Hydroxamic acids hydroxylamine reactions

A seemingly complex heterocycle which on close examination is in fact a latentiated derivative of a salicylic acid shows antiinflammatory activity. It might be speculated that this compound could quite easily undergo metabolic transformation to a salicylate and that this product is in fact the active drug. Condensation of acid 134 with hydroxylamine leads to the hydroxamic acid 135. Reaction of that with the ethyl acetal from 4-chlorobutyraldehyde then leads to the cyclic carbinolamine derivative 136. Treatment... 

Hydroxylamine is acylated by carboxylic esters, giving hydroxamic acids. The reaction often occurs in a few hours at room temperature if the components are mixed in alcoholic solution, preferably with addition of an equivalent of an alkoxide. In the latter case the hydroxamic acids are obtained as salts, but are easily liberated therefrom. 

Many investigators have studied the incorporation of labeled amino acids into proteins. The system involved in mammalian cells seems to be associated with microsomes, but requires factors from the soluble portion of the cell. Both ATP and GTP are required. The function of the ATP may be to form activated amino acids. Several systems have been found with relative specificity for various amino acids that catalyze the exchange of pyrophosphate with ATP in the presence of the amino acid, and presumably form adenyl-amino acid compounds. The activated amino acids react with hydroxylamine to form hydroxamic acids. Several reactions have been considered as models for peptide bond formation. These include the formation of hippuric acid, in which benzoyl CoA condenses with glycine in a reaction similar to the acetylation of... 

If primary or secondary amines are used, A/-substituted amides are formed. This reaction is called aminolysis. Hydra2ines yield the corresponding hydra2ides, which can then be treated with nitrous acid to form the a2ides used in the Curtius rearrangement. Hydroxylamines give hydroxamic acids. 

Earlier reported syntheses have been shown to give isoxazolin-5-ones. Other isoxazolin-3-ones have been prepared by the reaction of methylacetoacetic esters and hydroxylamine. An additional synthesis was reported by the action at 0°C of hydroxylamine on ethyl -benzoylpropionate to produce an insoluble hydroxamic acid which cyclized on acid treatment. The hydroxamic acid acetal was similarly transformed into the isoxazolin-3-one (Scheme 149) (71BSF3664, 70BSF1978). 

Indirect detection of an intermediate. The overall reaction of hydroxylamine with a carboxylic acid derivative yields a hydroxamic acid as the product, Eq. (3-176). 

When Jencks reacted hydroxylamine with p-nitrophenyl acetate, p-nitrophenolate ion was released at a rate faster than that at which acetohydroxamic acid was formed. This burst effect is evidence for a two-step reaction. In this case the intermediate is O-acetylhydroxylamine, which subsequently reacts with hydroxylamine to form the hydroxamic acid. 

Because of the great range of structures containing cyclic hydroxamic acid functions it is difficult to give a concise summary of the available synthetic methods. Nevertheless, the vast majority of published syntheses depend on condensation reactions involving only familiar processes of acylation or alkylation of hydroxylamine derivatives. The principles of such syntheses are outlined in a number of typical examples in Section III, A but no attempt has been made to cover all reported cases. 

Shaw and McDowellhave prepared imidazolone derivatives by cyclization of a-acylamino amides. In a variation of this reaction the azlactone (30) was gradually converted to the hydroxamic acid (31) by methanolic hydroxylamine. Sodium methoxide and hydroxylamine readily gave the acyclic hydroxamic acid (32) which could be cyclized to 31 by dilute acid. Benzyloxyurea has been used in the sjrnthesis of pyrimidine hydroxamic acids (33) by reaction with /S-diketones followed by catalytic hydrogenation of the benzyl group. Protection... 

A mixture of 3-hydroxy-4-phenylfurazan and 1,2,4-oxadiazole 243 was prepared from a-phenyl-a-hydroximino hydroxamic acid by acylation and subsequent treatment with 15% aqueous NaOH (Scheme 164) (25G201). The reaction of tetraacetate 244 with sodium acetate hydrate in glacial acetic acid at 70°C gives 3,4-dihydroxyfurazan (9%) (92URP1752734). a-Hydroximino ester 245 reacts with hydroxylamine to form furazan 246 in 25% yield (Scheme 164) (79JHC689). 

As in 10-55 hydrazides and hydroxamic acids can be prepared from carboxylic esters, with hydrazine and hydroxylamine, respectively. Both hydrazine and hydroxylamine react more rapidly than ammonia or primary amines (the alpha effect, p. 445). Imidates, RC(=NH)OR, give amidines, RC(=NH)NH2. Lactones, when treated with ammonia or primary amines, give lactams. Lactams are also produced from y- and 5-amino esters in an internal example of this reaction. 

Knobler Y, Bittner S, Frankel M (1964) Reaction of N-carboxy-alpha-amino-acid anhydrides with hydrochlorides of hydroxylamine O-alkylhydroxylamines + amines syntheses of amino-hydroxamic acids amido-oxy-peptides + alpha-amino-acid amides. J Chem Soc 3941... 

Any rational approach to the study of antidotes for nerve-gas poisoning must take this firm attachment into account. It has been observed that, in experiments in vitro, the D.F.P.-poisoned heart recovers to an appreciable extent in the presence of hydroxylamine. With this experiment in mind I. B. Wilson1 has examined the action of hydroxylamine derivatives and has had considerable success with nictonic hydroxamic acid meth-iodide (IV). The reaction envisaged here is a nucleophilic attack... 

Hydroxylamine can act as either a N-nucleophile or O-nucleophile, depending on which of the reactive centers is protected. For all reactions Ph-Pybox has been used as ligand, and moderate to high levels of selectivity have been achieved. Hydroxamic acid derivatives and oximes have also been probed as O-nucleophiles [63]. 

Formally, they can all be viewed as derivatives of hydroxylamine, H2N—OH indeed, oximes can be prepared by the addition of hydroxylamine to aldehydes and ketones (equations 1 and 2), and hydroxamic acids by its reactions with acetyl halides and esters (equations 3 and 4). ... 

Organic thermochemistry of hydroxylamines, oximes, and hydroxamic acids 71 The related solid phase reaction 36 is exothermic by but 5.0 kJmol ... 

The main first part of the review (Section HI) summarizes preparation of hydroxylamine derivatives through alkylation, arylation, and addition reaction of hydroxylamine, or its derivatives such as hydroxamic acids and A-oxysulfonamides. The second main part (Sections IV-VIII) describes methods of creation of hydroxyamino groups de novo from other functionalities. Due to easy interconversion outlined in Section II, syntheses of hydroxylamines and hydroxamic acids are considered together. For the same reason, the chapter also relates to synthesis of A-oxysulfonamides and V-oxyphosphonamides as far as these methods are of interest for the preparation of hydroxylamines. 

Hydrolysis of nitrones, oximes and hydroxamic acids is frequently used as a final step in the preparation of substituted hydroxylamines. Although hydrolysis is the most commonly utilized method for oximes, oxime ethers and nitrones, formation of sensitive hydroxylamines can also be achieved under milder reaction conditions by treatment of... 

Conversion of hydroxamic acids into hydroxylamines is usually performed by hydrolysis or alcoholysis under acidic"" or basic catalysis , although other methods like reaction with trimethylsilyl iodide have also been sparingly used. 

Preparation of hydroxamic acids through acylation of hydroxylamines is a common and straightforward reaction. However, acylation of iV-alkylhydroxylamines is known to proceed on both the oxygen and nitrogen atoms and can result in mixtures of N- and 0-acylation products . In iV-alkylhydroxylamines possessing a bulky " or an electron-poor substituent on nitrogen atom, O-acylation is predominant. 

Intermolecular reactions of hydroxylamines with secondary alkyl halides and mesylates proceed slower than with alkyl triflates and may not provide sufficiently good yield and/or stereoselectivity. A nseful alternative for these reactions is application of more reactive anions of 0-alkylhydroxamic acids or 0-alkoxysulfonamides ° like 12 (equation 8) as nucleophiles. The resulting Af,0-disubstituted hydroxamic acids or their sulfamide analogs of type 13 can be readily hydrolyzed to the corresponding hydroxylamines. The same strategy is also helpful for synthesis of hydroxylamines from sterically hindered triflates and from chiral alcohols (e.g. 14) through a Mitsunobu reaction (equation 9). 

Hydroxamic acids undergo facile nucleophilic Ai-arylation with activated aryl halides such as 31 (equation 22). While hydroxamates are known to be ambident nucleophiles in alkylation reactions, arylation of hydroxylamines results exclusively in Ai-substituted hydroxamates of type 32 (equation 22)". ... 

The general approach to 0-arylation of hydroxylamines involves N-protection followed by O-arylation. Activated aryl halides and heteroaryl halides easily alkylate oxime salts (equation 25), N-aUcyl hydroxamic acids and N-hydroxysuccinimide . N-Hydroxyph-thalimide can be also 0-phenylated through a reaction with diphenyliodonium salt, although in lower yield . ... 

Intramolecular addition of hydroxylamines and hydroxamic acids to the non-activated double bonds is possible through oxidative cyclization. Reaction of O-Acyl fi,y-unsaturated hydroxamates (e.g. 56, equation 38) with bromine provides 3,4-substituted iV-hydroxy -lactams such as 57 with high diastereoselectivity. Analogous reaction of O-benzyl hydroxylamine 58 (equation 39) with iodine results in five-membered cyclization with 2 1 ratio of diastereomers. ... 

The use of hydroxyamino instead of amino components in a Ugi reaction (equations 46 and 47) gives access to the corresponding hydroxamic acid of type 72 (equation 46), hydroxylamines such as 73 (equation 47) or Af-hydroxyamidines. ... 

Nowadays, the most economical way of preparing hydroxamic acid derivatives is the reaction of hydroxylamine with acid chlorides or esters . Unfortunately, the preparation of acid chlorides is often tedious. In addition, it is very difficult to avoid further acylation during the reaction with hydroxylamine. 

Although the simplest route to prepare hydroxamic acid derivatives remains the reaction of hydroxylamine with acid chlorides, this last method cannot be apphed to all Af-protected-a-amino acids. The synthesis of Fmoc-protected amino acid hydroxamates represents the only exception to this rule . In fact, Fmoc-amino acid hydroxamates 98 can be synthesized by the acylation of hydroxylamine using Fmoc-amino acid chlorides 97 in the presence of MgO (Scheme 52). The route is simple, efficient, and affords good yields of products. 

The reaction of dicyano epoxides 105 with hydroxylamines hydrochlorides 106 in acetonitrile represents a direct route via 107 to a number of new and higher a-chloro hydroxamic acids and their derivatives 108 (Scheme 55) . ... 

Multicomponent reactions have recently become one of the favored methods to prepare pharmacologically important compounds. Ugi condensations with O-protected hydroxylamines have been successfully performed in THE using ZnCl2 as activating agent (Scheme 56). This synthetic strategy opens up the route to a very convergent assembly of internal hydroxamic acid derivatives (A-acyl-A-hydroxypeptides 109)" . 

Ho and coworkers" have observed that the addition of small amounts of solid KCN (0.2 equivalents) can effectively accelerate the formation of hydroxamic acids 112 from methyl esters 111 (Scheme 58). The authors suggested that this reaction proceeds through an acylcyanide intermediate followed by nucleophilic substitution by 50% aqueous hydroxylamine at room temperature. The use and advantage of this methodology have been demonstrated for both solution-phase and solid-phase applications. 

The syntheses of the methyl esters 116-118 from 115 and hydroxamic acids 119-121 were carried out via a typical alkylation of the hydroxy function of methyl 4-hydroxyben-zoate 116 followed by either reaction with hydroxylamine to provide bishydroxamic acids 119-121 containing an alkyl spacer between two aromatic rings. 

An alternative procedure that generates stable, storable nitrone intermediates 175 from 170, mediated by dry m-CPBA as oxidating reagent, is shown in Scheme 77 ". Conversion of the nitrone 175 to the hydroxylamine by an exchange reaction with hydroxylamine hydrochloride was followed by EDC/HOAt-mediated cyclization to hydroxamic acid 174 (HOAt l-hydroxy-7-azabenzotriazole). 

In 2003, Devocelle and colleagues reported a convenient two-step procedure for the parallel synthesis of hydroxamic acids (or O-protected hydroxamic acids 207) from carboxylic acids and hydroxylamine. It involves the formation of a polymer-bound HOBt active ester 206 from 204 and the acid 205 and subsequent reaction with O-protected or free hydroxylamine (Scheme 89). The use of free hydroxylamine leads to increased yields while maintaining high purities. Recycling of the exhausted resin 204 to prodnce the same or a different hydroxamic acid has been achieved by a three-step protocol, which is easily amenable to automation and cost-economical. 

Rearrangement reactions of hydroxylamines (1), oximes (2) and hydroxamic acids (3) will be covered in this chapter with emphasis on the developments made during the last 15 years. All referred compounds possess a relatively low energy N—O bond ca 53 kcalmol" ) which facilitates the ability of these compounds to rearrange. 

Rearrangements of hydroxylamines, oximes and hydroxamic acids 441 Thiazoloazepines 402 may be produced by a ring-expansion reaction (equation 165). 

The most widely used rearrangement reactions of hydroxylamine, oximes and hydroxamic acids are reported in previous sections of this chapter, but there are other relevant rearrangements which were less exploited. 

This review describes electrochemical reactions of hydroxylamines, oximes and hydroxamic acids. In addition, utilization of hydroxylamines and hydroxamic acids as redox mediators are shown. Since the electroorganic chemistry of hydroxylamines, oximes and hydroxamic acids is rather a minor area in the electrochemistry of organic compounds, the reader is advised to refer to texts which are written for organic chemists unfamiliar with the electroorganic chemistry. 

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