Tautomerism hydroxamic acids

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). 

Hydroxamic acid exists in two tautomeric forms, (1) and (2), and such keto-enol tautomerism provides a number of sites for coordination and chelation. The keto form (1) predominates in acid media and the enol form (2) in alkaline media 25 this has been corroborated by the extraction of vanadium benzohydroxamic acid complexes in organic solvents.26... 

Benzonitrile oxide (C in Figure 15.44) is an isolable 1,3-dipole. It can be generated from benzaldoxime and anNaOH/Cl2 solution. Under these reaction conditions the oxime/nitroso anion (A B) is initially formed and chlorine disproportionates into Cl—O and chloride. An SN reaction of the negatively charged C atom of the anion A B at the Cl atom of Cl— O or of Cl—O—H affords the oc-chlorinated nitroso compound E, which tautomerizes to the hydroxamic acid chloride D. From that species, the nitrile oxide C is generated via a base mediated 1,3-elimination. Isoxazoles are formed in the reactions of C with alkynes (Figure 15.44), while isoxazolines would be formed in its reactions with alkenes. 

Clauson-Kaas and co-workers333 have carried out a different pyridine synthesis with acylfurans. 2-Acetylfuran (54) is first converted into the ketal (55). After electrolysis to 56, the pyridine-A-oxide (57) is formed via acid-catalyzed ring opening and treatment with hydroxylamine. A tautomeric pyridone form (58) (hydroxamic acid) has also been proposed334 ... 

The lactim/lactam tautomerism of hydroxamic acids and their O-alkyl and O-acyl derivatives has also been studied [146], Hydroxamic acids exist in the solid state and in polar solvents as the lactam tautomer only, whereas in nonpolar solvents the hydroximic tautomer is also present. Further analogous solvent-dependent lactim/lactam equilibria have been observed for certain Schiff bases (prepared from anilines and 2(4)-hydroxybenzaldehyde [256] or 2-hydroxynaphthaldehyde [257]), for 3-hydroxypyrazole [258], and for 3-methyl-l-phenylpyrazolin-5-one [259]. 

A-Acylated hydroxylamines may exist in several tautomeric forms the protonated hydroxamic acid may be reducible. Protonation of a hydroxamic acid takes place mainly on oxygen to R-C(OH)=NHOH [97]. Aromatic hydroxamic acids with electron-attracting groups, such as /j-cyanobenzhydroxamic acid or isonicotinic hydroxamic acid, are reducible to the amides [98]. 

The Lessen rearrangement differs from the Hofmaim rearrangement only in that the leaving group is carboxylate anion rather than bromide ion. The starting material is the ester of hydroxamic add (RCONHOH), which is decomposed in presence of base. The hydroxamic acids exhibit tautomerism - the keto-form is termed hydroxamic form while the enol-form is called hydroximic add. As the reaction is normally carried out in water, the process furnishes the amine directly (Scheme 6.39). 

Hydroxamic acids exist in two tautomeric modifications of A -hydroxylamide and AT-hydroxyimino acid (118). The phenomenon occurs for N,0-diacylhydroxamines also. 

Hydrophobic bonds see Noncovalent bonds Hydroxamic acids derivatives of carbonic acid containing the tautomeric group R-CO-NHOH R-C(OH) = N-OH.H.a. form stable, five-membered rings with metal ions and they are especially important in the iron metabolism of many organisms Well-known examples of H.a. are aspergillic acid (Fig.), synthesized from leucine and isoleucine by Aspergillus flavus, and the Siderochromes (see). 

The formed aldehyde, treated by phenylsulfohydroxamic acid in the presence of a ferric salt, offers the usual ferric hydroxamate complex. Phenylsulfohydroxamic acid gives sulfinic acid and nitroxyl (unstable intermediary of structure HNO) in basic medium. Nitroxyl is a nucleophilic species that attacks the carbonyl group of carboxaldehyde. A hydroxynitroso derivative forms. It is in tautomeric equilibrium with hydroxamic acid responsible for the color ... 

A positive reaction is obtained, however, with a series of nonphenolic compounds aromatic amines, ends or hydroxymethylene compounds, tautomeric jS-diketones, j -ketoesters, derivatives of malonic acid esters or of cyanoacetate with a negative substituent at the -- CH2 group, some isonitrites, hydroxylamine derivatives, oximes, hydroxamic acids, and certain derivatives of sulfur. 

---