Electrophilic activation of nitroalkanes in efficient synthesis of 1,3,4-oxadiazoles

An Electrosynthesis of 1,3,4-Oxadiazoles from N-Acyl Hydrazones

The 1,3,4-oxadiazole is a widely encountered motif in the areas of pharmaceuticals, materials, and agrochemicals. This work has established a mediated electrochemical synthesis of 2,5-disubstituted 1,3,4-oxadiazoles from N-acyl hydrazones. Using DABCO as the optimal redox mediator has enabled a mild oxidative cyclisation, without recourse to stoichiometric oxidants. In contrast to previous methods, this indirect electrochemical oxidation has enabled a broad range of substrates to be accessed, with yields of up to 83 %, and on gram scale. The simplicity of the method has been further demonstrated by the development of a one-pot procedure, directly transforming readily available aldehydes and hydrazides into valuable heterocycles.

Metal-Free, PPA-Mediated Fisher Indole Synthesis via Tandem Hydroamination-Cyclization Reaction between Simple Alkynes and Arylhydrazines

A new variant of Fisher indole synthesis involving Bronsted acid-catalyzed hydrohydrazination of unactivated terminal and internal acetylenes with arylhydrazines is reported. The use of polyphosphoric acid alone either as the reaction medium or in the presence of a co-solvent appears to provide the required balance for activating the C-C triple bond towards the nucleophilic attack of the hydrazine moiety without unrepairable reactivity loss of the latter due to competing amino group protonation. Additionally, the formal hydration of acetylenes to the corresponding ketones occurs under the same conditions, making it an alternative approach for generating carbonyl groups from alkynes.

Synthesis of β-Carbolines with Electrocyclic Cyclization of 3-Nitrovinylindoles

The β-carboline motif is common in drug discovery and among numerous biologically active natural products. However, its synthetic preparation relies on multistep sequences and heavily depends on the type of substitution required in the core of the desired β-carboline target. Herein, we demonstrate that this structural motif can be accessed with the microwave-assisted electrocyclic cyclization of heterotrienic aci (alkylideneazinic acid) forms of 3-nitrovinylindoles. The reaction can start with 3-nitrovinylindoles themselves under two sets of conditions. The first one involves microwave irradiation of butanolic solutions of 3-nitrovinylindoles, whereas the second one consists of prior Boc protection of indolic nitrogen, where the protecting group cleanly comes off during the course of the reaction. Alternatively, the reaction can start with 3-nitrovinylindoles prepared in situ using various processes. Finally, the reaction may utilize indoles with β-nitrostyrenes, likely involving the intermediacy of spirocyclic oxazolines, which rearrange to similar heterotrienic systems undergoing cyclization to β-carbolines. As part of this study, several natural products, namely, alkaloids norharmane, harmane, and eudistomin N, were synthesized.

DFT Study of Regio- and Stereoselective 13DC Reaction between Diazopropane and Substituted Chalcone Derivatives: Molecular Docking of Novel Pyrazole Derivatives as Anti-Alzheimer's Agents

In the present work, a combination of experimental and density functional theory (DFT) investigation of the (3+2) cycloaddition reactions of diazopropane with chalcone derivatives was reported. All calculations were performed using several DFT approaches (B3LYP, M06, M06-2X) and 6-311+G(d, p) basis set. Based on the NMR, MS analyses and IRC calculations, the pyrazole derivatives are the kinetic adducts over the oxadiazoles. The use of two equivalents of diazopropane leads to thermodynamical products. A molecular docking analysis was performed to investigate the efficiency of the obtained products against selected drug targets in anti-Alzheimer ligand-receptor interactions. We revealed that the ligands selected were bound mainly to the catalytic (CAS) and peripheral (PAS) anionic sites of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) inhibitors, respectively. The selected ligands 1, 3, 4 and P14 may act as the best inhibitors against Alzheimer's disease (AD).

Polyphosphoric acid-promoted one-pot synthesis and neuroprotective effects of flavanones against NMDA-induced injury in PC12 cells

We report herein an efficient polyphosphoric acid (PPA) promoted one-pot protocol for the synthesis of flavanone derivatives from 2-hydroxyacetophenones and benzaldehydes. A variety of flavanones were produced in moderate to excellent yields and evaluated for their neuroprotective effects against N-methyl-d-aspartate (NMDA)-induced excitotoxicity in PC12 cells. Derivatives bearing electron-donating groups exhibited better neuroprotective activity. Compound 3m demonstrated the best protective potency and reversed the intracellular calcium (Ca²⁺) influx caused by NMDA, suggesting that flavanones protected the PC12 cells against NMDA-induced neurotoxicity via inhibition of Ca²⁺ overload.

Nitrovinylindoles as Heterotrienes: Electrocyclic Cyclization En Route to β-Carbolines: Total Synthesis of Alkaloids Norharmane, Harmane, and Eudistomin N

Unusual cascade transformation was developed involving microwave assisted electrocyclic cyclization of aci (alkylideneazinic acid) forms of nitrovinylindoles acting as heterotrienes. Subsequent one-pot reduction allowed for efficient access to β-carbolines, including several natural products, alkaloids norharmane, harmane and eudistomin N.

Electrophilically Activated Nitroalkanes in Double Annulation of [1,2,4]Triazolo[4,3-a]quinolines and 1,3,4-Oxadiazole Rings

Nitroalkanes activated with polyphosphoric acid could serve as efficient electrophiles in reactions with amines and hydrazines, enabling various cascade transformations toward heterocyclic systems. This strategy was developed for an innovative synthetic protocol employing simultaneous or sequential annulation of two different heterocyclic cores, affording [1,2,4]triazolo[4,3-a]quinolines with 1,3,4-oxadiazole substituents.

Electrophilically Activated Nitroalkanes in Synthesis of 3,4-Dihydroquinozalines

Nitroalkanes activated with polyphosphoric acid serve as efficient electrophiles in reactions with various nucleophilic amines. Strategically placed second functionality allows for the design of annulation reactions enabling preparation of various heterocycles. This strategy was employed to develop an innovative synthetic approach towards 3,4-dihydroquinazolines from readily available 2-(aminomethyl)anilines.

Preparation of 3,5-diarylsubstituted 5-hydroxy-1,5-dihydro-2H-pyrrol-2-ones via base-assisted cyclization of 3-cyanoketones

A convenient preparative method is developed allowing for expeditious assembly of 3,5-diarylsubstituted 5-hydroxy-1,5-dihydro-2H-pyrrol-2-ones from routinely available inexpensive synthetic precursors. These compounds could not be prepared via the previously known protocols, as 2-aminofuran derivatives were produced instead.

Synthesis, Characterization, and Biological Evaluation of New Derivatives Targeting MbtI as Antitubercular Agents

Tuberculosis (TB) causes millions of deaths every year, ranking as one of the most dangerous infectious diseases worldwide. Because several pathogenic strains of Mycobacterium tuberculosis (Mtb) have developed resistance against most of the established anti-TB drugs, new therapeutic options are urgently needed. An attractive target for the development of new antitubercular agents is the salicylate synthase MbtI, an essential enzyme for the mycobacterial siderophore biochemical machinery, absent in human cells. A set of analogues of I and II, two of the most potent MbtI inhibitors identified to date, was synthesized, characterized, and tested to elucidate the structural requirements for achieving an efficient MbtI inhibition and a potent antitubercular activity with this class of compounds. The structure-activity relationships (SAR) here discussed evidenced the importance of the furan as part of the pharmacophore and led to the preparation of six new compounds (IV-IX), which gave us the opportunity to examine a hitherto unexplored position of the phenyl ring. Among them emerged 5-(3-cyano-5-(trifluoromethyl)phenyl)furan-2-carboxylic acid (IV), endowed with comparable inhibitory properties to the previous leads, but a better antitubercular activity, which is a key issue in MbtI inhibitor research. Therefore, compound IV offers promising prospects for future studies on the development of novel agents against mycobacterial infections.

Does electrophilic activation of nitroalkanes in polyphosphoric acid involve formation of nitrile oxides?

The mechanistic rationale involving activation of nitroalkanes towards interaction with nucleophilic reagents in the presence of polyphosphoric acid (PPA) was re-evaluated. Could nitrile oxide moieties be formed during this process? This experiment demonstrates that at least in some cases this could happen, as generated nitrile oxides were successfully intercepted as adducts of [3 + 2] cycloadditions.

The mechanistic rationale involving activation of nitroalkanes towards interaction with nucleophilic reagents in the presence of polyphosphoric acid (PPA) was re-evaluated. Could nitrile oxide moieties be formed during this process?

Synthesis of imidazo[1,5-a]pyridines via cyclocondensation of 2-(aminomethyl)pyridines with electrophilically activated nitroalkanes

Imidazo[1,5-a]pyridines were efficiently prepared via the cyclization of 2-picolylamines with nitroalkanes electrophilically activated in the presence of phosphorous acid in polyphosphoric acid (PPA) medium.

DEAD-Mediated Oxidative Ugi/Aza-Wittig Reaction for the Synthesis of 5-(1,2,3,4-Tetrahydroisoquinolin-1-yl)-1,3,4-oxadi­­azoles Starting from (N-Isocyanimine)triphenylphosphorane

A one-pot protocol for the synthesis of 5-(1,2,3,4-tetra­hydroisoquinolin-1-yl)-1,3,4-oxadiazoles via DEAD-mediated oxidative Ugi/aza-Wittig reaction has been developed. The reaction of (N-iso­cyanimine)triphenylphosphorane, carboxylic acids, and N-aryl-1,2,3,4-tetrahydroisoquinolines produced polysubstituted 5-(1,2,3,4-tetra­hydroisoquinolin-1-yl)-1,3,4-oxadiazoles directly in good yields in the presence of DEAD as an oxidant.

Nitroalkanes as electrophiles: synthesis of triazole-fused heterocycles with neuroblastoma differentiation activity

We discovered a reaction of nitroalkanes with 2-hydrazinylquinolines, 2-hydrazinylpyridines and bis-2,4-dihydrazinylpyrimidines in polyphosphoric acid (PPA) affording 1,2,4-triazolo[4,3-a]quinolines, 1,2,4-triazolo[4,3-a]pyridines and bis[1,2,4]triazolo[4,3-a:4',3'-c]pyrimidines, respectively. The reaction expands the scope of heterocyclic annulations involving phosphorylated nitronates, believed to be the electrophilic intermediates formed from nitroalkanes in PPA. Several of the synthesized triazoles showed promising anticancer activity by inducing differentiation in neuroblastoma cancer cells. Due to the urgent need for novel differentiation agents for neuroblastoma therapy, this finding warrants further evaluation of this class of compounds against neuroblastoma.

Crystal structure of (E)-4-hydroxy-3-((5-phenyl-1,3,4-oxadiazol-2-yl)thio)pent-3-en-2-one, C13H12N2O3S

C13H12N2O3S, monoclinic, P21/c (no. 14), a = 12.3541(8) Å, b = 14.3818(10) Å, c = 7.3757(5) Å, β = 96.471(2)°, V = 1302.12(15) Å3, Z = 4, R gt(F) = 0.0417, wR ref(F 2) = 0.1080, T = 293(2) K.

Electrochemical one-pot synthesis of five-membered azaheterocycles via [4 + 1] cyclization

Five-membered N-heterocycles, such as oxadiazoles, thiadiazoles, oxazolines and imidazoles, have been prepared via electrochemical oxidation/[4 + 1] cyclization/dehydrogenative aromatization in one pot from readily available materials.

Electrophilically activated nitroalkanes in reaction with aliphatic diamines en route to imidazolines

A novel synthetic methodology for the assembly of imidazolines via an unusual reaction between nitroalkanes and aliphatic 1,2-diamines in the presence of phosphorous acid is described. In contrast to the related highly efficient preparation of benzimidazoles from aromatic amines, this process represents a major synthetic challenge and for a long time was elusive. Analysis of the method limitations is provided.

Imidazolines were assembled via an unusual reaction between nitroalkanes and aliphatic 1,2-diamines in the presence of phosphorous acid.