but were insufficient for complete structure determination. However, it was discovered that the new compounds could all be produced by applying organic reactions to previously identified flea beetle sesquiterpenes, and the resulting, larger amounts of material permitted definitive structure analysis by NMR. Molecular modeling was used in conjunction with NMR to define relative configurations of several newly created stereogenic centers. The absolute configurations of natural 9-14 were established by chiral gas chromatography/mass spectrometry. In electrophysiological tests (GC-EAD) conducted with P. striolata, compound 9 was detected with high sensitivity by the beetle antennae, which is consistent with a pheromonal function. The research opens new possibilities for using behavioral chemicals to monitor or manage these pest species.

Male-produced aggregation pheromone compounds from the eggplant flea beetle (Epitrix fuscula): identification, synthesis, and field biossays

Volatiles from the eggplant flea beetle, Epitrix fuscula Crotch (Coleoptera: Chrysomelidae), feeding on host foliage, were investigated. Six male-specific compounds were detected and were identified through the use of mass spectrometry, nuclear magnetic resonance (NMR) spectrometry, chiral and achiral gas chromatography, high-performance liquid chromatography, electrophysiology (gas chromatography-electroantennography, GC-EAD), and microchemical tests. The two most abundant of the six compounds were (2E,4E,6Z)-2,4,6-nonatrienal (1) and (2E,4E,6E)-2,4,6-nonatrienal (2). The other four compounds, present in minor amounts, were identified as himachalene sesquiterpenes; two of these, 3 and 4, were hydrocarbons and two, 5 and 6, were alcohols. All four sesquiterpenes were previously encountered from male flea beetles of Aphthona spp. and Phyllotreta cruciferae. Synthetic 1 and 2 matched the natural products by GC retention times, mass spectra, and NMR spectra. Sesquiterpenes 3-6 similarly matched synthetic standards and natural samples from the previously studied species in all ways, including chirality. Both natural and synthetic 1 and 2 gave positive GC-EAD responses, as did sesquiterpenes 3, 5, and 6. Field trials were conducted with a mixture of 1 and 2, and the baited traps were significantly more attractive than control traps to both male and female E. fuscula. The E. fuscula pheromone has potential for monitoring or controlling these pests in eggplants.

Dimethylfuran-lactone pheromone from males of Galerucella calmariensis and Galerucella pusilla

Male Galerucella calmariensis and Galerucella pusilla (Coleoptera: Chrysomelidae) emit an aggregation pheromone while feeding on host foliage. Isolation of the compound from collected volatiles was guided by comparisons of gas chromatograms of extracts from males and females and by gas chromatography-electroantennographic detection. The compound was identified by a combination of spectrometric methods and microchemical tests as the novel dimethylfuran lactone, 12,13-dimethyl-5,14-dioxabicyclo[9.2.1]tetradeca-1(13),11-dien-4-one. The structure was confirmed by synthesis, and the synthetic compound attracted males and females of both species in field bioassays. These beetles were previously introduced into North America as biological control agents for the invasive wetland weed, purple loosestrife Lythrum salicaria, and the pheromone could become a tool for monitoring populations. A new method is described for distinguishing the two species based on the tibial spurs of the males.

Pheromonal Activity of Compounds Identified from Male Phyllotreta cruciferae: Field Tests of Racemic Mixtures, Pure Enantiomers, and Combinations with Allyl Isothiocyanate

Four himachalene sesquiterpenes and (+)-gamma-cadinene, previously identified as possible pheromone components from males of a North American population of Phyllotreta cruciferae Goeze (Coleoptera, Chrysomelidae), were tested for attractiveness in field trapping experiments in Hungary. A mixture of the four synthetic racemic himachalene derivatives and (+)-gamma-cadinene from a botanical source was slightly attractive to beetles, but much more attractive when blended with the known host-plant-derived attractant allyl isothiocyanate. This result was consistent with a previous study in North America. In tests with optically pure synthetic compounds, a blend of the same himachalene enantiomers found from male beetles was equivalent to the corresponding blend of racemic compounds, whereas a blend of the opposite enantiomers was not active. Through subtraction tests, it was found that the single compound, (6R,7S)-2,2,6,10-tetramethylbicyclo[5.4.0.]undeca-9,11-diene [compound (+)-A in this study], was as active as the whole mixture, suggesting that this compound is the key pheromone component of the European population of P. cruciferae. During field trials, several congeneric species, including P. vittula, P. nemorum, P. nodicornis, and P. ochripes, also were caught, suggesting that the same compound(s) may be relatively widespread as pheromone components in this genus.

Responses of Flea Beetle Phyllotreta cruciferae to Synthetic Aggregation Pheromone Components and Host Plant Volatiles in Field Trials

Male-specific compounds, previously identified from Phyllotreta cruciferae and synthesized or isolated from natural sources, attracted both sexes of the beetle in field trials and therefore function as components of a male-produced aggregation pheromone. Six field experiments of 7 to 10 d duration each were conducted over 2 yr using modified boll weevil traps and two doses of pheromone. Treatments containing two doses of allyl isothiocyanate (AITC), a breakdown product of glucosinolates in Brassica napus L., a host plant of the beetles, were included in the study. A dose response was observed for both the pheromone components and AITC, and combinations of the pheromone and AITC generally attracted greater numbers of flea beetles than did either component itself. This increased attraction to a combination of beetle-produced compounds and host odors has not been previously demonstrated in halticine beetles and could help explain patterns of movement by P. cruciferae into field crops.

Synthetic examination of incorrectly proposed structures of biomolecules

Many incorrect structures of biomolecules have been proposed for natural products. Synthesis of compounds having the proposed structures often enabled us to judge the correctness of the proposals. In some cases, we were able to revise the structures by synthesizing the biomolecules themselves. In other cases, we were able to definitely disprove the proposed structures. Some examples treated in this review include: auxin-a and b; the sex pheromone of Chlamydomonas; sex pheromones of the gypsy moth, the American cockroach, and the pink bollworm moth; Persoons' periplanone-A; orobanchol; naurol A; bifurcarenone; koninginin A; alpha-acoradiene; himachalene-type pheromones of the flea beetle; differolide; blattellastanoside A and B; etc.