Benzimidazoles and benzoxazoles via the nucleophilic addition of anilines to nitroalkanes

Electrophilic Intermediates in the Nef and Meyer Reactions: A Computational Study

The generation, interconversion, and reactivity of electrophilic species generated upon activation of nitroalkanes with protic acids (the Nef and Meyer reactions) were investigated by quantum-chemical calculations. N,N-Bis(hydroxy)iminium (R2C═N+(OH)2) and N-oxoiminium (R2C═N+═O) cations were shown to be produced independently from aci-nitroalkanes, while N-hydroxynitrilium cations (RC≡N+-OH) were formed via nearly barrierless C-H bond cleavage in N-oxoiminium cations. The N-oxoiminium and N-hydroxynitrilium cations whose formation is favored under highly acidic anhydrous conditions are strong electrophiles capable of reacting even with nonactivated arenes under ambient conditions. The N-oxoiminium cations R2C═N+═O are highly unusual ambident species containing three contiguous electrophilic centers (C, N, and O atoms). Nucleophilic addition at the oxygen atom is less preferred than the C- and N-attack yet possible in an intramolecular variant. These computational results shed light on some key aspects of the mechanisms of the Nef and Meyer reactions and predict the possibility of numerous interrupted versions of these reactions.

A mild and convenient protocol for the synthesis of quinoxalin-2(1H)-ones and benzimidazoles

We present a mild and simple method for the cyclization of N-protected o-phenylenediamines with carbonyl compounds in the presence of trifluoroacetic acid. This method reliably provides various substrates of benzimidazoles and quinoxalin-2(1H)-ones, with all reactions conducted at room temperature, demonstrating excellent substrate adaptability and a broad substrate scope.

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.

Interrupted Nef and Meyer Reactions: A Growing Point for Diversity-Oriented Synthesis Based on Nitro Compounds

The Nef reaction (nitro to carbonyl group conversion) and related Meyer reaction are among the key transformations of aliphatic nitro compounds. The interrupted versions of these reactions in which the normal pathway is redirected to a different end product by an external nucleophile are much less common, albeit these processes substantially increase the synthetic potential of nitro compounds. In this review, examples of interrupted Nef and Meyer reactions are summarized, and the prospects of this methodology in diversity-oriented organic synthesis are analyzed. The bibliography contains 90 references.

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.

Copper(II)-Dioxygen Facilitated Activation of Nitromethane: Nitrogen Donors for the Synthesis of Substituted 2-Hydroxyimino-2-phenylacetonitriles and Phthalimides

A simple and efficient method is explored for the synthesis of 2-hydroxyimino-2-phenylacetonitriles (2) and phthalimides (4), by using nitromethane as nitrogen donors. Both reactions are promoted by Cu(II) system with the participation of dioxygen as an oxidant. The scope of the method has been successfully demonstrated with a total of 51 examples. The flexible and diversified characteristics of reactions are introduced in terms of electronic effect, steric effect, position of substituted groups, and intramolecular charge transfer. Experimental studies suggest that the methyl nitrite could be a precursor in the path to the final products. A possible reaction mechanism is proposed, including the Cu(II)/O2-facilitated transformation of nitromethane to methyl nitrite, the base-induced formation of 2-hydroxyimino-2-phenylacetonitriles, and the base-dioxygen-promoted formation of phthalimides.

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.

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.

Electrophilic alkylation of arenes with 5-bromopyrimidine en route to 4-aryl-5-alkynylpyrimidines

A new synthetic protocol for preparation of medicinally important 4-aryl-5-alkynylpyrimidines is described. The featured approach involves a sequence of chemo- and regioselective Brønsted acid-catalyzed electrophilic alkylation of arenes with 5-bromopyrimidine, followed by oxidative re-aromatization of the formed dihydropyrimidine ring. Finally, palladium-catalyzed Sonogashira cross-coupling reaction provided an end-game strategy.

Nitro compounds as the core structures of promising energetic materials and versatile reagents for organic synthesis

This review addresses some promising areas of chemistry of nitro compounds extensively developed in recent years in Russia (particularly at the N.D.Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences) and worldwide. The most important results in the synthesis of novel energetic N-, C- and O-nitro compounds are summarized. New environmentally friendly approaches to the preparation of known compounds of this series, used as components of energetic compositions, are considered. Methods for selective transformations of various nitro compounds to valuable products of organic synthesis, primarily biologically active products and their precursors, are systematically analyzed.

The bibliography includes 446 references.

Employment of α-nitroketones in organic synthesis

This review summarizes the recent developments of the reactions of α-nitroketones and the consequent formation of a variety of structural frameworks is discussed.

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

A novel methodology for general and chemoselective preparation of non-symmetric 1,3,4-oxadiazoles is developed. This unusual reaction proceeds via polyphosphoric acid-assisted activation of nitroalkanes towards nucleophilic attack with acylhydrazides.

A novel methodology for chemoselective preparation of non-symmetric 1,3,4-oxadiazoles via PPA-assisted activation of nitroalkanes towards nucleophilic attack with acylhydrazides is developed.

Catalytic synthesis of benzimidazoles and organic carbamates using a polymer supported zinc catalyst through CO2 fixation

Zinc metal is attached to the organically modified polystyrene and the obtained catalyst is well characterized. The catalyst is very efficient for the formation of benzimidazoles and organic carbamates through carbon dioxide fixation.

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.

A convenient one-pot synthesis of N-substituted amidoximes and their application toward 1,2,4-oxadiazol-5-ones

The first direct one-pot approach for the synthesis of N-substituted amidoximes from secondary amides or the intermediate amides has been developed. Through the Ph3P-I2-mediated dehydrative condensation, a variety of N-aryl and N-alkyl amidoximes (R1(C[double bond, length as m-dash]NOH)NHR2, where R1 or R2 = aryl, alkyl, or benzyl) were readily afforded under mild conditions and short reaction times. The synthetic application of the obtained amidoximes has also been demonstrated through the formation of 1,2,4-oxadiazolones via base-mediated carbonylative cyclization with 1,1'-carbonyldiimidazole.

Nitrostyrenes as 1,4-CCNO-dipoles: diastereoselective formal [4+1] cycloaddition of indoles

An unusual reactivity of nitrostyrenes in phosphorous acid was discovered, which permits the employment of these readily available synthons as 1,4-CCNO-dipole surrogates in a highly diastereoselective (4+1)-cycloaddition of indoles to afford 4′H-spiro[indole-3,5′-isoxazoles] in a diastereomerically pure form.

A nitroalkane-based approach to one-pot three-component synthesis of isocryptolepine and its analogs with potent anti-cancer activities

A second generation polyphosphoric acid-mediated one-pot three-component synthesis of indoloquinoline scaffold is developed. This improved version of the process involves electrophilically activated nitroalkanes for the installation of strategic C–C and C–N bonds and ring C assembly. This modification allows the elimination of unnecessary solvent change operations and all steps are carried out in a true, uninterrupted one-pot manner. A further improvement involves the possibility to install an ortho-amino group in situ. A synthetic application of this method is showcased by the concise synthesis of an isocryptolepine alkaloid and its synthetic analogs with potent anticancer activities.

An improved one-pot three-component synthesis involving electrophilically activated nitroalkanes allowed for efficient preparation of indoloquinolines with potent anticancer activities.

Unexpected cyclization of 2-(2-aminophenyl)indoles with nitroalkenes to furnish indolo[3,2-c]quinolines

A novel synthetic route to the indoloquinoline core of the alkaloid isocryptolepine involving an unprecedented PPA-mediated reaction of 2-(2-aminophenyl)indenes with nitroalkenes is discovered.

The synthesis of benzimidazoles via a recycled palladium catalysed hydrogen transfer under mild conditions

An efficient synthesis of benzimidazoles was developed by virtue of a recycled palladium catalyzed hydrogen transfer.

Dual role of polyphosphoric acid-activated nitroalkanes in oxidative peri-annulations: efficient synthesis of 1,3,6,8-tetraazapyrenes

Electrophilically activated nitroalkanes play a dual role in the novel reaction with diaminoperimidines, promoting their oxidative peri-annulation to access tetraazapyrenes.

One-pot strategy of copper-catalyzed synthesis of 1,2-disubstituted benzimidazoles

A simple, one-pot and copper-catalyzed coupling reaction for the construction of 1,2-disubstituted benzimidazole derivatives is described. A low-cost copper salt and a weak base, K₃PO₄, were utilized in this reaction. A variety of 1,2-disubstituted benzimidazoles were obtained in moderate to excellent yields.

Rational design of an efficient one-pot synthesis of 6H-pyrrolo[2,3,4-gh]perimidines in polyphosphoric acid

Several highly efficient one-pot synthetic protocols were developed, enabling polyphosphoric acid-activated nitroalkanes to act as electrophiles in reactions with aminonapthalenes.

Direct metal-free synthesis of diarylamines from 2-nitropropane via the twofold C–H functionalization of arenes

Synthesis of symmetric diarylamines via a twofold intermolecular electrophilic C–H functionalization of electron-rich arenes by umpolung-activated nitroalkane in polyphosphoric acid is demonstrated.

One-pot synthesis of benzoxazoles via the metal-free ortho-C–H functionalization of phenols with nitroalkanes

PPA-activated nitroalkanes are employed in the design of a one-pot cascade transformation involvingortho-C–H functionalization, by Beckman rearrangement, and condensation to produce benzoxazoles and benzobisoxazoles directly from phenols.