Acetohydroxamic acids

From stability constant measurements, log K = 2.3 mol ) . (32,33) The equilibrium therefore favors the hexadentate chelate for all but extreme ligand concentrations. 

The second component of the chelate effect, the concentration factor, can be Illustrated by considering the ability of a drug (D) to sequester transferrin-bound iron  

The same property is illustrated by the data in Table II which compares the degrees of dissociation of tetracoordlnate complexes with 0, 2, and 3 chelate rings. A chelate effect of 10 per chelate ring is assumed as the basis for the arbitrary stability constants listed. The superior properties of the metal chelate in dilute solution are dramatically Illustrated by a simple calculation of the degrees of dlssoclaton in 1.0 molar and 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

Degrees of Dissociation of Complexes and Chelates in Dilute Solution 

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These are initial rates of acetohydroxamic acid formation from acetic acid, catalyzed by nickel chloride. 

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. 

Caracemide (3) is an antitumor agent. This simple molecule is constructed by reacting acetohydroxamic acid (1) with methylisocyanate (2) promoted by triethylamine. The resulting 0,N-biscarbamate (3), caracemide, is metabolized readily either by deacetylation or by decarba-moylation and its antitumor properties are believed to re.sult from the reactivity of the resulting metabolites with DNA [1]. 

Another prepn reported is by heating trifluoro-acetohydroxamic acid (F3CCONHOH) to 85° at 30—40mm of Hg, yield 63% (Ref 10)... 

Payne, A.N., Garland, L.G., Lees, I.W. and Salmon, J.A. (1988). Selective inhibition of arachidonate 5-lipoxygenase by novel acetohydroxamic acids effects on bronchial anaphylaxis in anaesthetised guinea-pigs. Br. J. Pharmacol. 94, 540-546. 

Reported redox potentials of laccases are lower than those of non-phenolic compounds, and therefore these enzymes cannot oxidize such substances [7]. However, it has been shown that in the presence of small molecules capable to act as electron transfer mediators, laccases are also able to oxidize non-phenolic structures [68, 69]. As part of their metabolism, WRF can produce several metabolites that play this role of laccase mediators. They include compounds such as /V-hvdi oxvacetan i I ide (NHA), /V-(4-cyanophenyl)acetohydroxamic acid (NCPA), 3-hydroxyanthranilate, syringaldehyde, 2,2 -azino-bis(3-ethylben-zothiazoline-6-sulfonic acid) (ABTS), 2,6-dimethoxyphenol (DMP), violuric acid, 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpipperidin-iV-oxide radical and acetovanillone, and by expanding the range of compounds that can be oxidized, their presence enhances the degradation of pollutants [3]. 

TABLE 15. Gas-phase acidities of acetohydroxamic acid, acetamides and their methyl derivatives... 

The gas-phase acidities of hydroxamic acid and its N- and O-methyl derivatives have been measured using FT ion cyclotron resonance (Table 15)160. The acidity order is the same as that found in DMSO although, in this solvent, the O-methylation of acetohydroxamic acid decreases the acidity by 1 pK unit and the IV-methylation by 3.6 pK units, respectively. 

The pentacoordinate silicon compounds 81, 8,54 82,54 83,54 and 8455 are spirocyclic zwitterionic A5S7-silicates with an Si04C skeleton. The chiral zwitterions contain two diolato(2—) ligands that formally derive from aceto-hydroximic acid and benzohydroximic acid (tautomers of acetohydroxamic acid and benzohydroxamic acid). 

We shall now compare some properties of prototypical examples of hydroxylamines, oximes and hydroxamic acids. These will be structures 1, 2 and 3 with methyl groups at the remaining positions, i.e. dimethylhydroxylamine (6), acetoxime (7) and acetohydroxamic acid (8). 

As points of reference, we will take two well-established hydrogen-bond donor/ acceptors, H2O and NH3. Their computed gas-phase Vs,max and Vs,mm are in Table 5, along with the same data for all of the molecules that have been discussed hydroxylamine (5), dimethylhydroxylamine (6), acetoxime (7), acetohydroxamic acid (8), and the isomeric pairs of oximes examined in the last section. Finally, we included an additional hydroxamic acid, 11, to see the effects of the strongly electron-withdrawing cyano group. 

Hydroxylamine and dimethylhydroxylamine, 5 and 6, are extremely weak acids, judging from how much less are their conjugate base /s.min than that of acetohydroxamic acid, 8. Accordingly the experimental p Ta of hydroxylamine, 5.94, must refer to its behavior as a base, with pX b = 8.06. Indeed its p Ta and its nitrogen /s.min are both quite similar to those of pyridine. 

Again in the case of acetohydroxamic acid, H3C—C(0)—N(H)—OH, the acetyl and hydroxyl oxygen lone pairs that dominate its Fs(r) (Figure 5) are only intermediate in terms of /s(r) and hence polarizability (Figure 6). The most polarizable region is associated with the nitrogen—which does not manifest itself at all in Fs(r)-... 

For example, the specific reaction of yellow Fe with acetohydroxamic acid to form a purple complex is therefore assumed to correspond to the scheme in equation 3. 

C80H56C14JN4O12 CPCHA (25,26,27,28-telrahydroxy-5,11,17,23-tetrakis(A -p-chlorophenyl)calix[4]arene-p,p, p", p "-tetrakis(amino-bis(acetohydroxamic acid))) Extraction Th, U, Zr 149-151... 

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