Oxidation Of Arylamines Research Articles

Revisiting Nitroaromatic Drugs: Mechanisms of Bioactivation, Metabolism and Toxicity and Methods to Synthesize Nitroaromatic Fragments

Nitroaromatic drugs represent an important class of therapeutic agents, endowed with different pharmacological properties such as antibacterial, antiprotozoal, anti-Parkinson, anticancer, etc. However, the nitroheterocycle and nitroarene fragments are considered potentially toxic, with reports of mutagenicity and genotoxicity. The bioreduction process of the nitro group plays a central role in the antimicrobial and toxicological mechanism of action of nitroaromatic drugs. This article will review the nitroaromatic drugs approved by the Food and Drug Administration (FDA) in the last 30 years, the mechanisms of bioactivation, metabolism and toxicity and, finally, it will revisit the main methodologies for obtaining the nitroaromatic fragment from aromatic nitration reaction and oxidation of arylamines and/or heteroarylamines.

Synthesis of Nitroarenes by Oxidation of Aryl Amines

Nitro compounds are an important class of organic molecules with broad application in organic synthesis, medicinal chemistry, and materials science. Among the variety of methodologies available for their synthesis, the direct oxidation of primary amines represents an attractive alternative route. Efforts towards the development of oxidative procedures for the synthesis of nitro derivatives have spanned over the past decades, leading to a wide variety of protocols for the selective oxidative conversion of amines to nitro derivatives. Methods for the synthesis of nitroarenes via oxidation of aryl amines, with particular emphasis on recent advances in the field, are summarised in this review.

The polyhedral nature of selenium-catalysed reactions: Se(iv) species instead of Se(vi) species make the difference in the on water selenium-mediated oxidation of arylamines

Selenium-mediated on water oxidation of arylamines with H2O2 is reported. Nitroarenes or azoxyarenes are selectively achieved by switching the selenium catalyst. Mechanistic analysis and 77Se NMR studies highlighted the key role of Se(iv) species.

Copper arylnitrene intermediates: formation, structure and reactivity.

The mechanism of oxidation of arylamines by copper enzymes is not clarified yet. Here, we explored a reaction between a possible high-valent copper(ii)-oxyl intermediate and arylamine. We have employed a TPA ligand (TPA = tris(2-pyridylmethyl)amine) with the NH2 group in position 2 of one of the pyridine rings (TPANH2). This model system allows generation of [(TPANH2)Cu(O)]+ in the gas phase, which immediately undergoes a reaction between the arylamino group and the copper oxyl moiety. The reaction leads to elimination of H2O and formation of a copper-nitrene complex. The structure of the resulting copper-nitrene complex was confirmed by infrared spectroscopy in the gas phase. We show that the copper-nitrene complex reacts by hydrogen atom transfer with 1,4-cyclohexadiene and by an order of magnitude faster by a double hydrogen atom transfer with ethanethiol and methanol. DFT calculations explain the formation of the copper nitrene as well as its reactivity in agreement with the experimental findings.

NO sorption, in-crystal nitrite and nitrate production with arylamine oxidation in gas-solid single crystal to single crystal reactions.

A bridging nitrite and a nitrate counter anion per Co2 site are generated in-crystal and an arylamine group on the ligand scaffold is oxidised to a nitro group when nitric oxide (NO) is chemisorbed by molecular crystals of cobalt complexes.

Computer-Aided Laccase Engineering: Toward Biological Oxidation of Arylamines

Oxidation of arylamines, such as aniline, is of high industrial interest, and laccases have been proposed as biocatalysts to replace harsh chemical oxidants. However, the reaction is hampered by the redox potential of the substrate at acid pH, and enzyme engineering is required to improve the oxidation. In this work, instead of trying to improve the redox potential of the enzyme, we aim toward the (transient) substrate’s potential and propose this as a more reliable strategy. We have successfully combined a computational approach with experimental validation to rationally design an improved biocatalyst. The in silico protocol combines classical and quantum mechanics to deliver atomic and electronic level detail on the two main processes involved: substrate binding and electron transfer. After mutant expression and comparison to the parent type, kinetic results show that the protocol accurately predicts aniline’s improved oxidation (2-fold kcat increase) in the engineered variant for biocatalyzed polyanili...

Proteomic profile of aminoglutethimide-induced apoptosis in HL-60 cells: Role of myeloperoxidase and arylamine free radicals

In this study, the cellular effects resulting from the metabolism of aminoglutethimide by myeloperoxidase were investigated. Human promyelocytic leukemia (HL-60) cells were treated with aminoglutethimide (AG), an arylamine drug that has a risk of adverse drug reactions, including drug-induced agranulocytosis. HL-60 cells contain abundant amounts of myeloperoxidase (MPO), a hemoprotein, which catalyzes one-electron oxidation of arylamines using H2O2 as a cofactor. Previous studies have shown that arylamine metabolism by MPO results in protein radical formation. The purpose of this study was to determine if pathways associated with a toxic response could be determined from conditions that produced protein radicals. Conditions for AG-induced protein radical formation (with minimal cytotoxicity) were optimized, and these conditions were used to carry out proteomic studies. We identified 43 proteins that were changed significantly upon AG treatment among which 18 were up-regulated and 25 were down-regulated. The quantitative proteomic data showed that AG peroxidative metabolism led to the down-regulation of critical anti-apoptotic proteins responsible for inhibiting the release of pro-apoptotic factors from the mitochondria as well as cytoskeletal proteins such as nuclear lamina. This overall pro-apoptotic response was confirmed with flow cytometry which demonstrated apoptosis to be the main mode of cell death, and this was attenuated by MPO inhibition. This response correlated with the intensity of AG-induced protein radical formation in HL-60 cells, which may play a role in cell death signaling mechanisms.

Synthetic Organic Electrochemistry in Ionic Liquids: The Viscosity Question

Ionic liquids are obvious candidates for use in electrochemical applications due to their ionic character. Nevertheless, relatively little has been done to explore their application in electrosynthesis. We have studied the Shono oxidation of arylamines and carbamates using ionic liquids as recyclable solvents and have noted that the viscosity of the medium is a major problem, although with the addition of sufficient co-solvent, good results and excellent recovery and recycling of the ionic liquid can be achieved.

Reply to “Comment on ‘Revised Mechanism of Boyland–Sims Oxidation’”

Recently, we reported the new computational insights into the mechanism of Boyland !Sims oxidation of arylamines with peroxydisulfate (S 2O8 2! ) in an alkaline aqueous solution. 1 The classic Behrman ’s mechanism 2 has been revised; i.e., we proposed the key role of arylnitrenium cations, in the case of primary and secondary arylamines, and arylamine dications and immonium cations, in the case of tertiary arylamines, in the formation of corresponding o-aminoaryl sulfates, as prevalent soluble products, and oligoarylamines, as prevalent insoluble products, based on the AM1 and RM1 semiempirical quantum chemical study of the Boyland !Sims oxidations of aniline, 12 ringsubstituted and 3 N-substituted anilines, which were experimentally studied by Boyland and Sims more than !ve decades ago. 3 Quantum chemical results indicate that arylnitrenium cations and sulfate anions (SO4 2! ) are generated by the rate-determining two-electron oxidation of primary and secondary arylamines with S2O8 2! , while arylamine dications/immonium cations and SO 4 2! are initially formed by two-electron oxidation of tertiary arylamines with S 2O8 2! . For the !rst time the regioselectivity of Boyland !Sims oxidation was explained. It was computationally found that the regioselectivity-determining reaction of arylnitrenium cations/arylamine dications/immonium cations and SO4 2! , within the solvent cage, leads to the prevalent formation of o-aminoaryl sulfates. The formation of insoluble oligomeric/ polymeric precipitates during the Boyland!Sims oxidation of arylamines was also computationally studied. We proposed that the reaction between arylamine and its nitrenium cation, whenever the formation of p-/ o-iminoquinonoid product is not possible, represents dimerization route in the oxidative oligomerization of primary and secondary arylamines with S 2O8 2! which generally leads to the formation of N !C4 coupled arylamine dimers. The oxidative co-oligomerizations of arylamines with more oxidizable aminoaryl sulfates and aminophenols, 4 formed by the reaction of arylnitrenium cations/ arylamine dications/immonium cations with the hydroxide anions in highly alkaline solution and with water molecules in highly diluted aqueous solutions, are proposed to lead to

Revised Mechanism of Boyland−Sims Oxidation

New computational insights into the mechanism of the Boyland-Sims oxidation of arylamines with peroxydisulfate (S(2)O(8)(2-)) in an alkaline aqueous solution are presented. The key role of arylnitrenium cations, in the case of primary and secondary arylamines, and arylamine dications and immonium cations, in the case of tertiary arylamines, in the formation of corresponding o-aminoaryl sulfates, as prevalent soluble products, and oligoarylamines, as prevalent insoluble products, is proposed on the basis of the AM1 and RM1 computational study of the Boyland-Sims oxidation of aniline, ring-substituted (2-methylaniline, 3-methylaniline, 4-methylaniline, 2,6-dimethylaniline, anthranilic acid, 4-aminobenzoic acid, sulfanilic acid, sulfanilamide, 4-phenylaniline, 4-bromoaniline, 3-chloroaniline, and 2-nitroaniline) and N-substituted anilines (N-methylaniline, diphenylamine, and N,N-dimethylaniline). Arylnitrenium cations and sulfate anions (SO(4)(2-)) are generated by rate-determining two-electron oxidation of primary and secondary arylamines with S(2)O(8)(2-), while arylamine dications/immonium cations and SO(4)(2-) are initially formed by two-electron oxidation of tertiary arylamines with S(2)O(8)(2-). The subsequent regioselectivity-determining reaction of arylnitrenium cations/arylamine dications/immonium cations and SO(4)(2-), within the solvent cage, is computationally found to lead to the prevalent formation of o-aminoaryl sulfates. The formation of insoluble precipitates during the Boyland-Sims oxidation of arylamines was also computationally studied.

Isoform-Selective Inactivation of Human Arylamine N-Acetyltransferases by Reactive Metabolites of Carcinogenic Arylamines

Human arylamine N-acetyltransferases (NATs) are expressed as two polymorphic isoforms, NAT1 and NAT2, that have toxicologically significant functions in the detoxification of xenobiotic arylamines by N-acetylation and in the bioactivation of N-arylhydroxylamines by O-acetylation. NAT1 also catalyzes the N-acetylation of 4-aminobenzoylglutamic acid, a product of folic acid degradation, and is associated with endogenous functions in embryonic development. On the basis of earlier studies with hamster NAT1, hamster NAT2, and human NAT1, we proposed that human NAT2 would be more susceptible than NAT1 to inactivation by N-arylhydroxamic acid metabolites of arylamines. Kinetic analyses of the inactivation of recombinant NAT1 and NAT2 by the N-arylhydroxamic acid, N-hydroxy-2-acetylaminofluorene (N-OH-AAF), as well as the inactivation of NAT2 by N-hydroxy-4-acetylaminobiphenyl (N-OH-4-AABP), resulted in second-order inactivation rate constants (k(inact)/K(I)) that were several fold greater for NAT2 than for NAT1. Mass spectrometric analysis showed that inactivation of NAT2 in the presence of the N-arylhydroxamic acids was due to formation of a sulfinamide adduct with Cys68. Treatment of HeLa cells with N-OH-4-AABP and N-OH-AAF revealed that the compounds were less potent inactivators of intracellular NAT activity than the corresponding nitrosoarenes, but unexpectedly, the hydroxamic acids caused a significantly greater loss of NAT1 activity than of NAT2 activity. Nitrosoarenes are the electrophilic products responsible for NAT inactivation upon interaction of the enzymes with N-arylhydroxamic acids, as well as being metabolic products of arylamine oxidation. Treatment of recombinant NAT2 with the nitrosoarenes, 4-nitrosobiphenyl (4-NO-BP) and 2-nitrosofluorene (2-NO-F), caused rapid and irreversible inactivation of the enzyme by sulfinamide adduct formation with Cys68, but the k(inact)/K(I) values for inactivation of recombinant NAT2 and NAT1 did not indicate significant selectivity for either isoform. Also, the IC(50) values for inactivation of HeLa cell cytosolic NAT1 and NAT2 by 4-NO-BP were similar, as were the IC(50) values obtained with 2-NO-F. Treatment of HeLa cells with low concentrations (1-10 microM) of either 4-NO-BP or 2-NO-F resulted in preferential and more rapid loss of NAT1 activity than NAT2 activity. Because of its wide distribution in human tissues and its early expression in developing tissues, the apparent high sensitivity of intracellular NAT1 to inactivation by reactive metabolites of environmental arylamines may have important toxicological consequences.

Identification and Characterization of the Flavin:NADH Reductase (PrnF) Involved in a Novel Two-Component Arylamine Oxygenase

Two-component oxygenases catalyze a wide variety of important oxidation reactions. Recently we characterized a novel arylamine N-oxygenase (PrnD), a new member of the two-component oxygenase family (J. Lee et al., J. Biol. Chem. 280:36719-36728, 2005). Although arylamine N-oxygenases are widespread in nature, aminopyrrolnitrin N-oxygenase (PrnD) represents the only biochemically and mechanistically characterized arylamine N-oxygenase to date. Here we report the use of bioinformatic and biochemical tools to identify and characterize the reductase component (PrnF) involved in the PrnD-catalyzed unusual arylamine oxidation. The prnF gene was identified via sequence analysis of the whole genome of Pseudomonas fluorescens Pf-5 and subsequently cloned and overexpressed in Escherichia coli. The purified PrnF protein catalyzes reduction of flavin adenine dinucleotide (FAD) by NADH with a k(cat) of 65 s(-1) (K(m) = 3.2 muM for FAD and 43.1 muM for NADH) and supplies reduced FAD to the PrnD oxygenase component. Unlike other known reductases in two-component oxygenase systems, PrnF strictly requires NADH as an electron donor to reduce FAD and requires unusual protein-protein interaction with the PrnD component for the efficient transfer of reduced FAD. This PrnF enzyme represents the first cloned and characterized flavin reductase component in a novel two-component arylamine oxygenase system.

Reconstitution and Characterization of Aminopyrrolnitrin Oxygenase, a Rieske N-Oxygenase That Catalyzes Unusual Arylamine Oxidation

Rieske oxygenases catalyze a wide variety of important oxidation reactions. Here we report the characterization of a novel Rieske N-oxygenase, aminopyrrolnitrin oxygenase (PrnD) that catalyzes the unusual oxidation of an arylamine to an arylnitro group. PrnD from Pseudomonas fluorescens Pf5 was functionally expressed in Escherichia coli, and the activity of the purified PrnD was reconstituted, which required in vitro assembly of the Rieske iron-sulfur cluster into the protein and the presence of NADPH, FMN, and an E. coli flavin reductase SsuE. Biochemical and bioinformatics studies indicated that the reconstituted PrnD contains a Rieske iron-sulfur cluster and a mononuclear iron center that are formed by residues Cys(69), Cys(88), His(71), His(91), Asp(323), His(186), and His(191), respectively. The enzyme showed a limited range of substrate specificity and catalyzed the conversion of aminopyrrolnitrin into pyrrolnitrin with K(m) = 191 microM and k(cat) = 6.8 min(-1). Isotope labeling experiments with (18)O(2) and H(2)(18)O suggested that the oxygen atoms in the pyrrolnitrin product are derived exclusively from molecular oxygen. In addition, it was found that the oxygenation of the arylamine substrates catalyzed by PrnD occurs at the enzyme active site and does not involve free radical chain reactions. By analogy to known examples of arylamine oxidation, a catalytic mechanism for the bioconversion of amino pyrrolnitrin into pyrrolnitrin was proposed. Our results should facilitate further mechanistic and crystallographic studies of this arylamine oxygenase and may provide a new enzymatic route for the synthesis of aromatic nitro compounds from their corresponding aromatic amines.

N-OXIDATION OF AROMATIC AMINES BY INTRACELLULAR OXIDASES

The introduction includes a literature review of DNA reactive species and DNA adduct formation that results from aromatic amine N-oxidation catalyzed by hepatic cytochrome P450 vs. that catalyzed by nonhepatic peroxidases. Experimental evidence is then described for a novel oxidative stress mechanism involving prooxidant N-cation radical formation by both oxidases, which is proposed as a contributing mechanism for aromatic amine induced cytotoxicity and carcinogenesis. Aromatic amine N-cation radicals formed by peroxidases were found to cooxidize GSH or NADH and form reactive oxygen species. The latter could explain the reported DNA oxidative damage found in vivo following methylaminoazobenzene administration [Hirano et al. Analyses of Oxidative DNA Damage and Its Repair Activity in the Livers of 3′-Methyl-4-dimethylaminoazobenzene-Treated Rodents. Jpn. J. Cancer Res. 2000, 91, 681–685]. It was also found that the prooxidant activity of the aromatic amine increased as its redox potential, i.e., ease of oxidation decreased with o-anisidine and aminofluorene being the most effective at forming reactive oxygen species. This suggests that the rate-limiting step in the cooxidation is the rate of arylamine oxidation by the peroxidase. Incubation of hepatocytes with aromatic amines caused a decrease in the mitochondrial membrane potential before cytotoxicity ensued. The CYP1A2-induced hepatocytes isolated from 3-methylcholanthrene administered rats were much more susceptible to some arylamines and were protected by CYP1A2 inhibitors. Hepatocyte GSH was also depleted by all arylamines tested and extensive GSH oxidation occurred with o-anisidine and aminofluorene, which was prevented by CYP1A2 inhibitors. This suggests that in intact hepatocytes CYP1A2 may also catalyze a one-electron oxidation of some arylamines to form prooxidant cation radicals, which cooxidize GSH to form the reactive oxygen species.

Oxidative dealkylation of aromatic amines by "bare" FeO+ in the gas phase

The gas-phase oxidations of aniline, N-methylaniline, and N,N-dimethylaniline by FeO+ cation are examined by using mass spectrometric techniques. Although bare FeO+ is capable of hydroxylating aromatic C—H bonds, the fate of the oxidation of arylamines is determined by docking of the FeO+ unit at nitrogen. The major reactions of the metastable aniline/FeO+ complex are losses of molecular hydrogen, ammonia, and water, all involving at least one N-H proton. N-alkylation results in a complete shift of the course of the reaction. The unimolecular processes observed can be regarded as initial steps of an oxidative dealkylation of the amines mediated by FeO+. More detailed mechanistic insight is obtained by examining the C—H(D) bond activation of N-methyl-N-([D3]-methyl)aniline by bare and ligated FeO+ species. The gas-phase reactions of FeO+ with methylanilines show some similarities to the enzymatic dealkylation of amines by cytochrome P-450. The kinetic isotope effects observed experimentally suggest an electron transfer mechanism for the gas-phase reaction.Key words: mass spectrometry, gas-phase chemistry, iron, dealkylation, N,N-dimethylaniline.

Oxidative dealkylation of aromatic amines by "bare" FeO<sup>+</sup> in the gas phase

The gas-phase oxidations of aniline, N-methylaniline, and N,N-dimethylaniline by FeO+ cation are examined by using mass spectrometric techniques. Although bare FeO+ is capable of hydroxylating aromatic C—H bonds, the fate of the oxidation of arylamines is determined by docking of the FeO+ unit at nitrogen. The major reactions of the metastable aniline/FeO+ complex are losses of molecular hydrogen, ammonia, and water, all involving at least one N-H proton. N-alkylation results in a complete shift of the course of the reaction. The unimolecular processes observed can be regarded as initial steps of an oxidative dealkylation of the amines mediated by FeO+. More detailed mechanistic insight is obtained by examining the C—H(D) bond activation of N-methyl-N-([D3]-methyl)aniline by bare and ligated FeO+ species. The gas-phase reactions of FeO+ with methylanilines show some similarities to the enzymatic dealkylation of amines by cytochrome P-450. The kinetic isotope effects observed experimentally suggest an electron transfer mechanism for the gas-phase reaction.Key words: mass spectrometry, gas-phase chemistry, iron, dealkylation, N,N-dimethylaniline.

Unexpected selectivity in the oxidation of arylamines with ferrate–preliminary mechanistic considerations

Ferrate rapidly converts aromatic amines exclusively into either nitro- or azo-compounds under relatively mild conditions.

Catalytic selective oxidation of amines with hydroperoxides over molecular sieves: Investigations into the reaction of alkylamines, arylamines, allylamines and benzylamines with H 2O 2 and TBHP over TS-1 and CrS-2 as the new catalyst

The liquid phase oxidation of various substituted amines with dil H2O2 and tert-butyl hydroperoxide (TBHP) has been investigated over titanium and chromium silicates respectively. While TS-1/H2O2 combination exhibits a remarkable activity and selectivity in the oxidation of arylamines to produce the symmetrical azoxybenzenes, CrS-2 catalyzes the selective oxidation of various substituted amines to the corresponding nitro compounds by oxidation with 70% TBHP. The nature of the reactive intermediates during the oxidation of anilines to nitrobenzenes has been established using cyclic voltammetry experiments. Further, amines possessing α C-H bonds are selectively oxidized to either oximes or the carbonyl compounds on reaction with H2O2 catalyzed by TS-1. fa† N.C.L. Communication No.6302

Selective catalytic oxidation of arylamines to azoxybenzenes with H2O2 over zeolites

Catalytic version of the H2O2–TS–1 combination has been shown to display a good reaction selectivity in the liquid-phase oxidation of anilines to symmetrical azoxybenzenes.

Alcohol dehydrogenase-dependent reduction of 2-nitrosoflurene and rearrangement of N-hydroxy-2-aminofluorene.

Various C-nitroso compounds are intermediates of arylamine oxidation or nitroarene reduction. Reductive metabolism of C-nitroso compounds to their corresponding hydroxylamines is a necessary step in the activation of these compounds to mutagenic end points. In this study, 2-nitrosofluorene (2-NOF) has been investigated as an aldehyde substrate analogue for horse liver alcohol dehydrogenase (HLADH). The reaction products are assigned on the basis of their UV/visible spectra and coelution with authentic standards in reversed-phase HPLC. The direct product of 2-NOF reduction is N-hydroxy-2-aminofluorene (N-OH-2-AF), which undergoes further reduction to 2-aminofluorene (2-AF) and rearrangement to 1- and 3-hydroxy-2-aminofluorene (1- and 3-OH-2-AF). The formation of these products is potently inhibited by pyrazole indicating the involvement of active-site zinc ion in the role of a Lewis acid catalyst. It is suggested that the rearrangement reaction occurs via an inner-sphere N-OH 2AF-Zn2+...E-coenzyme complex following the elimination of the hydroxyl group from the N-OH-2-AF intermediate and the hydrolysis of the fluorenyl nitrenium-derived carbocations to yield the hydroxy 2-AF products. Herein ADH is identified as a C-nitroso-reducing enzyme which must be considered in the mutagenic sequelae of nitro and nitrosoarenes.