The Mechanism of Selective Purine C-Nitration Revealed: NMR Studies Demonstrate Formation and Radical Rearrangement of an N7-Nitramine Intermediate

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

Synthesis of 2'-deoxyguanosine from 2'-deoxyadenosine via C2 nitration

An efficient synthetic method has been developed for the synthesis of 2'-deoxyguanosine from the more commercially available 2'-deoxyadenosine via late-stage C2 nitration in 48.7% total yield by a 5-step synthetic procedure. Crucially, 2'-deoxyadenosine was fully protected by bennzoyl groups and then nitrated at C2 by tetrabutylammonium nitrate/trifluoroacetic anhydride. The resulting 2-NO₂ moiety was converted into 2-NH₂ by Ni-catalyzed hydrogenolysis. Finally, 2'-deoxyguanosine was obtained from the diaminopurine intermediate by deaminase-catalyzed reaction. Furthermore, the 2-NO₂ moiety also appeared to be a versatile handle to introduce a variety of functional groups, resulting in a divergent access to 2-substituted 2'-deoxyadenosine analogues.

Recent advances in nitro-involved radical reactions

Significant progress in the chemistry of nitro radicals has been witnessed in the past decades, providing efficient and rapid access to nitro-containing compounds. This review describes recent advances in nitro-involved radical reactions, and summarizes various transformations.

15N labeling and analysis of 13C–15N and 1H–15N couplings in studies of the structures and chemical transformations of nitrogen heterocycles

This review provides a generalization of effective examples of ¹⁵N labeling followed by an analysis of J_CN and J_HN couplings in solution as a tool to study the structural aspects and pathways of chemical transformations in nitrogen heterocycles.

Underappreciated and Complex Role of Nitrous Acid in Aromatic Nitration under Mild Environmental Conditions: The Case of Activated Methoxyphenols

Many ambiguities surround the possible mechanisms of colored and toxic nitrophenols formation in natural systems. Nitration of a biologically and environmentally relevant aromatic compound, guaiacol (2-methoxyphenol), under mild aqueous-phase conditions (ambient temperatures, pH 4.5) was investigated by a temperature-dependent experimental modeling coupled to extensive ab initio calculations to obtain the activation energies of the modeled reaction pathways. The importance of dark nonradical reactions is emphasized, involving nitrous (HNO₂) and peroxynitrous (HOONO) acids. Oxidation by HOONO is shown to proceed via a nonradical pathway, possibly involving the nitronium ion (NO₂⁺) formation. Using quantum chemical calculations at the MP2/6-31++g(d,p) level, NO₂^• is shown capable of abstracting a hydrogen atom from the phenolic group on the aromatic ring. In a protic solvent, the corresponding aryl radical can combine with HNO₂ to yield OH^• and, after a subsequent oxidation step, nitrated aromatic products. The demonstrated chemistry is especially important for understanding the aging of nighttime atmospheric deliquesced aerosol. The relevance should be further investigated in the atmospheric gaseous phase. The results of this study have direct implications for accurate modeling of the burden of toxic nitroaromatic pollutants, and the formation of atmospheric brown carbon and its associated influence on Earth's albedo and climate forcing.

A Practical Synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from 2′-deoxyadenosine

A practical synthesis of 2-chloro-2′-deoxyadenosine (Cladribine) from 2′-deoxyadenosine is reported. Treatment of fully protected 2′-deoxyadenosine with 2,2,2-trifluoroacetic anhydride and tetrabutylammonium nitrate gave protected 2-nitro-2′-deoxyadenosine with high yield. 2-Chloro-2′-deoxyadenosine was synthesised in four steps and 44.8% yield after substitution by chloride and deprotection steps.

Novel potent inhibitors of A. thaliana cytokinin oxidase/dehydrogenase

The synthesis of a new group of 2-X-6-anilinopurines, including compounds with potential cytokinin-like activities, with various substitutions (X=H, halogen, amino, methylthio or nitro) on the phenyl ring is described. The prepared compounds have been characterized using standard physico-chemical methods, and the influence of individual substituents on biological activity has been compared in three different bioassays, based on the stimulation of tobacco callus growth, retention of chlorophyll in excised wheat leaves and the dark induction of betacyanin synthesis in Amaranthus cotyledons. The biological activity of the prepared compounds was also assessed in receptor assays, in which the ability of the compounds to activate the cytokinin receptors AHK3 and AHK4/CRE1 was studied. Finally, the interactions of the compounds with the Arabidopsis cytokinin oxidase/dehydrogenase AtCKX2 (heterologously expressed) were investigated. Systematic testing led to the identification of two very potent inhibitors of AtCKX2: 2-chloro-6-(3-methoxyphenyl)aminopurine and 2-fluoro-6-(3-methoxyphenyl)aminopurine.

Solid phase synthesis and antiprotozoal evaluation of di- and trisubstituted 5′-carboxamidoadenosine analogues

The rapid increase of resistance to drugs commonly used in the treatment of tropical diseases such as malaria and African sleeping sickness calls for the prompt development of new safe and efficacious drugs. The pathogenic protozoan parasites lack the capability of synthesising purines de novo and they take up preformed purines from their host through various transmembrane transporters. Adenosine derivatives constitute a class of potential therapeutics due to their selective internalisation by these transporters. Automated solid-phase synthesis can speed up the process of lead finding and we pursued the solid-phase synthesis of di- and trisubstituted 5'-carboxamidoadenosine derivatives by using a safety-catch approach. While efforts with Kenner's sulfonamide linker remained fruitless, successful application of the hydrazide safety-catch linker allowed the construction of two representative combinatorial libraries. Their antiprotozoal evaluation identified two compounds with promising activity: N(6)-benzyl-5'-N-phenylcarboxamidoadenosine with an IC(50) value of 0.91 microM against Trypanosoma brucei rhodesiense and N(6)-diphenylethyl-5'-phenylcarboxamidoadenosine with an IC(50) value of 1.8 microM against chloroquine resistant Plasmodium falciparum.