Toxicity and Sublethal Effects of Phthalides Analogs to Rhyzopertha dominica

Phthalides and their precursors have demonstrated a large variety of biological activities. Eighteen phthalides were synthesized and tested on the stored grain pest Rhyzopertha dominica. In the screening bioassay, compounds rac-(2R,2aS,4R,4aS,6aR,6bS,7R)-7-bromohexahydro-2,4-methano-1,6-dioxacyclopenta[cd]pentalen-5(2H)-one (15) and rac-(3R,3aR,4R,7S,7aS)-3-(propan-2-yloxy)hexahydro-4,7-methano-2-benzofuran-1(3H)-one (17) showed mortality similar to the commercial insecticide, Bifenthrin® (≥90 %). The time (LT₅₀ ) and dose (LD₅₀ ) necessary to kill 50 % of the R. dominica population were determined for the most efficacious phthalides 15 and 17. Compound 15 presented the lowest LD₅₀ (1.97 μg g^-1 ), being four times more toxic than Bifenthrin® (LD₅₀ =9.11 μg g^-1 ). Both compounds presented an LT₅₀ value equal to 24 h. When applied at a sublethal dose, both phthalides (especially compound 15), reduced the emergence of the first progeny of R. dominica. These findings highlight the potential of phthalides 15 and 17 as precursors for the development of insecticides for R. dominica control.

Toxicity to Diaphania hyalinata, selectivity to non-target species and phytotoxicity of furanones and phthalide analogues

BACKGROUND

Despite being of great importance to crop protection, the disadvantages of intensive and inappropriate use of pesticides have stimulated the search for more selective and less harmful agrochemicals. Thus, we have evaluated the effectiveness of 16 synthetic molecules (phthalides and precursors) to control the melonworm Diaphania hyalinata, a key pest in cucurbit crops of economic importance in Brazil. The selectivity to beneficial organisms Solenopsis saevissima and Tetragonisca angustula and the phytotoxicity to Cucumis sativus of the promising insecticides were also assessed.

RESULTS

In the screening assay, compounds 1 and 6 provided 91 and 88% mortality of the melonworm. Compound 1 presented higher toxicity (median lethal dose LD50 = 15.99 µmol g(-1) ) and higher speed on pest control (median survival time LT50 = 420 min) than compound 6 (LD50 = 44.51 µmol g(-1) and LT50 = 840 min). Both compounds inhibited less than 11% of host-plant growth and caused ≤36 and ≥93% mortality of predator and pollinator respectively.

CONCLUSION

Among the tested compounds, only compounds 1 and 6 were effective in melonworm control. Both compounds presented no considerable phytotoxicity and were selective to predator but non-selective to pollinator, which enables their application for pest control if the exposure of the bees is minimised. © 2015 Society of Chemical Industry.

Synthesis and insecticidal activity of an oxabicyclolactone and novel pyrethroids

Deltamethrin, a member of the pyrethroids, one of the safest classes of pesticides, is among some of the most popular and widely used insecticides in the World. Our objective was to synthesize an oxabicyclolactone 6 and five novel pyrethroids 8–12 from readily available furfural and D-mannitol, respectively, and evaluate their biological activity against four insect species of economic importance namely A. obtectus, S. zeamais, A. monuste orseis, and P. americana. A concise and novel synthesis of 6,6-dimethyl-3-oxabicyclo[3.1.0]hexan-2-one (6) from furfural is described. Photochemical addition of isopropyl alcohol to furan-2(5H)-one afforded 4-(1'-hydroxy-1'-methylethyl)tetrahydro-furan-2-one (3). The alcohol 3 was directly converted into 4-(1'-bromo-1'-methylethyl)-tetrahydrofuran-2-one (5) in 50% yield by reaction with PBr(3) and SiO(2). The final step was performed by cyclization of 5 with potassium tert-butoxide in 40% yield. The novel pyrethroids 8–12 were prepared from methyl (1S,3S)-3-formyl-2,2-dimethylcyclopropane-1-carboxylate (7a) by reaction with five different aromatic phosphorous ylides. Compounds 6–12 presented high insecticidal activity, with 6 and 11 being the most active. Compound 6 killed 90% of S. zeamais and 100% of all the other insects evaluated. Compound 11 killed 100% of all insects tested.

Psychopharmacological evidences for the involvement of muscarinic and nicotinic cholinergic receptors on sweet substance-induced analgesia in Rattus norvegicus

In order to investigate the effects of sweet substance intake on pain modulation, male albino Wistar rats weighing 180-200 g received either tap water or sucrose solutions (250 g/l) for 14 days as their only source of liquid. Each rat consumed an average of 15.6 g sucrose/day. Their tail withdrawal latencies in the tail-flick test (probably a spinal reflex) were measured immediately before and after this treatment. An analgesia index was calculated from the withdrawal latencies before and after treatment. The index (mean +/- SEM, N = 8) for the groups receiving sucrose solution plus saline (NaCl; 0.9%) for 14 days was 0.70 +/- 0.01. Atropine (1 and 2 mg/kg)-treated rats (N = 8) after intake of sucrose exhibited an analgesia index of 0.39 +/- 0.09 and 0.39 +/- 0.08, respectively, while mecamylamine (1 and 2 mg/kg)-treated rats (N = 10) after intake of sucrose had an index of -0.02 +/- 0.07 and 0.03 +/- 0.07, respectively. These results indicate that the effect of sucrose intake on nociceptive thresholds is controlled by neurotransmission of acetylcholine and depends on the nicotinic cholinergic receptors for its major analgesic effect, although muscarinic receptors were also involved in this antinociceptive process.

Opioid neurotransmission in the post-ictal analgesia: involvement of mu(1)-opioid receptor

Pentylenetetrazol (PTZ), a non-competitive antagonist that blocks GABA-mediated Cl(-) flux, was used in the present work to induce seizures in animals. The aim of this work is to study the neurochemical basis of the antinociception induced by convulsions elicited by peripheral administration of PTZ (64 mg/kg). The analgesia was measured by the tail-flick test, in eight rats per group. Convulsions were followed by significative increase in the tail-flick latencies (TFL), for at least 120 min of the post-ictal period. Peripheral administration of naltrexone (5 mg/kg, 10 mg/kg and 20 mg/kg) caused a significant decrease in the TFL in seizing animals, as compared to controls. These data were corroborated with peripheral administration of naloxonazine (10 mg/kg and 20 mg/kg), a mu(1)-opioid blocker, in the same doses used for non-specific antagonist. These results indicate that endogenous opioids may be involved in the post-ictal analgesia. The involvement of mu(1)-opioid receptor was also considered.