Cyclic hydroxamic acid

No simple pteridine 1- or 3-oxides are yet known. If the AT-atom of an amide function is formally oxidized, tautomerism favours the cyclic hydroxamic acid structure, as found for 3-hydroxypteridin-4-one (55JA3927), 1-hydroxylumazine (64JOC408) and 2,4-diamino-8-hydroxypteridin-7-ones (75JOC2332). 

Only one cyclic hydroxamic acid which contains the pyrido[2,3-d]-pyrimidine ring system has been reported.This is 2-methyl-3-hydroxypyrido[2,3-d]pyrimidin-4(3i/)-one (21) which was prepared by the action of acetic anhydride on 2-aminonieotinhydroxamic acid (20) or from ethyl 2-aeetamidonicotinate (22) and hydroxylamine. In view of the known antibacterial activity of certain cyclic hydroxamic acids further work on these compounds would be of interest. 

The cyclic hydroxamic acid (104) obtained by this route had previously been prepared by the two methods illustrated. These exemplify well the variants possible within route (ii). 

In three of the compounds (154, R2= H) examined the commonest loss from the molecular ion was the cyanide RjCN to give the most predominant ion at m/e= 120 in each case. The M-0 peak (M-16), was observed in cyclic hydroxamic acids (154, R = OH). 

Although cyclic hydroxamic acids were made as early as 1881 by Friedlander and Ostermeierd little attention was paid to this class... 

The following discussion of hydroxamic acids includes saturated systems, e.g., 2, compounds such as 3, derived from aromatic systems, 7V-hydroxyimides such as 7V-hydroxyglutarimide (78), and certain of their derivatives including thiohydroxamic acids. Naturally occurring cyclic hydroxamic acids are discussed to show the range of structural types that has been found, hut macrocyclic polyhydroxamic acids are mentioned very briefly, because several comprehensive reviews of these compounds are already available. The main purpose of this review is to summarize the methods available for the synthesis of cyclic hydroxamic acids, to outline their characteristic reactions, and to present some useful physical data. Their synthesis and some biological properties have previously been reviewed by Coutts. ... 

For the present purpose this is not a useful scheme and the naturally occurring compounds are arranged instead in order of increasing molecular complexity. Compounds that do not contain a cyclic hydroxamic acid structure are excluded. 

The first natural products shown to be cyclic hydroxamic acids belong to the group of aspergillic acids, formed by cultures of Aspergillus flavus and related organisms. 

This cyclic hydroxamic acid (10) and the related demethoxy compound have been isolated from corn seedlings. Both compounds are fungistatic agents. 

The ferrioxamines form a large group of ferric trihydroxamates composed of residues of acetic acid, succinic acid, l-amino-5-hydroxyl-aminopentane, and l-amino-4-hydroxylaminobutane. Of the.se, only ferrioxamines E (14) and D2 (15) are formally cyclic hydroxamic acids. 

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. 

A variety of condensation processes can lead to cyclic hydroxamic acids. These involve either the condensation of two molecules or the intramolecular cyclization of a single compound. In some cases, a primary hydroxamic acid function is already present and formation of a cyclic compound can arise by suitable reaction on nitrogen. These processes will be dealt with first. 

By this method, Chauveau and Mathis have prepared cyclic hydroxamic acids (41) containing a sulfur atom in the ring. The acyclic precursors (40) were formed by alkylation of a thiol anion. 

In the catalytic hydrogenation of ethyl 2-nitrophenyl acetate (43), Di Carlo observed a small amount of the cyclic hydroxamic acid (44) in addition to the expected lactam (45). Following this result, a... 

Perhaps the most reliable method for the reductive cyclization of a nitro ester to a hydroxamic acid is that which involves treatment with sodium horohydride in the presence of palladium on charcoal. Although under these conditions aromatic nitro compounds are reduced to amines, o-nitro esters such as 53, in which the ester group is suitably oriented with respect to the nitro group, give good yields of cyclic hydroxamic acids (54). Coutts and his co-... 

In one particular example, an interesting intramolecular acylation occurred. Reduction of cis-iVi-acetyl-o-nitrobenzylideneoxindole (55) gave rise to a cyclic hydroxamic acid, assigned structure 56. 

Several reactions are particularly applicable to the synthesis of cyclic hydroxamic acids as they involve some kind of ring expansion. Some are quite general reactions which are of high preparative utility, whereas others exist as fairly isolated examples which have not as yet been generalized. 

Robinson and co-workers - have studied the photoreari-angement of steroidal 17-nitrites (102) to cyclic hydroxamic acids (103) in good... 

When heated at or about their melting points cyclic hydroxamic acids commonly decompose to give the corresponding lactams. Thus,... 

Reduction of cyclic hydroxamic acids generally leads to lactams or the corresponding amines. Chemical methods have frequently been preferred to catalytic hydrogenation, probably because the choice of... 

Cyclic hydroxamic acids and V-hydroxyimides are sufficiently acidic to be (9-methylated with diazomethane, although caution is necessary because complex secondary reactions may occur. N-Hydroxyisatin (105) reacted with diazomethane in acetone to give the products of ring expansion and further methylation (131, R = H or CH3). The benzalphthalimidine system (132) could not be methylated satisfactorily with diazomethane, but the V-methoxy compound was readil3 obtained by alkylation with methyl iodide and potassium carbonate in acetone. In the pyridine series, 1-benzyl-oxy and l-allyloxy-2-pyridones were formed by thermal isomeriza-tion of the corresponding 2-alkyloxypyridine V-oxides at 100°. 

This alkyl migration is believed to proceed via ion-pair formation. These and many other simple 0-alkyIated cyclic hydroxamic acids are thermally stablebelow 180°. 

Acyl derivatives of cyclic hydroxamic acids, like all 0-acyI derivatives of hydroxylamine, show infrared absorption because of the acjd carbonyl at characteristically high frequencies, generally near 1800 The range of frequencies is similar to that found for... 

There has as yet been no systematic work on the mass spectra of cyclic hydroxamic acids, but from the limited information available the direct loss of 0 or OH from the molecular ion is to be expected. The fragmentation behavior of the 0-alkyl derivatives is rather unpredictable, although again processes involving fission of the N—0 bond are generally important. Table II shows the prominent first-generation fragment ions from a few hydroxamic acids and their ethers. 

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