Amino-substituted para-Benzoquinones as Potential Herbicides

Antiureolytic Activity of Substituted 2,5-Diaminobenzoquinones

A series of 2,5-bis(alkyl/arylamino)-1,4-benzoquinones (1-12) were investigated in vitro for their potential to inhibit the activity of jack bean urease. Compounds 1-6, 8, 9, 11 and 12 effectively inhibited the jack bean urease activity by 90.8 % when tested at 5 μm, whereas 7 and 10 had relatively little effect. The IC₅₀ for most compounds was in the nanomolar range (31.4 nm and 36.0 nm for 2 and 8, respectively). The mechanism of enzyme inhibition shown by 2 and 8 is typical of mixed-type inhibitors, whose affinity for the active site is over 6- and 2-fold higher (K_i =30.0 and 22.8 nm, for 2 and 8, respectively) than that of an allosteric site. Molecular docking studies revealed that both 2 and 8 establish hydrogen bonds with the amino acids residues Asp494, Met588, His593 and Ala636 in the active site of jack bean urease. These results indicate that such aminoquinones are useful leads for the development of more efficient urease inhibitors of wider utility.

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.

Tailoring Natural Abenquines To Inhibit the Photosynthetic Electron Transport through Interaction with the D1 Protein in Photosystem II

Abenquines are natural N-acetylaminobenzoquinones bearing amino acid residues, which act as weak inhibitors of the photosynthetic electron transport chain. Aiming to exploit the abenquine scaffold as a model for the synthesis of new herbicides targeting photosynthesis, 14 new analogues were prepared by replacing the amino acid residue with benzylamines and the acetyl with different acyl groups. The synthesis was accomplished in three steps with a 68-95% overall yield from readily available 2,5-dimethoxyaniline, acyl chlorides, and benzyl amines. Key steps include (i) acylation of the aniline, (ii) oxidation, and (iii) oxidative addition of the benzylamino moiety. The compounds were assayed for their activity as Hill inhibitors, under basal, uncoupled, or phosphorylating conditions, or excluding photosystem I. Four analogues showed high effectiveness (IC₅₀ = 0.1-0.4 μM), comparable with the commercial herbicide diuron (IC₅₀ = 0.3 μM). The data suggest that this class of compounds interfere at the reducing side of photosystem II, having protein D1 as the most probable target. Molecular docking studies with the plastoquinone binding site of Spinacia oleracea further strengthened this proposal.

Natural Abenquines and Their Synthetic Analogues Exert Algicidal Activity against Bloom-Forming Cyanobacteria

Abenquines are natural quinones, produced by some Streptomycetes, showing the ability to inhibit cyanobacterial growth in the 1 to 100 μM range. To further elucidate their biological significance, the synthesis of several analogues (4f-h, 5a-h) allowed us to identify some steric and electronic requirements for bioactivity. Replacing the acetyl by a benzoyl group in the quinone core and also changing the amino acid moiety with ethylpyrimidinyl or ethylpyrrolidinyl groups resulted in analogues 25-fold more potent than the natural abenquines. The two most effective analogues inhibited the proliferation of five cyanobacterial strains tested, with IC₅₀ values ranging from 0.3 to 3 μM. These compounds may be useful leads for the development of an effective strategy for the control of cyanobacterial blooms.