Heterocyclic chemistry in crop protection

Cobalt-Catalyzed Synthesis of Alkenyl Heterocycles via Regioselective Intramolecular 1,4-Hydrofunctionalization of Dienes

We report a novel cobalt-catalyzed intramolecular 1,4-hydrofunctionalization of dienes. The reaction proceeds under mild conditions and is amenable to N- and O-nucleophiles. The protocol exhibits exclusive regioselectivity, yielding a number of different alkenyl heterocycles, including but not limited to dihydroisobenzofurans, isochromanes, tetrahydrofurans, morpholines, lactones, and isoindolines. Experimental studies were performed to offer some insight into the different mechanistic pathways and to rationalize the regio- and stereoselectivities of the reaction.

Derisking Future Agrochemicals before They Are Made: Large-Scale In Vitro Screening for In Silico Modeling of Thyroid Peroxidase Inhibition

Inhibition of thyroid peroxidase (TPO) is a known molecular initiating event for thyroid hormone dysregulation and thyroid toxicity. Consequently, TPO is a critical off-target for the design of safer agrochemicals. To date, fewer than 500 structurally characterized TPO inhibitors are known, and the most comprehensive result set generated under identical conditions encompasses approximately 1000 compounds from a subset of the ToxCast compound collection. Here we describe a collaboration between wet lab and data scientists combining a large in vitro screen and the subsequent development of an in silico model for predicting TPO inhibition. The screen encompassed more than 100,000 diverse drug-like agrochemical compounds and yielded more than 6000 structurally novel TPO inhibitors. On this foundation, we applied different machine learning techniques and compared their performance. We discuss use cases for in silico TPO models in agrochemical research and explain that model recall is of particular importance when selecting compounds from large virtual compound collections. Furthermore, we show that due to the higher structural diversity of our training data, our final model allowed better generalization than models trained on the ToxCast data set. We now have a tool to predict TPO inhibition even for molecules that are only available virtually, such as hits from virtual screenings, or compounds under consideration for inclusion in our screening collection. Structures and activity data for 34,524 compounds are provided. This data set includes almost all inhibitors, including more than 3000 proprietary structures, and a large proportion of the inactives.

Isothiourea-Catalysed Acylative Dynamic Kinetic Resolution of Tetra-substituted Morpholinone and Benzoxazinone Lactols

The development of methods to allow the selective acylative dynamic kinetic resolution (DKR) of tetra-substituted lactols is a recognised synthetic challenge. In this manuscript, a highly enantioselective isothiourea-catalysed acylative DKR of tetra-substituted morpholinone and benzoxazinone-derived lactols is reported. The scope and limitations of this methodology have been developed, with high enantioselectivity and good to excellent yields (up to 89 %, 99 : 1 er) observed across a broad range of substrate derivatives incorporating substitution at N(4) and C(2), di- and spirocyclic substitution at C(5) and C(6), as well as benzannulation (>35 examples in total). The DKR process is amenable to scale-up on a 1 g laboratory scale. The factors leading to high selectivity in this DKR process have been probed through computation, with an N-C=O⋅⋅⋅isothiouronium interaction identified as key to producing ester products in highly enantioenriched form.

Design, Synthesis, and Acaricidal Activity of 2,5-Diphenyl-1,3-oxazoline Compounds

By using a scaffold hopping/ring equivalent and intermediate derivatization strategies, a series of compounds of 2,5-diphenyl-1,3-oxazoline with substituent changes at the 5-phenyl position were prepared, and their acaricidal activity was studied. However, the synthesized 2,5-diphenyl-1,3-oxazolines showed lower activity against mite eggs and larvae compared to the 2,4-diphenyl-1,3-oxazolines with the same substituents. We speculate that there is a significant difference in the spatial extension direction of the substituents between the two skeletons of compounds, resulting in differences in their ability to bind to the potential target chitin synthase 1. This work is helpful in inferring the internal structure of chitin synthase binding pockets.

Investigation of 2,4-Dihydroxylaryl-Substituted Heterocycles as Inhibitors of the Growth and Development of Biotrophic Fungal Pathogens Associated with the Most Common Cereal Diseases

Climate change forces agriculture to face the rapidly growing virulence of biotrophic fungal pathogens, which in turn drives researchers to seek new ways of combatting or limiting the spread of diseases caused by the same. While the use of agrochemicals may be the most efficient strategy in this context, it is important to ensure that such chemicals are safe for the natural environment. Heterocyclic compounds have enormous biological potential. A series of heterocyclic scaffolds (1,3,4-thiadiazole, 1,3-thiazole, 1,2,4-triazole, benzothiazine, benzothiadiazine, and quinazoline) containing 2,4-dihydroxylaryl substituents were investigated for their ability to inhibit the growth and development of biotrophic fungal pathogens associated with several important cereal diseases. Of the 33 analysed compounds, 3 were identified as having high inhibitory potential against Blumeria and Puccinia fungi. The conducted research indicated that the analysed compounds can be used to reduce the incidence of fungal diseases in cereals; however, further thorough research is required to investigate their effects on plant-pathogen systems, including molecular studies to determine the exact mechanism of their activity.

Multicomponent Synthesis of New Fluorescent Boron Complexes Derived from 3-Hydroxy-1-phenyl-1H-pyrazole-4-carbaldehyde

Novel fluorescent pyrazole-containing boron (III) complexes were synthesized employing a one-pot three-component reaction of 3-hydroxy-1-phenyl-1H-pyrazole-4-carbaldehyde, 2-aminobenzenecarboxylic acids, and boronic acids. The structures of the novel heterocyclic compounds were confirmed using 1H-, 13C-, 15N-, 19F-, and 11B-NMR, IR spectroscopy, HRMS, and single-crystal X-ray diffraction data. The photophysical properties of the obtained iminoboronates were investigated using spectroscopic techniques, such as UV-vis and fluorescence spectroscopies. Compounds display main UV-vis absorption maxima in the blue region, and fluorescence emission maxima are observed in the green region of the visible spectrum. It was revealed that compounds exhibit fluorescence quantum yield up to 4.3% in different solvents and demonstrate an aggregation-induced emission enhancement effect in mixed THF-water solutions.

De-epimerizing DyKAT of β-lactones generated by isothiourea-catalysed enantioselective [2 + 2] cycloaddition

An enantioselective isothiourea-catalysed [2 + 2] cycloaddition of C(1)-ammonium enolates with pyrazol-4,5-diones is used to construct spirocyclic β-lactones in good yields, excellent enantioselectivity (99 : 1 er) but with modest diastereocontrol (typically 70 : 30 dr). Upon ring-opening with morpholine or alternative nucleophilic amines and alcohols β-hydroxyamide and β-hydroxyester products are generated with enhanced diastereocontrol (up to >95 : 5 dr). Control experiments show that stereoconvergence is observed in the ring-opening of diastereoisomeric β-lactones, leading to a single product (>95 : 5 dr, >99 : 1 er). Mechanistic studies and DFT analysis indicate a substrate controlled Dynamic Kinetic Asymmetric Transformation (DyKAT) involving epimerisation at C(3) of the β-lactone under the reaction conditions, coupled with a hydrogen bond-assisted nucleophilic addition to the Si-face of the β-lactone and stereodetermining ring-opening. The scope and limitations of a one-pot protocol consisting of isothiourea-catalysed enantio-determining [2 + 2] cycloaddition followed by diastereo-determining ring-opening are subsequently developed. Variation within the anhydride ammonium enolate precursor, as well as N(1) and C(3) within the pyrazol-4,5-dione scaffold is demonstrated, giving a range of functionalised β-hydroxyamides with high diastereo- and enantiocontrol (>20 examples, up to >95 : 5 dr and >99 : 1 er) via this DyKAT.

The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic?

Heteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs (nσ, herein n) and use qualitative theory and quantum chemical computations, starting at Mandado's 2n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n + 2)π-electron species has 2n + 2 πα-electrons and 2n + 1 πβ-electrons (or vice versa) and becomes πα-antiaromatic and πβ-aromatic. Yet, the antiaromatic πα- and aromatic πβ-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the πβ-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in 3n,π* is often of the lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,π* state and the energy gap between the lowest n,π* and π,π* states (R2 = 0.42), while it is stronger for monosubstituted pyrazines (R2 = 0.84).

Copper-Catalyzed Three-Component 1,5-Carboamination of Vinylcyclopropanes

The 1,5-copper-catalyzed carboamination of vinylcyclopropanes is presented. A carbon-centered radical, formed upon reduction of an alkyl halide by Cu(I), adds across the alkene of a vinylcyclopropane, triggering ring opening to generate a benzylic radical, which, finally, undergoes copper-mediated amination to afford a homoallylic amine. The reaction occurs with outstanding regio- and good to very good diastereoselectivities. The scope of the reaction is demonstrated with respect to all three components: alkyl halide, vinylcyclopropane, and amine nucleophile. A total of 38 examples are presented with an average yield of 60%.

Breaking the Base Barrier: Cu(II)-Mediated C-H Heteroarylation of o-Carboranes with Base-Sensitive Heteroaryl Halides

While cage C-arylation reactions using strong bases are among the most frequently used transformations in carborane chemistry, there has been no general solution to allow for the use of weak bases in the reaction. Moreover, base-metal-promoted C-H heteroarylation with base-sensitive heteroaryl halides remained elusive. Herein, copper-mediated cage C-H (hetero)arylation has been achieved without the need for strong bases, leading to the facile synthesis of a wide range of C-(hetero)arylated carboranes in good to excellent yields with a broad substrate scope and good functional group compatibility.

Photo- and Cobalt-Catalyzed Synthesis of Heterocycles via Cycloisomerization of Unactivated Olefins

We report a general, intramolecular cycloisomerization of unactivated olefins with pendant nucleophiles. The reaction proceeds under mild conditions and tolerates ethers, esters, protected amines, acetals, pyrazoles, carbamates, and arenes. It is amenable to N-, O-, as well as C-nucleophiles, yielding a number of different heterocycles including, but not limited to, pyrrolidines, piperidines, oxazolidinones, and lactones. Use of both a benzothiazinoquinoxaline as organophotocatalyst and a Co-salen catalyst obviates the need for stoichiometric oxidant or reductant. We showcase the utility of the protocol in late-stage drug diversification and synthesis of several small natural products.

Generation and reactivity of unsymmetrical strained heterocyclic allenes

Strained cyclic allenes are short-lived intermediates that confine a functional group with a preferred linear geometry, an allene, into a small ring, inducing strain-driven reactivity. Nitrogen-containing variants, or azacyclic allenes, have proved valuable for the assembly of complex nitrogen-containing compounds. Whereas 3,4-azacyclic allenes, which bear a symmetrical core, have been the focus of multiple studies, their unsymmetrical 2,3-azacyclic counterparts have remained underexplored. In the present study, we report density functional theory studies investigating the structure of such unsymmetrical azacyclic allenes and experimental efforts to access and engage them in strain-promoted cycloadditions under mild conditions. Control experiments support either concerted or stepwise diradical mechanisms for these reactions, depending on the type of cycloaddition examined. Moreover, we generate the corresponding 2,3-oxacyclic allene and demonstrate its reactivity in cycloadditions and a metal-catalysed process. Given the scaffolds accessed, coupled with the observed selectivity trends, these results are expected to encourage the application of unsymmetrical heterocyclic allenes for the synthesis of heterocycles that bear a high fraction of sp3-hybridized atoms.

Efficient construction of functionalized pyrroloindolines through cascade radical cyclization/intermolecular coupling

Pyrroloindolines are important structural units in nature and the pharmaceutical industry, however, most approaches to such structures involve transition-metal or photoredox catalysts. Herein, we describe the first tandem SET/radical cyclization/intermolecular coupling between 2-azaallyl anions and indole acetamides. This method enables the transition-metal-free synthesis of C3a-substituted pyrroloindolines under mild and convenient conditions. The synthetic utility of this transformation is demonstrated by the construction of an array of C3a-methylamine pyrroloindolines with good functional group tolerance and yields. Gram-scale sequential one-pot synthesis and hydrolysis reactions demonstrate the potential synthetic utility and scalability of this approach.

Cu(II)-Catalyzed Aminocyclization of N-Propargyl Hydrazones to Substituted Pyrazolines

An efficient route for the copper(II)-catalyzed synthesis of substituted pyrazolines from readily accessible N-propargyl hydrazones has been reported under open flask conditions via intramolecular C-N bond formation. N-acyl and N-tosyl-substituted pyrazolines have been prepared in moderate to excellent yields. Mechanistic investigations using NMR, high-resolution mass spectrometry (HRMS), and Hammett analyses suggest that the Cu(II) catalyst generally acts as a Lewis acid to form an iminium-ion intermediate via cyclization, which afforded the desired pyrazolines upon hydrolysis. One progesterone receptor antagonist has also been synthesized utilizing this reaction methodology.

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.

Antifungal Function Oriented Scaffold Hopping for the Discovery of Oxazolyl-oxazoline as a Novel Model against Fusarium graminearum

Discovery of novel structural models is extremely important in agrochemical innovation. Scaffold hopping was conducted, and 16 kinds of novel models were synthesized and biologically evaluated. Oxazolyl-oxazoline 25 showed a promising in vitro potential against Fusarium graminearum with EC50 value of 18.25 μM, which was 2.4 times more potent than that of carbendazim (EC50 = 43.06 μM). The antifungal structure-activity relationship (SAR) revealed that compound 25am had the most promising antifungal activity against F. graminearum, with an EC50 value of 13.46 μM, which was 3.2 more potent than that of carbendazim. Different from carbendazim, the candidate 25am could form five hydrogen bonds with the amino acid residues in β-tubulin in the molecular docking and could effectively inhibit the carbendazim-resistant F. graminearum strain. Scanning electron microscopy (SEM) revealed that compound 25am induced the mycelia of F. graminearum slight collapse. This work suggests that compound 25am should be prioritized for further evaluation for new antifungal agents.

Ring Closure and Ring Opening as Useful Scaffold Hopping Tools in Agrochemistry

Ring closing acyclic parts of a molecular scaffold or the opposite manipulation, opening rings to produce pseudo-ring structures, is an important scaffold hopping manipulation. Analogues derived from biologically active compounds through the utilization of such strategies are often similar in shape and physicochemical properties and, therefore, likely to exhibit similar potency. This review will demonstrate how several different ring closure techniques, such as replacing carboxylic functions by cyclic peptide mimics, incorporating double bonds into aromatic rings, tying back ring substituents to a bicyclic structure, cyclizing adjacent ring substituents to an annulated ring, bridging annulated ring systems to tricyclic scaffolds, and exchanging gem-dimethyl groups by cycloalkyl rings, but also ring opening led to the discovery of highly active agrochemicals.

Design, Synthesis and Bioactivity Evaluation of Heterocycle-Containing Mono- and Bisphosphonic Acid Compounds

Fosmidomycin (FOS) is a naturally occurring compound active against the 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) enzyme in the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, and using it as a template for lead structure design is an effective strategy to develop new active compounds. In this work, by replacing the hydroxamate unit of FOS with pyrazole, isoxazole and the related heterocycles that also have metal ion binding affinity, while retaining the monophosphonic acid in FOS or replacing it with a bisphosphonic acid group, heterocycle-containing mono- and bisphosphonic acid compounds as FOS analogs were designed. The key steps involved in the facile synthesis of these FOS analogs included the Michael addition of diethyl vinylphosphonate or tetraethyl vinylidenebisphosphonate to β-dicarbonyl compounds and the subsequent cyclic condensation with hydrazine or hydroxylamine. Two additional isoxazolinone-bearing FOS analogs were synthesized via the Michaelis-Becker reaction with diethyl phosphite as a key step. The bioactivity evaluation on model plants demonstrated that several compounds have better herbicidal activities compared to FOS, with the most active compound showing a 3.7-fold inhibitory activity on Arabidopsis thaliana, while on the roots and stalks of Brassica napus L. and Echinochloa crus-galli in a pre-emergence inhibitory activity test, the activities of this compound were found to be 3.2- and 14.3-fold and 5.4- and 9.4-fold, respectively, and in a post-emergency activity test on Amaranthus retroflexus and Echinochloa crus-galli, 2.2- and 2.0-fold inhibition activities were displayed. Despite the significant herbicidal activity, this compound exhibited a DXR inhibitory activity lower than that of FOS but comparable to that of other non-hydroxamate DXR inhibitors, and the dimethylallyl pyrophosphate rescue assay gave no statistical significance, suggesting that a different target might be involved in the inhibiting process. This work demonstrates that using bioisosteric replacement can be considered as a valuable strategy to discover new FOS analogs that may have high herbicidal activities.

Morpholine Derivatives in Agrochemical Discovery and Development

Derivatives of morpholine are biologically active organic compounds with special structures discovered in multiple drugs. As a result of the terminal pharmacophore of action and extraordinary activity, they attracted fair attention with regard to pesticide innovation and development. Analysis of brief structure-activity relationships and the summarization of the characteristics of pesticides containing morpholine fragments with efficient activity are key steps in the development of novel pesticides. This review primarily overviews morpholine compounds with insecticidal, fungicidal, herbicidal, antiviral, and plant growth regulation properties to provide educational insight for the creation of new morpholine-containing compounds.

Fungicidal activity of novel quinazolin-6-ylcarboxylates and mode of action on Botrytis cinerea

BACKGROUND

Fungal diseases remain important causes of crop failure and economic losses. As the resistance toward current selective fungicides becomes increasingly problematic, it is necessary to develop efficient fungicides with novel chemotypes.

RESULTS

A series of novel quinazolin-6-ylcarboxylates which combined the structures of pyridine or heterocyclic motif and the N-(3-chloro-4-fluorophenyl)quinazolin-4-amine moiety, a binding group of ATP-binding site of gefitinib, were evaluated for their fungicidal activity on different phytopathogenic fungi. Most of these compounds showed excellent fungicidal activities against Botrytis cinerea and Exserohilum rostratum, especially compound F17 displayed the highest activity with EC50 values as 3.79 μg mL-1 against B. cinerea and 2.90 μg mL-1 against E. rostratum, which was similar to or even better than those of the commercial fungicides, such as pyraclostrobin (EC50 , 3.68, 17.38 μg mL-1 ) and hymexazol (EC50 , 4.56, 2.13 μg mL-1 ). Moreover, compound F17 significantly arrested the lesion expansion of B. cinerea infection on tomato detached leaves and strongly suppressed grey mold disease on tomato seedlings in greenhouse. The abilities of compound F17 to induce cell apoptosis of the non-germinated spores, to limit oxalic acid production, to reduce malate dehydrogenase (MDH) expression, and to block the active pocket of MDH protein were demonstrated in B. cinerea.

CONCLUSION

The novel quinazolin-6-ylcarboxylates containing ATP-binding site-directed moiety, especially compound F17, could be developed as a potential fungicidal candidate for further study. © 2023 Society of Chemical Industry.

A decade update on the application of β-oxodithioesters in heterocyclic synthesis

The diverse synthesis of heterocyclic compounds has always been one of the popular subjects of organic chemistry. To this end, great efforts have been devoted to developing new reagents and establishing new strategies and methods concerning efficiency, selectivity and sustainability. β-Oxodithioesters and their enol tautomers (i.e., α-enolic dithioesters), as a class of simple and readily accessible sulfur-containing synthons, have been widely applied in the construction of various five- and six-membered heterocycles (e.g., thiophenes, thiopyrans, thiazoles, pyridines and quinolines) and other useful open-chain frameworks. Due to their unique chemical structures, β-oxodithioesters bear multiple reaction sites, which enable them to participate in two-component or multicomponent reactions to construct various heterocyclic compounds. In the past decade, the application of β-oxodithioesters in the synthesis of heterocycles has made remarkable progress. Herein, an update on the recent advances in the application of β-oxodithioesters in the synthesis of heterocycles during the period from 2013 to 2023/06 is provided. According to the different types of rings concerning heteroatoms in products, this review is divided into five sections under discussion including (i) synthesis of sulfur-containing heterocycles, (ii) synthesis of sulfur and nitrogen-containing heterocycles, (iii) synthesis of nitrogen-containing heterocycles, (iv) synthesis of nitrogen and oxygen-containing heterocycles, and (v) modification to other open-chain frameworks.

Friedel-Crafts Reactions with N-Heterocyclic Alcohols

N-Heterocyclic alcohols are shown to be excellent substrates for superacid-promoted Friedel-Crafts reactions. The N-heterocyclic alcohols ionize to produce reactive, dicationic intermediates which provide good to excellent yields of arylation products.

Total Synthesis of Phenanthroindolizidines Using Strained Azacyclic Alkynes

We report a concise approach to phenanthroindolizidine alkaloids, wherein strained azacyclic alkynes are intercepted in Pd-catalyzed annulations. Two types of strained intermediates were evaluated: a functionalized piperidyne and a new strained intermediate, an indolizidyne. We show that each can be employed, ultimately allowing access to three natural products: tylophorine, tylocrebine, and isotylocrebine. These efforts demonstrate the successful merger of strained azacyclic alkyne chemistry with transition-metal catalysis for the construction of complex heterocycles.

Heterocyclic Molecular Targeted Drugs and Nanomedicines for Cancer: Recent Advances and Challenges

Cancer is a top global public health concern. At present, molecular targeted therapy has emerged as one of the main therapies for cancer, with high efficacy and safety. The medical world continues to struggle with the development of efficient, extremely selective, and low-toxicity anticancer medications. Heterocyclic scaffolds based on the molecular structure of tumor therapeutic targets are widely used in anticancer drug design. In addition, a revolution in medicine has been brought on by the quick advancement of nanotechnology. Many nanomedicines have taken targeted cancer therapy to a new level. In this review, we highlight heterocyclic molecular-targeted drugs as well as heterocyclic-associated nanomedicines in cancer.

Design and Synthesis of Novel Oxathiapiprolin Derivatives as Oxysterol Binding Protein Inhibitors and Their Application in Phytopathogenic Oomycetes

Oomycetes, particularly those from the genus Phytophthora, are significant threats to global food security and natural ecosystems. Oxathiapiprolin (OXA) is an effective oomycete fungicide that targets an oxysterol binding protein (OSBP), while the binding mechanism of OXA is still unclear, which limits the pesticide design, induced by the low sequence identity of Phytophthora and template models. Herein, we generated the OSBP model of the well-reported Phytophthora capsici using AlphaFold 2 and studied the binding mechanism of OXA. Based on it, a series of OXA analogues were designed. Then, compound 2l, the most potent candidate, was successfully designed and synthesized, showing a control efficiency comparable to that of OXA. Moreover, field trial experiments showed that 2l exhibited nearly the same activity (72.4%) as OXA against cucumber downy mildew at 25 g/ha. The present work indicated that 2l could be used as a leading compound for the discovery of new OSBP fungicides.

Photo- and Metal-Mediated Deconstructive Approaches to Cyclic Aliphatic Amine Diversification

Described herein are studies toward the core modification of cyclic aliphatic amines using either a riboflavin/photo-irradiation approach or Cu(I) and Ag(I) to mediate the process. Structural remodeling of cyclic amines is explored through oxidative C-N and C-C bond cleavage using peroxydisulfate (persulfate) as an oxidant. Ring-opening reactions to access linear aldehydes or carboxylic acids with flavin-derived photocatalysis or Cu salts, respectively, are demonstrated. A complementary ring-opening process mediated by Ag(I) facilitates decarboxylative Csp³-Csp² coupling in Minisci-type reactions through a key alkyl radical intermediate. Heterocycle interconversion is demonstrated through the transformation of N-acyl cyclic amines to oxazines using Cu(II) oxidation of the alkyl radical. These transformations are investigated by computation to inform the proposed mechanistic pathways. Computational studies indicate that persulfate mediates oxidation of cyclic amines with concomitant reduction of riboflavin. Persulfate is subsequently reduced by formal hydride transfer from the reduced riboflavin catalyst. Oxidation of the cyclic aliphatic amines with a Cu(I) salt is proposed to be initiated by homolysis of the peroxy bond of persulfate followed by α-HAT from the cyclic amine and radical recombination to form an α-sulfate adduct, which is hydrolyzed to the hemiaminal. Investigation of the pathway to form oxazines indicates a kinetic preference for cyclization over more typical elimination pathways to form olefins through Cu(II) oxidation of alkyl radicals.

Tunable Regio- and Stereoselective Synthesis of Z-Acrylonitrile Indoles and 3-Cyanoquinolines from 2-Alkynylanilines and Alkynylnitriles

The merger of two bifunctional moieties, 2-alkynylaniline and alkynylnitriles, in the presence of ZnBr₂ offers the tunable synthesis of two biologically important motifs: acrylonitrile indoles and 3-cyanoquinolines. The group present on the terminal alkyne of 2-alkynylaniline regulates the reaction pathways, intra- versus intermolecular, which thereby adds stereoselectivity and regioselectivity in this protocol. The conversion of an acrylonitrile indole ring to quinoline is an intriguing synthetic utility of this methodology.

Synthesis, Structural Determination, and Antifungal Activity of Novel Fluorinated Quinoline Analogs

A series of new fluorinated quinoline analogs were synthesized using Tebufloquin as the lead compound, 2-fluoroaniline, ethyl 2-methylacetoacetate, and substituted benzoic acid as raw materials. Their structures were confirmed by ¹H NMR, ¹³C NMR, and HRMS. The compound 8-fluoro-2,3-dimethylquinolin-4-yl 4-(tert-butyl)benzoate (2b) was further determined by X-ray single-crystal diffraction. The antifungal activity was tested at 50 μg/mL, and the bioassay results showed that these quinoline derivatives had good antifungal activity. Among them, compounds 2b, 2e, 2f, 2k, and 2n exhibited good activity (>80%) against S. sclerotiorum, and compound 2g displayed good activity (80.8%) against R. solani.

Recent Advances in Synthetic Routes to Azacycles

A heterocycle is an important structural scaffold of many organic compounds found in pharmaceuticals, materials, agrochemicals, and biological processes. Azacycles are one of the most common motifs of a heterocycle and have a variety of applications, including in pharmaceuticals. Therefore, azacycles have received significant attention from scientists and a variety of methods of synthesizing azacycles have been developed because their efficient synthesis plays a vital role in the production of many useful compounds. In this review, we summarize recent approaches to preparing azacycles via different methods as well as describe plausible reaction mechanisms.

Fluorosulfonamide-Directed Heteroarylation of Aliphatic C(sp3)-H Bonds

Herein, we describe a formal dehydrogenative cross coupling of heterocycles with unactivated aliphatic amines. The resulting transformation enables the direct alkylation of common heterocycles by merging N-F-directed 1,5-HAT with Minisci chemistry, leading to predictable site selectivity. The reaction provides a direct route for the transformation of simple alkyl amines to value-added products under mild reaction conditions, making this an attractive option for C(sp³)-H heteroarylation.

A Copper-Catalyzed Interrupted Domino Reaction for the Synthesis of Fused Triazolyl Benzothiadiazine-1-oxides

Copper(I)-catalyzed domino reactions of 2-azido sulfoximines with 1-iodoalkynes yield fused triazolyl-containing benzothiadiazine-1-oxides. The protocol features the synthesis of two fused heterocyclic rings in one step with good to excellent yields and a broad functional group tolerance. Detailed mechanistic investigations indicate that a copper π-complex initiates a cycloaddition and oxidative C-N coupling reaction sequence. The results suggest an interrupted domino process involving an iodinated triazole as a key intermediate.

Pd-Catalyzed Amination of Base-Sensitive Five-Membered Heteroaryl Halides with Aliphatic Amines

We report a versatile and functional-group-tolerant method for the Pd-catalyzed C-N cross-coupling of five-membered heteroaryl halides with primary and secondary amines, an important but underexplored transformation. Coupling reactions of challenging, pharmaceutically relevant heteroarenes, such as 2-H-1,3-azoles, are reported in good-to-excellent yields. High-yielding coupling reactions of a wide set of five-membered heteroaryl halides with sterically demanding α-branched cyclic amines and acyclic secondary amines are reported for the first time. The key to the broad applicability of this method is the synergistic combination of (1) the moderate-strength base NaOTMS, which limits base-mediated decomposition of sensitive five-membered heteroarenes that ultimately leads to catalyst deactivation, and (2) the use of a GPhos-supported Pd catalyst, which effectively resists heteroarene-induced catalyst deactivation while promoting efficient coupling, even for challenging and sterically demanding amines. Cross-coupling reactions between a wide variety of five-membered heteroaryl halides and amines are demonstrated, including eight examples involving densely functionalized medicinal chemistry building blocks.

cis-Selective Hydrogenation of Aryl Germanes: A Direct Approach to Access Saturated Carbo- and Heterocyclic Germanes

A catalytic approach of synthesizing the cis-selective saturated carbo- and heterocyclic germanium compounds (3D framework) is reported via the hydrogenation of readily accessible aromatic germanes (2D framework). Among the numerous catalysts tested, Nishimura's catalyst (Rh2O3/PtO2·H2O) exhibited the best hydrogenation reactivity with an isolated yield of up to 96%. A broad range of substrates including the synthesis of unprecedented saturated heterocyclic germanes was explored. This selective hydrogenation strategy could tolerate several functional groups such as -CF3, -OR, -F, -Bpin, and -SiR3 groups. The synthesized products demonstrated the applications in coupling reactions including the newly developed strategy of aza-Giese-type addition reaction (C-N bond formation) from the saturated cyclic germane product. These versatile motifs can have a substantial value in organic synthesis and medicinal chemistry as they show orthogonal reactivity in coupling reactions while competing with other coupling partners such as boranes or silanes, acquiring a three-dimensional structure with high stability and robustness.

Regioselective Synthesis of Fully Substituted Fused Pyrroles through an Oxidant-Free Multicomponent Reaction

The synthesis of fully substituted fused pyrroles through a multicomponent reaction between a thioamide, an aldehyde, and ammonium acetate is described. This process improves on a route commonly employed in the patent literature by avoiding the use of potentially hazardous oxidants, which cause the formation of side products and require a stringent process of derisking to be utilized on scale. The reaction proceeds under mild conditions, displays excellent functional group tolerance, and facilitates diversification through multiple vectors.

Design, Synthesis and Fungicidal Activity of N-(thiophen-2-yl) Nicotinamide Derivatives

Based on the modification of natural products and the active substructure splicing method, a series of new N-(thiophen-2-yl) nicotinamide derivatives were designed and synthesized by splicing the nitrogen-containing heterocycle natural molecule nicotinic acid and the sulfur-containing heterocycle thiophene. The structures of the target compounds were identified through ¹H NMR, ¹³C NMR and HRMS spectra. The in vivo bioassay results of all the compounds against cucumber downy mildew (CDM, Pseudoperonospora cubensis (Berk.et Curt.) Rostov.) in a greenhouse indicated that compounds 4a (EC₅₀ = 4.69 mg/L) and 4f (EC₅₀ = 1.96 mg/L) exhibited excellent fungicidal activities which were higher than both diflumetorim (EC₅₀ = 21.44 mg/L) and flumorph (EC₅₀ = 7.55 mg/L). The bioassay results of the field trial against CDM demonstrated that the 10% EC formulation of compound 4f displayed excellent efficacies (70% and 79% control efficacies, respectively, each at 100 mg/L and 200 mg/L) which were superior to those of the two commercial fungicides flumorph (56% control efficacy at 200 mg/L) and mancozeb (76% control efficacy at 1000 mg/L). N-(thiophen-2-yl) nicotinamide derivatives are significant lead compounds that can be used for further structural optimization, and compound 4f is also a promising fungicide candidate against CDM that can be used for further development.

Construction of bi(hetero)aryls via dicyanopyrazine-mediated photochemical cross-coupling

A photochemical cross-coupling protocol towards bi(hetero)aryls has been developed. The coupling reactions were mediated by dicyanopyrazine photoredox catalyst, while a photoinduced disproportionation process has been identified as an accompanying mechanism, especially for pyrrole derivatives. The developed method allows the cross-coupling of five-membered rings such as pyrrole, imidazole, thiazole and oxazole as well as various diazines (pyridine and pyrimidine) and benzene derivatives. A plausible mechanism of the reaction has also been disclosed. The practical application and relevance of the developed method were demonstrated by constructing an atorvastatin core or by the gradual functionalization of benzo[c][1,2,5]thiadiazole. In total, twenty-one bi(hetero)aryls were prepared in yields ranging from 19 to 95%.

Insight into designing of 2-pyridone derivatives for COVID-19 drug discovery - A computational study

Presently, the prime global focus is on SARS-CoV-2, as no fully established, licensed medicine has been found thus far, in spite of the existence of various reports and administration of partially proven certain class of natural products. However, in case of natural products, the extraction and purification limit their application. This situation drives researchers to explore synthetically viable drugs. In the present investigation, twenty-three 2-pyridone synthetic derivatives (P1-P23) have been theoretically tested for their suitability as potential inhibitors for COVID-19 main protease through DFT, molecular docking, and molecular dynamics simulations. DFT calculations offer insights into structure-property relationships, while ADMET studies indicate the pharmacological characteristics of these molecules. Molecular docking studies ascertain the nature and mode of interactions of these entities with COVID-19 main protease. Furthermore, covalent docking has been carried out to verify the feasibility of the formation of a covalent bond with the active site. The top protein-inhibitor complexes, such as P18, P11, and P12, were identified based on their glide score. These molecules, along with the covalent docked complexes, namely P18 and P16, were selected and subjected to molecular dynamics simulations. The 100 ns simulation process exhibited that the covalent docked ones, due to their stable form could serve as lead compounds against SARS-CoV-2. Hence, this study affirms the potential candidature of 2-pyridone-based inhibitors.

The Road Less Traveled: Unconventional Site Selectivity in Palladium-Catalyzed Cross-Couplings of Dihalogenated N-Heteroarenes

The vast majority (≥90%) of literature reports agree on the regiochemical outcomes of Pd-catalyzed cross-coupling reactions for most classes of dihalogenated N-heteroarenes. Despite a well-established mechanistic rationale for typical selectivity, several examples reveal that changes to the catalyst can switch site selectivity, leading to the unconventional product. In this Perspective, we survey these unusual cases in which divergent selectivity is controlled by ligands or catalyst speciation. In some cases, the mechanistic origin of inverted selectivity has been established, but in others the mechanism remains unknown. This Perspective concludes with a discussion of remaining challenges and opportunities for the field of site-selective cross-coupling. These include developing a better understanding of oxidative addition mechanisms, understanding the role of catalyst speciation on selectivity, establishing an explanation for the influence of ring substituents on regiochemical outcome, inverting selectivity for some "stubborn" classes of substrates, and minimizing unwanted over-reaction of di- and polyhalogenated substrates.

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Bioisosterism and Scaffold Hopping in Modern Nematicide Research

The application of agrochemicals is critical to global food safety. Nowadays, environmentally friendly green agrochemicals are the trend in field crop protection. The research and development of nematicides absorbed more attention as a typical representation of agrochemicals. This review describes the origin of recently commercialized nematicides, the application of bioisosterism and scaffold hopping in the discovery and optimization of agrochemicals, especially nematicides, and novel bioisosteric design strategies for the identification of fluensulfone analogues. Pesticide repurposing, high-throughput screening, computer-aided drug design, and incorporation of known pharmacophoric fragments have been the most successful approach for the discovery of new nematicides. As outlined, the strategies of bioisosteric replacements and scaffold hopping have been very successful approaches in the search for new nematicides for sustainable crop protection. In the exploration of novel fluensulfone analogues with nematicidal activity, bioisosteric replacement of sulfone by amide, chain extension by insertion of a methylene group, and reversal of the amide group have proven to be successful approaches and yielded new and highly active fluensulfone analogues. These attempts might result in compounds with an optimal balance of steric, hydrophobic, electronic, and hydrogen-bonding properties and contribute to deal with the complex problem during the research and development of new nematicides. Further ideas are also put forward to provide new approaches for the molecular design of nematicides.

3-Hydroxy-3-alkylindolin-2-ones: regioselective synthesis, molecular docking and nematicidal studies against Meloidogyne incognita

Regioselective nucleophilic addition of unsubstituted isatin (1) was carried out for the synthesis of pharmaceutically and to be agrochemically important 3-hydroxy-3-akylindolin-2-ones (3a-f) using discrete nucleophiles via generation of Grignard reagent. The synthesized derivatives were characterized by spectral techniques and were evaluated for nematicidal activity against Meloidogyne incognita. The nematicidal assay revealed that 1-ethyl-3-hydroxyindolin-2-one (3a) exhibited potent nematicidal activity against M. incognita. The most active member (3a) exhibited reasonably good ovicidal (LC₅₀ = 0.077 mg/mL) and larvicidal activity (LC₅₀ = 0.058 mg/mL), respectively. In support of the nematicidal activity, molecular docking of isatin (1) and its derivatives (3a-f) was performed using three parasitic proteins viz., carboxylic ester hydrolase, cytochrome c oxidase and aspartyl protease which revealed maximum interaction with amino acid residues Tyr 356, Tyr 170, Glu 238, Glu 327, Arg 271, Arg 112, Ser 29, Ser 31, Ser 368, Asn 115, Leu 326 and His 51 which act as supporting factors for compounds to curb the parasite.

One-Pot Diastereoselective Synthesis of Pyrrolopiperazine-2,6-diones by a Ugi/Nucleophilic Substitution/N-Acylation Sequence

The diastereoselective synthesis of two families of pyrrolopiperazine-2,6-diones is presented. These compounds were prepared by one-pot Ugi/nucleophilic substitution/N-acylation/debenzoylation/(elimination) sequences. This novel route provides straightforward access to a wide variety of pyrrolopiperazine-2,6-diones with high chemical yields and complete diastereoselectivities. The proposed synthetic strategy poses a significant improvement compared to the syntheses of pyrrolopiperazine-2,6-diones previously described, as it allows introduction of different substituents to the C4 position and the diastereoselective generation of a new stereogenic center on the bridgehead carbon (C8a).

Development of 1,3,4-Oxadiazole Derived Antifungal Agents and Their Application in Maize Diseases Control

Maize is an important food crop and its fungal disease has become a limiting factor to improve the yield and quality of maize. In the control of plant pathogens, commercial fungicides have no obvious effect on corn diseases due to the emergence of drug resistance. Therefore, it is of great significance to develop new fungicides with novel structure, high efficiency, and low toxicity to control maize diseases. In this paper, a series of 1,3,4-oxadiazole derivatives were designed and synthesized from benzoyl hydrazine and aromatic aldehydes through condensation and oxidation cyclization reaction. The antifungal activity of oxadiazole derivatives against three maize disease pathogens, such as Rhizoctonia solani (R. solani), Gibberella zeae (G. zeae), and Exserohilum turcicum (E. turcicum), were evaluated by mycelium growth rate method in vitro. The results indicated that most of the synthesized derivatives exhibited positive antifungal activities. Especially against E. turcicum, several compounds demonstrated significant antifungal activities and their EC 50 values were lower than positive control carbendazim. The EC 50 values of compounds 4k, 5e, and 5k were 50.48, 47.56, 32.25 μg/ml, respectively, and the carbendazim was 102.83 μg/ml. The effects of active compounds on E. turcicum microstructure were observed by scanning electron microscopy (SEM). The results showed that compounds 4k, 5e, and 5k could induce the hyphae of E. turcicum to shrink and collapse obviously. In order to elucidate the preliminary mechanism of oxadiazole derivatives, the target compounds 5e and 5k were docked with the theoretical active site of succinate dehydrogenase (SDH). Compounds 5e and 5k could bind to amino acid residues through hydrophobic contact and hydrogen bonds, which explained the possible mechanism of binding between the inhibitor and target protein. In addition, the compounds with antifungal activities had almost no cytotoxicity to MCF-7. This study showed that 1,3,4-oxadiazole derivatives were worthy for further attention as potential antifungal agents for the control of maize diseases.

High-Level Expression of Nitrile Hydratase in Escherichia coli for 2-Amino-2,3-Dimethylbutyramide Synthesis

In the synthesis of imidazolinone herbicides, 2-Amino-2,3-dimethylbutyramide (ADBA) is an important intermedium. In this study, the recombinant production of nitrile hydratase (NHase) in Escherichia coli for ADBA synthesis was explored. A local library containing recombinant NHases from various sources was screened using a colorimetric method. NHase from Pseudonocardia thermophila JCM3095 was selected, fused with a His-tag and one-step purified. The enzymatic properties of recombinant NHase were studied and indicated robust thermal stability and inhibition of cyanide ions due to substrate degradation. After systematic optimization of fermentation conditions, the OD600 (optical density at 600 nm), enzyme activity and specific activity of recombinant strain E. coli BL21(DE3)/pET-28a+NHase reached 19.4, 3.72 U/mL and 1.04 U/mg protein at 42 h, representing 5.86-, 26.6- and 4-fold increases, respectively. These results offered an efficient recombinant whole-cell biocatalyst for ADBA synthesis.