Activating Pyrimidines by Pre-distortion for the General Synthesis of 7-Aza-indazoles from 2-Hydrazonylpyrimidines via Intramolecular Diels-Alder Reactions

Synthesis of 3-Aminoazaindazoles via Cu-Catalyzed Cross Coupling of Isocyanides

Nitrogen-containing heterocycles are commonly encountered in drug discovery, but the synthesis of such ring structures is not always efficient. Fused heterocyclic rings, in particular, can be challenging to synthesize. Herein, we report a highly convergent synthesis of 3-aminoazaindazoles via a Cu-catalyzed reaction between isocyanides and 3-halo-2-hydrazineylpyridines (and analogues). Reaction optimization through high-throughput experimentation (HTE) identified a novel set of exogenous ligand-free Cu conditions utilizing a cheap and readily available catalyst. The reaction displays high functional group tolerance and has the potential to be highly enabling for medicinal chemistry efforts. A putative mechanism is described as well as preliminary mechanistic experiments.

Synthesis of Highly Functionalized 4-Iodo-7-azaindazoles via Condensation/Diels-Alder/Retro-Diels-Alder Cyclization of Iodoalkynones and 2-Hydrazineylpyrimidines

A method for the preparation of highly functionalized 4-iodo-7-azaindazoles is reported. These valuable heterocycles are synthesized via condensation of 2-hydrazineylpyrimidines with various iodoalkynones followed by Diels-Alder/retro-Diels-Alder cyclization. The method is general to the formation of products with a variety of C3, C5, and C6 substituents while preserving the C4 iodide functional handle for further late-stage functionalization. The utility of this transformation is demonstrated through the rapid synthesis of several bioactive azaindazole targets.

Divergent Synthesis of 6- or 7-Aza-Indazoles via Intramolecular Diels-Alder Cascade of Pyrazines

Pyrazines are reactive 4π partners in intermolecular Diels-Alder cycloaddition with exceptionally activated dienophiles or in an intermolecular version at elevated temperatures. Herein, it is shown that an intramolecular cascade could occur even at room temperature, delivering a collection of 6- or 7-aza-indazoles. An interesting substituent effect of the cycloaddition precursor on the product distribution was uncovered, and in situ NMR studies were conducted to gain insights into this unexpected selectivity.

A General Synthesis of Cross-Conjugated Enynones through Pd Catalyzed Sonogashira Coupling with Triazine Esters

The palladium-catalyzed Sonogashira coupling of α, β-unsaturated acid derivatives offers a diversity-oriented synthetic strategy for cross-conjugated enynones. However, the susceptibility of the unsaturated C-C bonds adjacent to the carbonyl group toward Pd catalysts makes the direct conversion of α, β-unsaturated derivatives as acyl electrophiles to cross-conjugated ketones rare. This work presents a highly selective C-O activation approach to prepare cross-conjugated enynones using α, β-unsaturated triazine esters as acyl electrophiles. Under base and phosphine ligand-free conditions, NHC-Pd(II)-Allyl precatalyst alone catalyzed the cross-coupling of α, β-unsaturated triazine esters with terminal alkynes efficiently, yielding 31 cross-conjugated enynones with diverse functional groups. This method demonstrates the potential of triazine-mediated C-O activation for preparing highly functionalized ketones.

Exploration of Fungicidal Activity and Mode of Action of Ferimzone Analogs

Lead discovery and molecular target identification are important for developing novel pesticides. Scaffold hopping, an effective approach of modern medicinal and agrochemical chemistry for a rational design of target molecules, is aiming to design novel molecules with similar structures and similar/better biological performance. Herein, 24 new ferimzone derivatives were designed and synthesized by a scaffold-hopping strategy. In vitro bioassays indicated that compound 5o showed similar potency to ferimzone against Cercospora arachidicola and 2-fold higher potency than ferimzone against Alternaria solani. Compounds 5q, 6a, and 6d displayed fungicidal activity with EC₅₀ values ranging from 1.17 to 3.84 μg/mL against Rhizoctonia solani, and compounds 5q and 6a displayed 1.6-1.8-fold higher activity than ferimzone against Fusarium graminearum. The in vivo bioassays at 200 μg/mL indicated that compound 5q was more potent than ferimzone against Pyricularia oryzae (90% vs 70% efficacy, respectively). Density functional theory (DFT) calculations elucidated the structure-energy relationship. Although the mode of action of ferimzone is still unclear, studies suggested that compound 5q significantly inhibited the growth and reproduction of R. solani, and its energy metabolism pathways (e.g., starch, sucrose, lipids, and glutathione) were seriously downregulated after a 5q treatment.

Fluoro, Trifluoromethyl, and Trifluoroacetyl Substituent Effects on Cycloaddition Reactivities: Computations and Analysis

The importance of fluoro and trifluoromethyl substituents in drug effectiveness prompted the computational exploration of fluorine-containing substituents in valuable synthetic cycloadditions. Diels-Alder or 1,3-dipolar cycloaddition reactions of typical reactants, cyclopentadiene, N-phenyldiazoacetamide, tetrazine, and N-phenylsydnone involving fluorine-containing substituents (F, CF₃, and COCF₃) were studied with M06-2X density functional theory. Inductive and conjugative effects influence normal and inverse electron-demand reactions differently. These results provide a guide to the design and use of cycloadditions for the introduction of fluoro and trifluoromethyl substituents in synthetic processes.

Hydrazones as Substrates in the Synthesis of Isoxazolidines via a KOH-Promoted One-Pot Three-Component Cycloaddition with Nitroso Compounds and Olefins

Hydrazones have been employed as the starting materials in a KOH-mediated one-pot three-component cycloaddition with readily accessible nitroso compounds and olefins to construct various isoxazolidines. Compared with diazo compounds as starting materials, this methodology could afford a wider range of products in good to excellent yields and diastereoselectivities for most substrates, and hydrazones are cheaper, more accessible, and safer substrates. The experimental study shows that the choice of suitable hydrazones is crucial.

Application of 1,4-Oxazinone Precursors to the Construction of Pyridine Derivatives by Tandem Intermolecular Cycloaddition/Cycloreversion

This study reveals a new method for the preparation of 1,4-oxazinone derivatives by Staudinger reductive cyclization of functionalized vinyl azide precursors. The resulting oxazinone derivatives prepared in this manner were intercepted with terminal alkyne substrates through an intermolecular cycloaddition/cycloreversion sequence to afford polysubstituted pyridine products. Alkyne substrates bearing propargyl oxygen substitution showed good regioselectivity in the cycloaddition operation selectively affording 2,4,6-substituted pyridines. Application of this chemistry to the synthesis of an ErbB4 receptor inhibitor is also described.

DFT investigation of solvent, substituent, and catalysis effects on the intramolecular Diels-Alder reaction

In this study, we report on a DFT investigation of two intramolecular Diels-Alder furan reactions. Optimizations of the studied structures, TS and IRC calculations, were carried out at B3LYP/6-31G(d) level. We have studied the effect of substituent, solvent and Lewis acid catalyst on cyclization-retrocyclization equilibria, activation energies, and stability of the desired products. The analysis of orbital coefficients, IRC curves, and Wiberg indices have proved that both reactions are under orbital control. We have found that for the reaction I (2↔4 + 5), where R = H, the exo attack is favored by hydrogen bond interaction, while for R = t-Bu, the steric hindrance leads to the endo attack. For the reaction II (3 → 6 + 7), the t-Bu-substituted products are the most stable ones. At another level, we have found that it is recommended to use polar organic solvents as DMSO with Lewis acid catalyst BF₃. The latest leads to accelerate the reaction II with stabilization of the desired products.

CuII-Catalyzed Coupling with Two Ynone Units by Selective Triple and Sigma C-C and C-H Bond Cleavages

We report a new copper-catalyzed [2 + 2 + 1] annulation process through the selective cleavage of sigma and triple C-C and C-H bonds using two ynone units. This new methodology involves breaking multiple chemical bonds in a single operation, including C≡C, C-C, C-H, and N-O. These high-value adducts lead to a diverse collection of synthetically challenging trisubstituted indolizines by the simultaneous engagement of different bond-breaking events and show excellent fluorescence in green aqueous solutions.

A study of antituberculosis activities and crystal structures of (E)-2-[2-(arylidene)hydrazinyl]pyrimidine and (E)-N 1-(arylidene)pyrimidine-2-carbohydrazide derivatives

A study of the anti-tuberculosis activity against Mycobacterium tuberculosis ATTC 27294 and an X-ray structural determination of (E)-2-[2-(arylidene)hydrazinyl]pyrimidine, 1, and (E)-N 1-(arylidene)pyrimidine-2-carbohydazide, 2, derivatives are presented. The effect of the substituents in the aryl moiety on the antituberculosis (anti-TB) activities of 1 and 2 is compared with that of other heteroaryl hydrazonyl and acylhydrazonyl derivatives. The biological activities of 1 do not depend on the coordinating ability of the substituted aryl group: in 2, the most effective aryl group is 5-nitrofuranyl. The structure determinations of (E)-2-((2-(pyrimidin-2-yl)hydrazono)methyl)-phenol, (E)-N′-(2,5-dihydroxybenzylidene)pyrimidine-2-carbohydrazide and of the hydrate of (E)-N′-(2-hydroxy-4-methylbenzylidene)pyrimidine-2-carbohydrazide, and a literature search of related structures in the CCDC data base, allowed an examination of the more important interactions, including the occurrence of X–Y⋯π interactions.