Alkyne chemistry in crop protection

Stereoselective polar radical crossover for the functionalization of strained-ring systems

Radical-polar crossover of organoborates is a poweful tool that enables the creation of two C-C bonds simultaneously. Small ring systems have become essential motifs in drug discovery and medicinal chemistry. However, step-economic methods for their selective functionalization remains scarce. Here we present a one-pot strategy that merges a simple preparation of strained organoboron species with the recently popularized polar radical crossover of borate derivatives to stereoselectively access tri-substituted azetidines, cyclobutanes and five-membered carbo- and heterocycles.

Radical revelations: the pnictogen effect in linear acetylenes

Acetylenes are essential building blocks in modern chemistry due to their remarkable modularity. The introduction of heteroatoms, such as pnictogens (X), is one of the simplest approaches to altering the C≡C bond. However, the chemistry of the resultant dipnictogenoacetylenes (DXAs) is strongly dependent on the nature of X. In this work, rigorous theoretical chemistry tools are employed to shed light on the origin of these differences, providing a detailed evaluation of the impact of X on the geometrical and electronic features of DXAs. Special emphasis is made on the study of the carbene character of the systems through the analysis of the interconversion mechanism between the linear and zigzag isomers. Our results show that second-period atoms behave drastically differently to the remaining X: down the group, a zwitterionic resonance form emerges at the expense of decreasing the carbenoid role, eventually resulting in an electrostatically driven ring closure. Furthermore, our findings pave the way to potentially unveiling novel routes for the promotion of free-radical chemistry.

Isosteric Ring Exchange as a Useful Scaffold Hopping Tool in Agrochemistry

Replacing one ring in a molecule by a different carba- or heterocycle is an important scaffold hopping manipulation, because biologically active compounds and their analogues, which underwent such a transformation, are often similar in size, shape, and physicochemical properties and, therefore, likely in their potency as well. This review will demonstrate, how isosteric ring exchange led to the discovery of highly active agrochemicals and which ring interchanges have proven to be most successful.

Strain-release arylations for the bis-functionalization of azetidines

The addition of nucleophilic organometallic species onto in situ generated azabicyclobutanes enables the selective formation of 3-arylated azetidine intermediates through strain-release. Single pot strategies were further developed for the N-arylation of resulting azetidines, employing either S_NAr reactions or Buchwald-Hartwig couplings.

Electro-alkynylation: Intramolecular Rearrangement of Trialkynylorganoborates for Chemoselective C(sp2)-C(sp) Bond Formation

An alternative and complementary transformation for the synthesis of aryl- and heteroaryl-substituted alkynes is presented that relies on a chemoselective electrocoupling process. Tetraorganoborate substrates were logically designed and simply accessed by transmetalations using readily or commercially available organotrifluoroborate salts.

Electron-Deficient Alkynes as Powerful Tools against Root-Knot Nematode Melodogyne incognita: Nematicidal Activity and Investigation on the Mode of Action

The present study reports on the powerful nematicidal activity of a series of electron-deficient alkynes against the root-knot nematode Meloidogyne incognita (Kofoid and White) Chitwood. Interestingly, we found that the conjugation of electron-withdrawing carbonyl groups to an alkyne triple bond was extremely proficient in inducing nematode paralysis and death. In particular, dimethylacetylenedicarboxylate (10), 3-butyn-2-one (1), and methyl propiolate (4), with EC_50/48 h of 1.54 ± 0.16, 2.38 ± 0.31, and 2.83 ± 0.28 mg/L, respectively, were shown to be the best tested compounds. Earlier studies reported on the ability of alkynoic esters and alkynones to induce a chemoselective cysteine modification of unprotected peptides. Thus, also following our previous findings on the impairment of vacuolar-type proton translocating ATPase functionality by activated carbonyl derivatives, we speculate that the formation of a vinyl sulfide linkage might be responsible for the nematicidal activity of the presented electron-deficient alkynes.

Multicomponent reactions in crop protection chemistry

An overview is given of the significance of multicomponent reactions in the synthesis of agrochemicals. The most important applications of multicomponent condensations, such as the Biginelli reaction, Bucherer-Bergs reaction, Hantzsch dihydropyridine synthesis, Kabachnik-Fields reaction, Mannich reaction, Passerini reaction, Petasis reaction, Strecker reaction, Ugi reaction and Willgerodt-Kindler reaction, to the synthesis of herbicidally, fungicidally and insecticidally active compounds are presented. Also the mode of action and biological activity of these multicomponent reaction products are reported.

Pre- and Postharvest Strategies to Minimize Mycotoxin Contamination in the Rice Food Chain

Rice is part of many people's diet around the world, being the main energy source in some regions. Although fewer reports exist on the occurrence of mycotoxins in rice compared to other cereals, fungal contamination and the associated production of toxic metabolites, even at lower occurrence levels compared to other crops, are of concern because of the high consumption of rice in many countries. Due to the diversity of fungi that may contaminate the rice food chain, the co-occurrence of mycotoxins is frequent. Specific strategies to overcome these problems may be applied at the preharvest part of the crop chain, while assuring good practices at harvest and postharvest stages, since different fungi may find suitable conditions to grow at the various stages of the production chain. Therefore, the aim of this review is to present the state-of-the-art knowledge on such strategies in an integrated way, from the field to the final products, to reduce mycotoxin contamination in rice.

Palladium-Catalyzed Cross-Coupling Reactions: A Powerful Tool for the Synthesis of Agrochemicals

Pd-catalyzed cross-coupling reactions have become essential tools for the construction of carbon-carbon and carbon-heteroatom bonds. Over the last three decades, great efforts have been made with cross-coupling chemistry in the discovery, development, and commercialization of innovative new pharmaceuticals and agrochemicals (mainly herbicides, fungicides, and insecticides). In view of the growing interest in both modern crop protection and cross-coupling chemistry, this review gives a comprehensive overview of the successful applications of various Pd-catalyzed cross-coupling methodologies, which have been implemented as key steps in the synthesis of agrochemicals (on R&D and pilot-plant scales) such as the Heck, Suzuki, Sonogashira, Stille, and Negishi reactions, as well as decarboxylative, carbonylative, α-arylative, and carbon-nitrogen bond bond-forming cross-coupling reactions. Some perspectives and challenges for these catalytic coupling processes in the discovery of agrochemicals are briefly discussed in the final section. The examples chosen demonstrate that cross-coupling chemistry approaches open-up new, low-cost, and more efficient industrial routes to existing agrochemicals, and such methods also have the capability to lead the new generation of pesticides with novel modes of action for sustainable crop protection.

Broad-Scope Rh-Catalyzed Inverse-Sonogashira Reaction Directed by Weakly Coordinating Groups

We report the alkynylation of C(sp2)-H bonds with bromoalkynes (inverse-Sonogashira reaction) directed by synthetically useful ester, ketone, and ether groups under rhodium catalysis. Other less common directing groups such as amine, thioether, sulfoxide, sulfone, phenol ester, and carbamate are also suitable directing groups. Mechanistic studies indicate that the reaction proceeds by a turnover-limiting C-H activation step via an electrophilic-type substitution.

Agrochemical lead optimization by scaffold hopping

Scaffold hopping, the exchange of a specific portion of a potential active ingredient with another substructure with the aim of finding isofunctional molecular structures with significantly different molecular backbones, often offers the chance in lead discovery or optimization to mitigate problems related to toxicity, intellectual property, and insufficient potency or stability. Scaffold hopping tools such as isosteric ring replacement including 1,3 nitrogen shift and cyclic imine-amide isosterism, but also ring opening and ring closure approaches, functional group isosterism, reversion of functional groups, chain shortening, chain lengthening, and scaffolds delivered by natural products, have become a permanent fixture of the innovation and optimization process in crop protection research. Their appropriate use will be explained through examples of success stories in the field of agrochemistry. Analogies to, but also differences from, the main categories of scaffold hopping in medicinal drug discovery are discussed. © 2017 Society of Chemical Industry.

Novel Triazole Hybrids of Betulin: Synthesis and Biological Activity Profile

Betulin derivatives containing a 1,2,3-triazole ring possess a wide spectrum of biological activities, including antiviral, anticancer, and antibacterial activity. A series of novel triazoles were prepared by the 1,3-dipolar cycloaddition reaction between the alkyne derivatives of betulin and organic azides. The chemical structures of the obtained compounds were defined by ¹H and ¹³C NMR, IR, and high-resolution mass spectrometry (HR-MS) analysis. The target triazoles were screened for their antiviral activity against DNA and RNA viruses. The cytotoxic activity of the obtained compounds 5a-k and 6a-h was determined using five human cancer cell lines (T47D, MCF-7, SNB-19, Colo-829, and C-32) by a WST-1 assay. The bistriazole 6b displayed a promising IC₅₀ value (0.05 μM) against the human ductal carcinoma T47D (500-fold higher potency than cisplatin). The microdilution method was applied for an evaluation of the antimicrobial activity of all of the compounds. The triazole 5e containing a 3'-deoxythymidine-5'-yl moiety exhibited antibacterial activity against two gram-negative bacteria vz. Klebsiellapneumoniae and Escherichia coli (minimal inhibitory concentration (MIC) range of 0.95-1.95 μM).

Progress of modern agricultural chemistry and future prospects

Agriculture is facing an enormous challenge: it must ensure that enough high-quality food is available to meet the needs of a continually growing population. Current and future agronomic production of food, feed, fuel and fibre requires innovative solutions for existing and future challenges, such as climate change, resistance to pests, increased regulatory demands, renewable raw materials or requirements resulting from food chain partnerships. Modern agricultural chemistry has to support farmers to manage these tasks. Today, the so-called 'side effects' of agrochemicals regarding yield and quality are gaining more importance. Agrochemical companies with a strong research and development focus will have the opportunity to shape the future of agriculture by delivering innovative integrated solutions. This review gives a comprehensive overview of the innovative products launched over the past 10 years and describes the progress of modern agricultural chemistry and its future prospects.

Heterocyclic chemistry in crop protection

An overview is given of the significance of heterocycles in crop protection chemistry, which is enormous as more than two-thirds of all agrochemicals launched to the market within the last 20 years belong to this huge group of chemicals. This review focuses on two important aspects of heterocyclic agrochemistry: the different roles of heterocyclic scaffolds in crop protection agents and the major possibilities for their synthesis.

Current Challenges and Trends in the Discovery of Agrochemicals

Crop protection chemistry has come a long way from its “alchemic” beginnings in the late 19th century to a high-tech science that supports the sustainable production of food, feed, and fiber for a rapidly growing population. Cutting-edge developments in the design and synthesis of agrochemicals help to tackle today’s challenges of weed and pest resistance, higher regulatory safety margins, and higher cost of goods with the invention of selective, environmentally benign, low use rate, and cost-effective active ingredients.

Synthesis, structure and cytotoxic activity of new acetylenic derivatives of betulin

A new series of betulin derivatives containing one or two pharmacophores bearing an acetylenic and carbonyl function at the C-3 and/or C-28 positions has been synthesized and characterized by ¹H- and ¹³C-NMR, IR, MS and elemental analyses. The crystal structure of 28-O-propynoylbetulin was determined by X-ray structural analysis. All new compounds, as well as betulin, were tested in vitro for their antiproliferative activity against human SW707 colorectal, CCRF/CEM leukemia, T47D breast cancer, and against murine P388 leukemia and Balb3T3 normal fibroblasts cell lines. Most of the compounds showed better cytotoxicity than betulin and cisplatin used as reference agent. 28-O-Propynoylbetulin was the most potent derivative, being over 500 times more potent than betulin and about 100 times more cytotoxic than cisplatin against the human leukemia (CCRF/CEM) cell line, with an ID₅₀ value of 0.02 μg/mL.