Chemoselective nitration of aromatic sulfonamides with tert-butyl nitrite

Steric-Hindrance-Induced Diastereoselective Radical Nitration of 3-Alkylidene-2-oxindoles Followed by Tosylhydrazine-Mediated Sulfonation

Metal-free radical nitration of the β C-H bond of 3-alkylidene-2-oxindoles with tert-butyl nitrite (TBN) has been explored. Interestingly, (E)-3-(2-(aryl)-2-oxoethylidene)oxindole and (E)-3-ylidene oxindole give different diastereomers on nitration. The mechanistic investigation revealed that the diastereoselectivity was controlled by the size of the functional group. Another transformation of 3-(nitroalkylidene) oxindole into 3-(tosylalkylidene) oxindole was performed through metal and oxidant-free tosylhydrazine-mediated sulfonation. Both methods have the advantages of readily available starting materials and operational simplicity.

Organic Nitrating Reagents

Nitro compounds are vital raw chemicals that are widely used in academic laboratories and industries for the preparation of various drugs, agrochemicals, and materials. Thus, nitrating reactions are of great importance for chemists and are even taught in schools as one of the fundamental transformations in organic synthesis. Since the discovery of the first nitrating reactions in the 19th century, progress in this field has been constant. Yet, for many years the classical electrophilic nitration approach using a mixture of strong mineral acids dominated the field. However, in recent decades, the attention of researchers has focused on new reactivity and new reagents that can provide access to nitro compounds in a practical and straightforward way under mild reaction conditions. Organic nitrating reagents have played a special role in this field since they have enhanced reactivity. They also allow nitration to be carried out in an ecofriendly and sustainable manner. This review examines the development and application of organic nitrating reagents.

1 Introduction

2 Organic Nitrating Reagents

2.1 Alkyl Nitrites

2.2 Nitroalkanes

2.3 Alkyl Nitrates

2.4 N-Nitroamides

2.5 N-Nitropyrazole

2.6 N-Nitropyridinium Salts

3 Organic Nitrating Reagents Generated In Situ

3.1 Acyl Nitrates

3.2 Trimethylsilyl Nitrate

3.3 Nitro Onium Salts

4 Organic Nitronium Salts

5 Organic Nitrates and Nitrites

5.1 Ammonium Nitrates

5.2 Heteroarylium Nitrates

5.3 Other Organic Nitrates

5.4 Organic Nitrites

6 Conclusion and Outlook

Acid-Free Copper-Catalyzed Electrophilic Nitration of Electron-Rich Arenes with Guanidine Nitrate

A practical copper-catalyzed nitration of electron-rich arenes with trimethylsilyl chloride and guanidine nitrate is reported. A variety of nitrated products were generated in moderate to excellent yields (32-99%) at ambient temperature under acid-free, open-flask, and operationally simple conditions.

Selective C3–H nitration of 2-sulfanilamidopyridines with tert-butyl nitrite

A selective C3–H nitration of bioactive 2-sulfanilamidopyridine derivatives, including corticosteroid 11-β-dehydrogenase isozyme, secretory phospholipase A2 inhibitor and human neutrophil elastase inhibitor, has been reported.

Electrochemical Nitration with Nitrite

Aromatic nitration has tremendous importance in organic chemistry as nitroaromatic compounds serve as versatile building blocks. This study represents the electrochemical aromatic nitration with NBu₄ NO₂ , which serves a dual role as supporting electrolyte and as a safe, readily available, and easy-to-handle nitro source. Stoichiometric amounts of 1,1,1-3,3,3-hexafluoroisopropan-2-ol (HFIP) in MeCN significantly increase the yield by solvent control. The reaction mechanism is based on electrochemical oxidation of nitrite to NO₂ , which initiates the nitration reaction in a divided electrolysis cell with inexpensive graphite electrodes. Overall, the reaction is demonstrated for 20 examples with yields of up to 88 %. Scalability is demonstrated by a 13-fold scale-up.

Site-selective and metal-free C-H nitration of biologically relevant N-heterocycles

The site-selective and metal-free C-H nitration reaction of quinoxalinones and pyrazinones as biologically important N-heterocycles with t-butyl nitrite is described. A wide range of quinoxalinones were efficiently applied in this transformation, providing C7-nitrated quinoxalinones without undergoing C3-nitration. From the view of mechanistic point, the radical addition reaction exclusively occurred at the electron-rich aromatic region beyond electron-deficient N-heterocycle ring. This is a first report on the C7-H functionalization of quinoxalinones under metal-free conditions. In contrast, the nitration reaction readily takes place at the C3-position of pyrazinones. This transformation is characterized by the scale-up compatibility, mild reaction conditions, and excellent functional group tolerance. The applicability of the developed method is showcased by the selective reduction of NO₂ functionality on the C7-nitrated quinoxalinone product, providing aniline derivatives. Combined mechanistic investigations aided the elucidation of a plausible reaction mechanism.

3-Nitro-coumarin synthesis via nitrative cyclization of aryl alkynoates using tert-butyl nitrite

We report a one-pot metal-free synthesis of 3-nitro-coumarins from aryl alkynoates using TBN (tert-butyl nitrite) as the sole reagent. The radical triggered cyclization and nitration of the aryl alkynoates proceeded in a cascaded manner via nitro radical addition to alkynoates, then 5-exo-trig spirocyclization and ester migration.

Development of an Antibiotic Resistance Breaker to Resensitize Drug-Resistant Staphylococcus aureus: In Silico and In Vitro Approach

Efflux pumps are one of the predominant microbial resistant mechanisms leading to the development of multidrug resistance. In Staphylococcus aureus, overexpression of NorA protein enables the efflux of antibiotics belonging to the class of fluoroquinolones and, thus, makes S. aureus resistant. Hence, NorA efflux pumps are being extensively exploited as the potential drug target to evade bacterial resistance and resensitize bacteria to the existing antibiotics. Although several molecules are reported to inhibit NorA efflux pump effectively, boronic acid derivatives were shown to have promising NorA efflux pump inhibition. In this regard, the current study exploits 6-(3-phenylpropoxy)pyridine-3-boronic acid to further improve the activity and reduce cytotoxicity using the bioisostere approach, a classical medicinal chemistry concept. Using the SWISS-Bioisostere online tool, from the parent compound, 42 compounds were obtained upon the replacement of the boronic acid. The 42 compounds were docked with modeled NorA protein, and key molecular interactions of the prominent compounds were assessed. The top hit compounds were further analyzed for their drug-like properties using ADMET studies. The identified potent lead, 5-nitro-2-(3-phenylpropoxy)pyridine (5-NPPP), was synthesized, and in vitro efficacy studies have been proven to show enhanced efflux inhibition, thus acting as a potent antibiotic breaker to resensitize S. aureus without elucidating any cytotoxic effect to the host Hep-G2 cell lines.

Oxidation of the inert sp3 C-H bonds of tetrahydroisoquinolines through C-H activation relay (CHAR): construction of functionalized isoquinolin-1-ones

A TBN/O₂-initiated oxidation of the relatively inert 3,4-C-H bonds of THIQs was accomplished, in which the existence of an α-phosphoric ester group is crucial to enable dioxygen trapping and intramolecular HAT (C-H activation relay, CHAR), realizing the synthesis of a series of isoquinolin-1-ones in high yields. The mechanistic study confirmed that the formation of the 3,4-double bond is mediated by the CHAR process. This work provides a new strategy to achieve remote C-H bond activation.

Recent Advances in the Synthesis and Development of Nitroaromatics as Anti-Infective Drugs

Infectious diseases commonly occur in tropical and sub-tropical countries. The pathogens of such diseases are able to multiply in human hosts, warranting their continual survival. Infections that are commonplace include malaria, chagas, trypanosomiasis, giardiasis, amoebiasis, toxoplasmosis and leishmaniasis. Malaria is known to cause symptoms, such as high fever, chills, nausea and vomiting, whereas chagas disease causes enlarged lymph glands, muscle pain, swelling and chest pain. People suffering from African trypanosomiasis may experience severe headaches, irritability, extreme fatigue and swollen lymph nodes. As an infectious disease progresses, the human host may also experience personality changes and neurologic problems. If left untreated, most of these diseases can lead to death. Parasites, microbes and bacteria are increasingly adapting and generating strains that are resistant to current clinical drugs. Drug resistance creates an urgency for the development of new drugs to treat these infections. Nitro containing drugs, such as chloramphenicol, metronidazole, tinidazole and secnidazole had been banned for use as antiparasitic agents due to their toxicity. However, recent discoveries of nitrocontaining anti-tuberculosis drugs, i.e. delamanid and pretonamid, and the repurposing of flexinidazole for use in combination with eflornithine for the treatment of human trypanosomiasis, have ignited interest in nitroaromatic scaffolds as viable sources of potential anti-infective agents. This review highlights the differences between old and new nitration methodologies. It furthermore offers insights into recent advances in the development of nitroaromatics as anti-infective drugs.

Recent developments in photoredox-catalyzed remote ortho and para C-H bond functionalizations

In recent years, the research area of direct C-H bond functionalizations was growing exponentially not only due to the ubiquity of inert C-H bonds in diverse organic compounds, including bioactive natural and nonnatural products, but also due to its impact on the discovery of pharmaceutical candidates and the total synthesis of intricate natural products. On the other hand, more recently, the field of photoredox catalysis has become an indispensable and unparalleled research topic in modern synthetic organic chemistry for the constructions of challenging bonds, having the foremost scope in academia, pharmacy, and industry. Therefore, the development of green, simpler, and effective methodologies to accomplish direct C-H bond functionalization is well overdue and highly desirable to the scientific community. In this review, we mainly highlight the impact on, and the utility of, photoredox catalysts in inert ortho and para C-H bond functionalizations. Although a surge of research papers, including reviews, demonstrating C-H functionalizations have been published in this vital area of research, to our best knowledge, this is the first review that focuses on ortho and para C-H functionalizations by photoredox catalysis to provide atom- and step-economic organic transformations. We are certain that this review will act as a promoter to highlight the application of photoredox catalysts for the functionalization of inert bonds in the domain of synthetic organic chemistry.

Facile access to nitroarenes and nitroheteroarenes using N-nitrosaccharin

Nitroaromatics and nitroheteroaromatics serve as key building blocks and intermediates in synthesis, and form the core scaffold of a vast number of materials, dyes, explosives, agrochemicals and pharmaceuticals. However, their synthesis relies on harsh methodologies involving excess mineral acids, which present a number of critical drawbacks in terms of functional group compatibility and environmental impact. Modern, alternative strategies still suffer from significant limitations in terms of practicality, and a general protocol amenable to the direct C-H functionalization of a broad range of aromatics has remained elusive. Herein we introduce a bench-stable, inexpensive, easy to synthesize and recyclable nitrating reagent based on saccharin. This reagent acts as a controllable source of the nitronium ion, allowing mild and practical nitration of both arenes and heteroarenes displaying an exceptional functional group tolerance.

Tapping the therapeutic potential of protein tyrosine phosphatase 4A with small molecule inhibitors

Protein tyrosine phosphatases (PTPs) are emerging new targets for drug discovery. PTPs and protein tyrosine kinases (PTKs) maintain cellular homeostasis through opposing roles: tyrosine O-dephosphorylation and -phosphorylation, respectively. An imbalance in the phosphorylation equilibrium results in aberrant protein signaling and pathophysiological conditions. PTPs have historically been considered 'undruggable', in part due to a lack of evidence defining their relationship to disease causality and a focus on purely competitive inhibitors. However, a better understanding of protein-protein interfaces and shallow active sites has recently renewed interest in the pursuit of allosteric and orthosteric modulators of targets outside the major druggable protein families. While their biological mechanism of action still remains to be clarified, PTP4A1-3 (also referred to as PRL1-3) are validated oncology targets and play an important role in cell proliferation, metastasis, and tumor angiogenesis. In this Digest, recent syntheses and structure-activity relationships (SAR) of small molecule inhibitors (SMIs) of PTP4A1-3 are summarized, and enzyme docking studies of the most potent chemotype are highlighted. In particular, the thienopyridone scaffold has emerged as a potent lead structure to interrogate the function and druggability of this dual-specificity PTP.

Regioselective Functionalization of 9,9-Dimethyl-9-silafluorenes by Borylation, Bromination, and Nitration

Despite the utility of 9-silafluorenes as functional materials and as building blocks, methods for efficient functionalization of their backbone are rare, probably because of the presence of easily cleavable C-Si bonds. Although controlling the regioselectivity of iridium-catalyzed direct borylation of C-H bonds is difficult, we found that bromination and nitration of 2-methoxy-9-silafluorene under mild conditions occurred predominantly at the electron-rich position. The resulting product having methoxy and bromo groups can be utilized as a building block for the synthesis of unsymmetrically substituted 9-silafluorene-containing π-conjugated molecules.

In-flow photooxygenation of aminothienopyridinones generates iminopyridinedione PTP4A3 phosphatase inhibitors

A continuous flow photooxygenation of 7-aminothieno[3,2-c]pyridin-4(5H)-ones to produce 7-iminothieno[3,2-c]pyridine-4,6(5H,7H)-diones has been developed, utilizing ambient air as the sole reactant. N-H Imines are formed as the major products, and excellent functional group tolerance and conversion on gram-scale without the need for chromatographic purification allow for facile late-stage diversification of the aminothienopyridinone scaffold. Several analogs exhibit potent in vitro inhibition of the cancer-associated protein tyrosine phosphatase PTP4A3, and the SAR supports an exploratory docking model.

Reactions of 2-aza-21-carbaporphyrin with aniline derivatives

Post-synthetic modification of N-confused porphyrin (NCP) yields chiral 21-arylazo- or 3,21-bis(arylamino)-NCP in one-step, one-pot reactions of NCP with anilines.

tert-Butyl nitrite promoted transamidation of secondary amides under metal and catalyst free conditions

Transamidation of secondary amides with various amines is demonstrated using tert-butyl nitrite. The reaction proceeds through the N-nitrosamide intermediate and provides the desired products in excellent yields.

Multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite: facile synthesis of 2H-azirines

A new, catalyst-free tandem annulation route to 2,2-disulfonyl-2H-azirines via multiple-functionalizations of terminal alkynes with sodium sulfinates and tert-butyl nitrite is described.

Intramolecular nitration–aminocarbonylation of unactivated olefins: metal-free synthesis of γ-lactams

A novel, metal-free synthesis of valuable γ-lactams through the radical-mediated nitration–aminocarbonylation of unactivated olefins is reported.

Silver-catalyzed nitration/annulation of 2-alkynylanilines for a tunable synthesis of nitrated indoles and indazole-2-oxides

Two new types of silver-catalyzed nitration–annulations of 2-alkynylanilines with tert-butyl nitrite (TBN) were reported, leading to the selective formation of a variety of nitrated indoles and indazole-2-oxides.

One-step construction of molecular complexity by tert-butyl nitrite (TBN)-initiated cascade α,β-sp3 C-H bond difunctionalization and C-N bond cleavage

TBN-initiated cascade multifunctionalization of inert C-H and C-N bonds in N-arylazacycles has been realized under transition-metal free conditions, in which the synthetically useful oxime, N-NO and aldehyde functional groups were constructed in only one synthetic step.

Fe-Catalyzed tandem cyclization for the synthesis of 3-nitrofurans from homopropargylic alcohols and Al(NO3)3·9H2O

Al(NO3)3·9H2O as a nitro source for the synthesis of 3-nitrofurans from homopropargylic alcohols through Fe-catalyzed tandem cyclization is described. In this transformation, the substituted nitrofurans are obtained through nitration and cyclization. The substrate homopropargylic alcohols with different groups participate smoothly in this process and the desired substituted nitrofurans were obtained in moderate yields.