Good pyridine hunting: a biomimic compound, a modifier and a unique pharmacophore in agrochemicals

Characterization of the Species of Pythium Associated with Corn Seedlings in Delaware

Corn (Zea mays L.) is the top grain crop by hectares grown in Delaware (DE). Increased pre- and postemergence damping-off in corn caused by Pythium species have been observed in recent seasons. To date, no characterization studies have been performed to understand Pythium species diversity in DE. Symptomatic corn seedlings were collected from 33 fields across DE in 2019 and 2020. Isolates were obtained from infected root tissues and identified by molecular sequencing. In total, 14 species were recovered: P. aristosporum, P. arrhenomanes, P. attrantheridium, P. catenulatum, P. dissotocum, P. graminicola, P. inflatum, P. irregulare, P. oligandrum, P. periplocum, P. spinosum, P. sylvaticum, P. tardicrescens, and P. torulosum. The dominant species recovered was P. graminicola, accounting for 66% of the isolates. Five species were screened for fungicide sensitivity to calculate the EC50 of mefenoxam, ethaboxam, and picarbutrazox which are the most commonly used active ingredients to manage species of Pythium in corn seed treatment packages. All species tested were sensitive to mefenoxam and picarbutrazox. For ethaboxam, P. torulosum was the only species with an EC50 value >10 μg ml-1. Ten species were screened for fungicide sensitivity to mefenoxam, ethaboxam, or picarbutrazox at 18, 21, and 25°C. As temperatures increased, percent inhibition was reduced in 40, 70, and 80% of the species screened for sensitivity to ethaboxam, picarbutrazox, and mefenoxam, respectively. Nine of these species were screened for isolate aggressiveness in greenhouse panels to examine impact on emergence, shoot height, shoot weight, and root weight. While all species were recovered from inoculated roots, P. arrhenomanes, P. attrantheridium, P. spinosum, and P. sylvaticum had the greatest root weight reduction.

Visible-Light-Mediated Three-Component Alkene 1,2-Alkylpyridylation Reaction Using Alkylboronic Acids as Radical Precursors for the Synthesis of 4-Alkylpyridines

We report the photocatalyzed three-component alkene 1,2-alkylpyridylation reaction between alkylboronic acids, 4-cyanopyridine, and an olefin to achieve the pyridination and alkylation of the olefin and the synthesis of structurally diversified 4-alkylpyridines. The readily available and easily manipulated alkylboronic acids were used as alkyl radical precursors. The reactions take place under mild conditions with a broad substrate scope and are easy to scale up to gram level, and they are therefore of potential practical value for the synthesis and structural modification of biologically active alkylpyridine derivatives.

Photochemical Skeletal Editing of Pyridines to Bicyclic Pyrazolines and Pyrazoles

We present an efficient one-pot photochemical skeletal editing protocol for the transformation of pyridines into diverse bicyclic pyrazolines and pyrazoles under mild conditions. The method requires no metals, photocatalysts, or additives and allows for the selective removal of specific carbon atoms from pyridines, allowing for unprecedented versatility. Our approach offers a convenient and efficient means for the late-stage modification of complex drug molecules by replacing the core pyridine skeleton. Moreover, we have successfully scaled up this procedure in stop-flow and flow-chemistry systems, showcasing its applicability to intricate transformations such as the Diels-Alder reaction, hydrogenation, [3 + 2] cycloaddition, and Heck reaction. Through control experiments and DFT calculations, we provide insights into the mechanistic underpinnings of this skeletal editing protocol.

Multicomponent synthesis via acceptorless alcohol dehydrogenation: an easy access to tri-substituted pyridines

Herein, we report palladium supported on a hydroxyapatite catalyst for synthesizing tri-substituted pyridines using ammonium acetate as the nitrogen source via acceptorless alcohol dehydrogenation strategy. The strategy offers a broad substrate scope using inexpensive and readily available alcohols as the starting material. The catalyst was prepared using a simple method and analyzed by several techniques, including FE-SEM, EDS, HR-TEM, BET, XRD, FT-IR, UV-visible spectroscopy, and XPS, demonstrating the anchoring of Pd nanoparticles on hydroxyapatite in the zero oxidation state. Moreover, several controlled experiments were carried out to understand the reaction pathway and a suitable mechanism has been proposed.

Synthesis of 4H-Pyrazolo[3,4-d]pyrimidin-4-one Hydrazine Derivatives as a Potential Inhibitor for the Self-Assembly of TMV Particles

Tobacco mosaic virus coat protein (TMV-CP), as a potential target for the development of antiviral agents, can assist in the long-distance movement of viruses and plays an extremely important role in virus replication and propagation. This work focuses on the synthesis and the action mechanism of novel 4H-pyrazolo[3,4-d] pyrimidin-4-one hydrazine derivatives. The synthesized compounds exhibited promising antiviral activity on TMV. Specifically, compound G2 exhibited high inactivating activity (93%) toward TMV, slightly better than commercial reagent NNM (90%). The action of mechanism was further explored by employed molecular docking, molecular dynamics simulation, microscale thermophoresis, qRT-PCR, and transmission electron microscopy. Results indicated that G2 had the capability to interact with amino acid residues such as Trp352, Tyr139, and Asn73 in the active pocket of TMV-CP, creating strong hydrophobic interactions and thus obstructing the virus's self-assembly.

Synthesis of 15N-Pyridines and Higher Mass Isotopologs via Zincke Imine Intermediates

Methods to incorporate stable radioisotopes are integral to pharmaceutical and agrochemical development. However, despite the prevalence of pyridines in candidate compounds, methods to incorporate 15N atoms within their structures are limited. Here, we present a general approach to pyridine 15N-labeling that proceeds via ring-opening to NTf-Zincke imines and then ring-closure with commercially available 15NH4Cl salts. This process functions on a range of substituted pyridines, from simple building block-type compounds to late-stage labeling of complex pharmaceuticals, and 15N-incorporation is >95% in most cases. The reactivity of the Zincke imine intermediates also enables deuteration of the pyridine C3- and C5-positions, resulting in higher mass isotopologs required for LCMS analysis of biological fluids during drug development.

Photoredox-Catalyzed Three-Component 1,2-Cyanoalkylpyridylation of Styrenes with Nonredox-Active Cyclic Oximes

Three-component alkene 1,2-difunctionalizations have emerged as a powerful strategy for rapid buildup of diverse and complex alkylpyridines, but the distal functionalized alkyl radicals for the alkene 1,2-alkylpyridylations were still rare. Herein, we report an example of regioselective three-component 1,2-cyanoalkylpyridylation of feedstock styrenes with accessible nonredox-active cyclic oximes through visible-light photoredox catalysis, providing a series of structurally diverse β-cyanoalkylated alkylpyridines. This protocol proceeds through a radical relay pathway including the generation of iminyl radicals enabled by phosphoranyl radical-mediated β-scission, radical transposition through C-C bond cleavage, highly selective radical addition, and precise radical-radical cross-coupling sequence, thus facilitating the regioselective formation of two distinct C-C single bonds in a single-pot operation. This synthetic strategy features mild conditions, broad compatibility of functional groups and substrate scope, diverse product derivatization, and late-stage modification.

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.

1,2,4-Oxadiazole as a potential scaffold in agrochemistry: a review

N,O-containing heterocycles have been incorporated into various approved pesticides and pesticide candidates. The persistent challenge in contemporary crop protection lies in the continuous pursuit of novel N,O-heterocycle-containing compounds with pesticidal properties. Among them, the 1,2,4-oxadiazole scaffold is one of the most extensively explored heterocycles in new pesticide discovery and development. This review focuses on elucidating the molecular design strategy employed along with highlighting the bioactivity of 1,2,4-oxadiazole derivatives since 2012. Throughout this time frame, tioxazafen and flufenoxadiazam have emerged as prominent examples in which 1,2,4-oxadiazole derivatives were utilized as the core active structure within numerous applications. Additionally, the preparation methods for substituted 1,2,4-oxadiazole derivatives are elaborated upon, and their potential value within agrochemistry is discussed.

Late-Stage C-H Functionalization of Azines

Azines, such as pyridines, quinolines, pyrimidines, and pyridazines, are widespread components of pharmaceuticals. Their occurrence derives from a suite of physiochemical properties that match key criteria in drug design and is tunable by varying their substituents. Developments in synthetic chemistry, therefore, directly impact these efforts, and methods that can install various groups from azine C-H bonds are particularly valuable. Furthermore, there is a growing interest in late-stage functionalization (LSF) reactions that focus on advanced candidate compounds that are often complex structures with multiple heterocycles, functional groups, and reactive sites. Because of factors such as their electron-deficient nature and the effects of the Lewis basic N atom, azine C-H functionalization reactions are often distinct from their arene counterparts, and the application of these reactions in LSF contexts is difficult. However, there have been many significant advances in azine LSF reactions, and this review will describe this progress, much of which has occurred over the past decade. It is possible to categorize these reactions as radical addition processes, metal-catalyzed C-H activation reactions, and transformations occurring via dearomatized intermediates. Substantial variation in reaction design within each category indicates both the rich reactivity of these heterocycles and the creativity of the approaches involved.

Recent Advances in Pyridine Scaffold: Focus on Chemistry, Synthesis, and Antibacterial Activities

Multidrug-resistant (MDR) pathogens have created a fatal problem for human health and antimicrobial treatment. Among the currently available antibiotics, many are inactive against MDR pathogens. In this context, heterocyclic compounds/drugs play a vital role. Thus, it is very much essential to explore new research to combat the issue. Of the available nitrogen-bearing heterocyclic compounds/drugs, pyridine derivatives are of special interest due to their solubility. Encouragingly, some of the newly synthesized pyridine compounds/drugs are found to inhibit multidrug-resistant S. aureus (MRSA). Pyridine scaffold bearing poor basicity generally improves water solubility in pharmaceutically potential molecules and has led to the discovery of numerous broad-spectrum therapeutic agents. Keeping these in mind, we have reviewed the chemistry, recent synthetic techniques, and bacterial preventative activity of pyridine derivatives since 2015. This will facilitate the development of pyridine-based novel antibiotic/drug design in the near future as a versatile scaffold with limited side effects for the next-generation therapeutics.

Design, Synthesis, and Bioactivity of Spiro Derivatives Containing a Pyridine Moiety

We designed and synthesized a series of pyridine spiro derivatives and evaluated their insecticidal and antiviral activities. Some compounds exhibited good insecticidal and antiviral activities. Notably, the E series of compounds displayed good insecticidal activity against Tetranychus urticae. Compounds E20 (EC₅₀ = 63.68 mg/L) and F4 (EC₅₀ = 47.81 mg/L) exhibited inactivation activities against the tobacco mosaic virus (TMV), which were similar to that of Ningnanmycin (EC₅₀ = 58.01 mg/L). Molecular docking showed that compounds E20 and F4 exhibited satisfactory affinities for the TMV coat protein (TMV-CP), with binding energies (-6.7 and -6.4 kcal/mol, respectively) slightly lower than that of Ningnanmycin (-6.3 kcal/mol). Further, molecular dynamics analysis revealed that compounds E20 and F4 exhibited better binding stability values than Ningnanmycin. Microscale thermophoresis showed that compounds E20 (K_d = 0.053 ± 0.016 μM) and F4 (K_d = 0.045 ± 0.022 μM) bound more strongly to TMV-CP than Ningnanmycin (K_d = 0.10 ± 0.029 μM). The results of transmission electron microscopy showed that these two compounds hindered the self-assembly and growth of TMV. In summary, we showed that these pyridine spiro derivatives could be used as a basis for the research and development of novel pesticides.

Synthesis and Pesticidal Activity of New Niacinamide Derivatives Containing a Flexible, Chiral Chain

Natural products are a source for pesticide or drug discovery. In order to discover lead compounds with high fungicidal or herbicidal activity, new niacinamide derivatives derived from the natural product niacinamide, containing chiral flexible chains, were designed and synthesized. Their structures were confirmed by ¹H NMR, ¹³C NMR and HRMS analysis. The fungicidal and herbicidal activities of these compounds were tested. The fungicidal activity results demonstrated that the compound (S)-2-(2-chloronicotinamido)propyl-2-methylbenzoate (3i) exhibited good fungicidal activity (92.3% inhibition) against the plant pathogen Botryosphaeria berengriana at 50 μg/mL and with an EC₅₀ of 6.68 ± 0.72 μg/mL, which is the same as the positive control (fluxapyroxad). Compound 3i was not phytotoxic and could therefore be used as a fungicide on crops. Structure-activity relationships (SAR) were studied by molecular docking simulations with the succinate dehydrogenase of the fungal mitochondrial respiratory chain.

Halogenation of the 3-position of pyridines through Zincke imine intermediates

Pyridine halogenation reactions are crucial for obtaining the vast array of derivatives required for drug and agrochemical development. However, despite more than a century of synthetic endeavors, halogenation processes that selectively functionalize the carbon-hydrogen bond in the 3-position of a broad range of pyridine precursors remain largely elusive. We report a reaction sequence of pyridyl ring opening, halogenation, and ring closing whereby the acyclic Zincke imine intermediates undergo highly regioselective halogenation reactions under mild conditions. Experimental and computational mechanistic studies indicate that the nature of the halogen electrophile can modify the selectivity-determining step. Using this method, we produced a diverse set of 3-halopyridines and demonstrated late-stage halogenation of complex pharmaceuticals and agrochemicals.

Recent Advances in Synthesis of Multiply Arylated/Alkylated Pyridines

Multiply aryl/alkyl-substituted pyridines are some of the untapped synthetic targets because of the challenge in regioselectively introducing less polar aryl/alkyl groups at the desired positions in the pyridine framework. Interestingly, the importance of this family of compounds has increased annually, particularly in biological and materials engineering applications. The syntheses of such pyridines have been extensively reported, but there is a lack of comprehensive review articles. Hence, this review discusses recent advances by grouping reaction patterns that generally deliver tri-, tetra-, and penta-aryl/alkyl pyridines.

The Expanding Role of Pyridine and Dihydropyridine Scaffolds in Drug Design

Pyridine-based ring systems are one of the most extensively used heterocycles in the field of drug design, primarily due to their profound effect on pharmacological activity, which has led to the discovery of numerous broad-spectrum therapeutic agents. In the US FDA database, there are 95 approved pharmaceuticals that stem from pyridine or dihydropyridine, including isoniazid and ethionamide (tuberculosis), delavirdine (HIV/AIDS), abiraterone acetate (prostate cancer), tacrine (Alzheimer's), ciclopirox (ringworm and athlete's foot), crizotinib (cancer), nifedipine (Raynaud's syndrome and premature birth), piroxicam (NSAID for arthritis), nilvadipine (hypertension), roflumilast (COPD), pyridostigmine (myasthenia gravis), and many more. Their remarkable therapeutic applications have encouraged researchers to prepare a larger number of biologically active compounds decorated with pyridine or dihydropyridine, expandeing the scope of finding a cure for other ailments. It is thus anticipated that myriad new pharmaceuticals containing the two heterocycles will be available in the forthcoming decade. This review examines the prospects of highly potent bioactive molecules to emphasize the advantages of using pyridine and dihydropyridine in drug design. We cover the most recent developments from 2010 to date, highlighting the ever-expanding role of both scaffolds in the field of medicinal chemistry and drug development.

Synthesis of 3-Amino-5-fluoroalkylfurans by Intramolecular Cyclization

A synthesis to access rarely described 3-amino-5-fluoroalkylfurans has been developed by cyclization of easily accessible fluorovinamides. This method is rapid and simple and affords the desired furans as hydrochloride salts in quantitative or nearly quantitative yields. It is compatible with four different fluorinated groups (-CF₃, -CF₂CF₃, -CHF₂, and -CF₂Cl) and a wide range of substituents on the amine.