Hydroxamic acids complexes with bases

Ring substituents show enhanced reactivity towards nucleophilic substitution, relative to the unoxidized systems, with substituents a to the fV-oxide showing greater reactivity than those in the /3-position. In the case of quinoxalines and phenazines the degree of labilization of a given substituent is dependent on whether the intermediate addition complex is stabilized by mesomeric interactions and this is easily predicted from valence bond considerations. 2-Chloropyrazine 1-oxide is readily converted into 2-hydroxypyrazine 1-oxide (l-hydroxy-2(l//)-pyrazinone) (55) on treatment with dilute aqueous sodium hydroxide (63G339), whereas both 2,3-dichloropyrazine and 3-chloropyrazine 1-oxide are stable under these conditions. This reaction is of particular importance in the preparation of pyrazine-based hydroxamic acids which have antibiotic properties. 

Ortho-directed metallations also allowed the synthesis of ferrocene-based hydroxamic acids such as 17 and 18 [23] as well as the preparation of planar-chiral carbenes 19 and 21 (which were trapped as chromium and rhodium complexes, 20 and 22, respectively) [24], In this context it is noteworthy that 19 was the first carbene with planar chirality ever reported. 

This method is based on the fact that only esters, but not free carboxylic acids, react with hydroxylamine to form hydroxamic acids, which can be determined colorimetrically as complex with ferric chloride (8). The method—in contrast to most other procedures—measures the concentration of remaining substrate instead of products of hydrolysis. It also requires purified enzymes because of the interference of colored contaminants in the colorimetric measurements. 

Application and Principle This procedure is used to determine transglutaminase activity in preparations derived from Streptoverticillium mobaraense var. The assay is based on the enzymatic formation of a glutamic acid y-hydroxamate in a glutaminyl residue in the substrate peptide with another substrate, hydroxylamine. The amount of the glutamic acid y-hydroxamate formed as a red complex with ferric ion in acidic conditions at 37° is measured spectrophotometrically. 

One subset of Schiff base complexes are the hydrazone complexes which have a R—C=N—N functional group instead of the R C=N—C functionality. These compounds have been prepared as models for bromoperoxidase389 and other biological systems.390 In the solid state, the hydrazones have coordination patterns similar to those of the Schiff bases and the majority of the complexes are oxovanadium(V) hexadentate or pentacoordinate complexes with tridentate 02N donor sets (see Table 5). While most structures contain alkoxide donors,390-403 complexes have been reported with diols and catechols,129,404,405 hydroxamic acids,288,391 hydroxy quinolinate,406, 7 and benzoylhydrazine.408 In addition, dioxo,375,409-412 oxo-bridged... 

Acetyl and Feruloyl Esters in Pectin. A colorimetric method for determining degrees of acetylation in pectins from various sources ( ), has been shown to be rapid and quite sensitive. Hydroxy-lamine is reactive toward both the methyl and acetyl esters in pectin, and ferric ion complexes with the resulting hydroxamic acids are red. The pectin complex is insoluble and removed by filtration the intensity at 520nm in the soluble fraction, consisting of the ferric complex with acetohydroxamic acid, is a measure of acetyl content. The accuracy of the method was demonstrated in determinations of 0-acetyl levels in standard per-ace-tylated polysaccharides. Another method ( ) involves alkaline hydrolysis of the acetyl groups from pectin, followed by distillation of acetic acid and its titration with standard base. 

The determination of cephalexin, cefixime, ceftriaxone, cefotaxime based on the Bent-French method, in which degradation products of P-lactam analogs with metal ions form the colored complexes, was developed. Hydroxamic acids formed by hydroxiaminolysis of cephem analogs (1 3), formed complexes with iron (II) ions (Fig. 13) [44]. 

Fe3 due to its high charge density, small ion radius and low polarisability is a hard Lewis acid and forms stable bonds with hard Lewis bases as oxygen. Its octahedral complexes possess six coordination sites. They can accomodate three two-dentate ligands which for entropic reasons are frequently connected by more or less mobile chains keeping them in the right position. There are three main types of ligands, viz. a) catecholates (which form very stable complexes at pH 7, but they are acid labile), b) hydroxamic acids (ref. 15) and c) carboxylic acid derivatives with a suitably located O- or N-functionality as the second coordination site (refs. 13, 14, 16, 17). 

Titrimetric and colorimetric methods are two classical methods for determining the ester contents of alcoholic beverages. The titrimetric procedure involves the treatment of esters with an excess of base followed by determination of the quantity of base required to hydrolyze the esters present in the sample. The colorimetric procedure involves the esters reacting with hydroxylamine in an alkaline solution to form a hydroxamic acid, which forms a colored complex with iron(III). 

This test is based on the following reactions where R and R are di- or poly functional. The hydroxyamic acid from the polyester reacts with the ferric ion to form an inner complex salt which is water-soluble and violet or purple in colour for most hydroxamic acids. 

Ln = La) and [Ln(phen)2(N03)3] (Ln = La-Lu, Y) are obtained in isomorphous and isostructural series. Both series have 10-coordinate lanthanides with all nitrates present as bidentate ligands. A few other cases are known. Thus, a family of isostructural eight-coordinate lanthanide complexes [Ln (TREN-l,2-HOIQO)] (Ln = La-Lu, except Pm), utilizing a hexadentate tripod ligand based on hydroxamic acid donors, has shown them to be isostructural. ... 

---