Nitrones photochemical rearrangement

Photochemical Rearrangement Isomerization of nitrones to oxaziri-dines is a general reaction of various cyclic and acyclic nitrones (447-449). When this reaction is reversible, many transformations of nitrone to oxaziridine and back to nitrone can be carried out without decomposition. This reaction is of special interest in view of light energy accumulation (450, 451). 

To illustrate the synthetic use of photochemical rearrangement, the photolysis of nitrones (249) leading to the formation of bicyclic lactams (250) is an example (Scheme 2.88) (459). 

An alternative method in the synthesis of alkaloids, photochemical rearrangement of endocyclic nitrones into bicyclic lactams has drawn special attention. Analyses of photochemical rearrangement and application of modified conditions of the Barton reaction testify to the comparability of results obtained in these approaches (Scheme 2.89) (460). 

A transformation of this type also occurs in the photochemical rearrangement of the acyclic nitrone (177). to the amide (178) and an intermediate oxaziridine may. be, involved in the photochemical conversion149 of aryl oximes into the corresponding amides [Eq. (42)]. 

The photochemical rearrangement of nitrones to amides often involves initial formation of oxaziridine intermediates. These oxaziridines can be isolated in good to excellent yields (Table 31). 

The rearrangement of phenyl- er -butyl nitrone to the isomeric oxazirane (9) occurred in 95% yield on irradiation in acetonitrile for 2 hr. Because 2-ieri-butyl-3-phenyloxazirane (9) can be reconverted into the more stable nitrone, the photochemical reaction involves the conversion of radiation energy into chemical energy. 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

Rearrangements of nitrones due to migration of the A-oxide oxygen can be induced both, photochemically and by various reagents, but in specific conditions it can proceed spontaneously. On one hand, such transformations are caused by the O-nucleophilic character of nitrones able to react easily with acid anhydrides, their halo anhydrides, sulfonyl chloride and other agents on the other hand, by a significant CH-acidity of a-alkyl groups. 

The transformation of endocyclic nitrone 56 (made from N,0-bis-protected hydroxylamine 55) to lactam 20 can be carried out by photochemical activation or by a two-step modification of Barton s protocol, that is, by trapping the nitrone oxygen followed by an alkali-promoted, semi-pinacol-like rearrangement (03JOC8065). 

Other relevant described reactions, which affect the stability [72] and are related to some adverse effects of these compounds (see below) [73], have been the photochemical transformations. In fact, QDO undergoes rearrangements in the presence of UV-irradiation, passing through an oxaziridine intermediate, like an acyclic nitrone (Fig. 8). The final products depend on the QDO 2,3-substitutions and reaction conditions [72,74,75]. 

Azinemonoxides are cleaved by two distinct photochemical pathways. One involves oxygen migration and the other involves a pericyclic ring closure. 4) Both processes are represented in the mass spectra of the compounds. Similar rearrangements occur in both the photochemistry and mass spectrometry of aryl nitrones, aryl N-oxides and aromatic azoxybenzenes. )... 

The remarkable stability reportedfor 2-phenyl-3,3-dibenzoyloxaziridine in the face of Padwa s inability to isolate oxaziridines but only amides upon oxidation of jV-phenylacylimine derivatives and the lability of nitrones 80 and 82 has recently been shown to be in error.The product from the thermal or photochemical isomerization of A -phenyl-C,C-dibenzoylnitrone is the expected rearrangement product, A -benzoylphenylglyoxanilide. 

The reported synthesis and properties of an oxaziridine by the thermal isomerization of the C7-trityl ester of 3-ac/nitrocamphor bears reinvestigation, particularly in view of the recent report that irradiation of nitronate salts yields hydrox-amic acids but that oxaziridine intermediates could not be detected.) A -Sulfonyl-oxaziridines also rearrange, thermally to nitrones that further decompose to products, but photochemically they yield amides. ... 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

Although further examples of photochemically induced rearrangement of nitrones and /V-oxides have been described this year, little novelty can be attached to most of these reports. The N-oxide (76), on irradiation in the presence of a proton source such as persulphate or dilute sulphuric acid, is converted into the corresponding 4-aryl-2 -l,4-benzoxazin-3(4iZ>one (77) 5 evidence for the pathway outlined in Scheme 3 involving the oxaziridine (78) as an intermediate is presented. 

Rearrangement. The considerable number of reports associated with the photochemically induced rearrangement of nitrones, oximes, and oxaziridines reflects the continuing interest in this area. The ratio of caprolactam to hexanoamide formed from irradiation of the oxaziridine (659 R = H) was different from that obtained by irradiation of cyclohexanone oxime. Photo-... 

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