Arylhydroxylamines esters

Many organic chemicals are analyzed by RPC. These include various arylhydroxylamines as the N-hydroxyurea derivative with methyl isocyanate (614) alkyl- and alkoxy-disubstituted azoxybenzenes (6t5), n-alkyl-4-nitrophenylcarbonate esters ranging in length from methyl to octyl (616), 4-nitrophenol in the presence of 4-nitrophenyl phosphate (617), ben-zilic acid, and benactyzine-HCI using ion-pair chromatography (618), as well as aniline and its various metabolites (619), stereoisomers of 4,4 -dihydroxyhydrobenzoin (620), and aldehydes and ketones as the 2,4-dinitrophenylhydrazones (621). The technique has also been used to analyze propellants and hydrazine and 1,1-dimethylhydrazine were quantitita-vely determined (622, 623). 

Since differences were often reported in product yields from photochemical and thermal reactions, it was not clear that the same intermediate was generated in both cases. This issue was complicated by the fact that the temperatures under whieh the two experiments were run were usually quite different. The acid-base chemistry of nitrenium ions was largely unexplored so it was not known under what conditions these species could be protonated or deprotonated. It had also not been demonstrated that nitrenium ions played any role in the biological activity of mutagenic and carcinogenic esters of N-arylhydroxylamines or hydroxamic acids, particularly in their reactions with the DNA bases. Over the next decade these issues would be resolved but many questions about nitrenium ion chemistry would remain unanswered. 

The structures of the N-substitution products are reminiscent of the C-8 adduct that is the major product of the reaction of 2-fluorenyl-, 4-biphe-nylyl- and other N-arylhydroxylamine and hydroxamic acid esters with 2 -deoxyguanosine, (d-G) 2 deoxyguanosine-5 -phosphate (d-GMP), guano-sine, (G) or DNA in an aqueous environment." The mechanism of this reaction was not seriously investigated for many years because of the mistaken impression that the reaction was inefficient and could not compete with... 

N-Substituted arylhydroxylamines add to methyl propynoate and rearrangement occurs to give indolc-3-carboxylate esters[3]. With unsubstituted... 

Aryl sulfotransferases catalyze the transfer of the sulfuryl moiety from 3 -phosphoadenosine 5 -phosphosulfate to phenols, catechols, benzylic alcohols, arylhydroxylamines, and arylhydroxamic acids. This assay measures adenosine 3, 5 -diphosphate and is thus suited to quantitate enzyme activity when the sulfate esters formed are chemically unstable. 

Synthesis.— The preparation of a-amino-acids from a-halogeno-acids and ammonia is often unsatisfactory owing to the occurrence of multiple alkylations etc. A way around this problem is to treat a-halogeno-esters with alkali-metal cyanates and an alcohol this gives N-alkoxycarbonyl-a-amino-esters in >90% yield. " The O-arylhydroxylamine (148) is a useful reagent for aminating enolates, especially those derived from malonic esters, which can thus be... 

Figure 13. S l vs 8 2 mechanism for the reaction of an arylhydroxylamine-O-ester. (Nu = nucleophile).

The formation of a covalent bond to the nitrenium nitrogen atom by an oxygen-centered nucleophile is not a favorable reaction on the basis of HSAB and would in many cases be a futile reaction in that the arylhydroxylamine (or arylhydroxamic acid) compound or its 0-ester is regenerated. Hydration of a primary arylnitrenium ion at the nitrogen results in an arylhydroxylamine (or arylhydroxamic acid) compound. However, experimental observations suggest that this is a minor reaction in many cases (53, 82). The formation of arylhydroxylamine or arylhydroxamic acid compounds in reactions of analogous O-esters is most often the result of solvolysis of the ester and not the result of hydration of the nitrenium ion at nitrogen. 

Most of what is now known about the heterolysis of arylhydroxylamine-O-esters and arylhydroxamic acid-O-esters has resulted from studies of the mechanism(s) by which arylhydroxylamine and arylhydroxamic acid compounds form covalent bonds with nucleic acid bases. The literature prior to 1985 has been reviewed by Hanna and Banks (50) however, since then, some major new advances have been made, particularly concerning enzymatic 0-esterification of arylhydroxylamine compounds. 

In recent years, several organic reagents have been found which rather selectively carry out 0-acylation of arylhydroxylamine compounds (e.g. Fig. 17). This apparent violation of the general rule was first reported for acetyl cyanide (36, 68, 78). Recently, aspirin was reported to facilitate the covalent binding of carcinogenic arylhydroxylamine compounds to DNA, and it was proposed that an intermolecular transfer of the acetyl group from the phenolic ester of aspirin to the -OH of the arylhydroxylamine compound occurred to produce the arylhydroxylamine-O-acetyl ester (75). Unfortunately, the direct detection of this intermediate was not accomplished. Explanations of these unusual and specific 0-acylations are not yet available. 

An unusual enzymatic action has been described in which arylhydroxamic acids were converted to arylhydroxylamine-O-ester (Fig. 16). This is thermodynamically unfavorable in most cases and is probably achieved as the result of the further rapid reactions of the latter which serve to displace the enzyme-mediated equilibrium between the arylhydroxamic acid compound and arylhydroxylamine-O-ester. The enzyme that catalyzes this reaction is called arylhydroxamic acid-N,0-acyl transferase (50, 51) and is either closely associated with or identical to the enzyme which catalyzes 0-acetylation of arylhydroxylamine compounds (64). In turn, it appears that the enzyme which catalyzes these two reactions may be the same as N-acetyltransferase, which is a rather non-specific enzyme that catalyzes reversible N-acetylations of arylamine and arylhydroxylamine compounds. These functions are depicted in Figure 16 for arylhydroxylamine metabolism, and their contributions to the bioactivation of arylhydroxylamine and aryl hydroxamic acid compounds are a major research area in arylamine toxicity (51). 

Enzymatic conversion of arylhydroxylamine and arylhydroxamic acid compounds to highly reactive 0-esters is often accompanied by progressive inactivation of the responsible enzyme (66). This suicide inhibition has been extensively investigated, particularly in the case of N,0-acyltransferase (6) and more recently for sulfotransferase (70,85). The probable cause is covalent binding of some of the activated metabolite to the enzyme. Such adduction is to be expected when a highly reactive metabolite is produced in close proximity to a biomolecule that possesses nucleophilic groups. The responsible enzyme often becomes the first biomolecule to be damaged by the electrophilic species that it produced. 

In research soon to be published, we have found that one or more of the potential electrophilic species generated from nitrosoarene compounds by both pyruvate dehydrogenase and a-KGD can effect extensive covalent adduction of nitrosoarene compounds to DNA. The arylhydroxylamine-O-esters are likely candidates for these phenomena. 

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