Hydroxamic acids, cyclic formation

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. 

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°. 

The literature contains several references to reductive cyclizations producing cyclic hydroxamic acids like those described above, though not in solid-phase chemistry.43-47 We showed that this reaction competes well with lactam formation. The relative extent of these two reaction pathways is highly dependent on the presence and the nature of other heteroatoms and substituents around the ring system as well as on the conditions used to effect the reduction. 

MisceUaneous Reactions.—Details of the formation of nitrous oxide by photofragmentation of methyl nitrite have been reported.Photorearrangement is observed, however, on irradiation of the nitrites (183) derived from 6-methyl- and 4,4,6-trimethyl-cholest-5-en-3-ol to give the cyclic hydroxamic acids (184). There is ample precedent for these transformations, the likely pathway for which is outlined in Scheme 12. An alkoxyl radical (185) is also thought to be involved in the photochemicaUy induced conversion of O-nitrobenzoin (186) into benzaldehyde (187) and 2-phenylbenzo[b]furan (188). Reductive photoelimination of vicinal dinitro-groups takes place by... 

O, D) A hypothetical pathway is the formation of an acylimino acid unit by N-hydroxylation of the amino acid within the peptide chain followed by loss of water. a-Acylimino carboxylic acid derivatives readily rearrange to acylenamino carboxylic acid derivatives 314). N-Hydroxy compounds, such as mycelianamide and pulcherrimic acid [for a review on cyclic hydroxamic acids see ref. (26)], have been found within the class of dehydrocyclopeptides. Although N-hydroxypyra-zinones are common, it is not known whether these compounds are biogenetically linked with the piperazinediones. It is appropriate to point out that N-hydroxylation of amides is a familiar degradative pathway in the metabolic processes of higher organisms 412). The facile elimination of water from N-acylhydroxylamino acids to form acylenamino acids 377) should also be noted in this context. 

A further type of nitro-group rearrangement gives rise to a cyclic hydroxamic ether. Noland e.t aL describe the action of cold, dilute sulfuric acid on the sodium salt of 5-nitronorbornene (98), which results in conversion to the oxazinone (101). This complex rearrangement is rationalized by the sequence 98 101 involving intermediate formation of the nitrile oxide (99) and the hydroxamic acid (100). 

Nitro-compounds fRNOj) are isomeric with nitrites, but their electronic structure, excited states and photochemistry are very different. There is no very low-lying (n.jt ) state, and nitroalkanes show n — 3i absorption with a maximum around 275 nm ( —201 mol - cm In cyclohexane solution, nitromethane (CH1NOi) is photoreduced to nitrosomethane(CH,NO, but nitroethane under the same conditions gives rise to a nitroso-dimer derived from the solvent CS.47). The latter process is probably initiated by cleavage of the carbon-nitrogen bond in the nitroalkane. In basic solution (when the nitroalkane is converted to a nitronate anion) irradiation can lead to efficient formation of a hydroxamic acid (S.48), and this reaction most likely proceeds through formation of an intermediate three-mem bered cyclic species. 

---