Photoisomerization of nitrones

Oxaziridine formation by photoisomerization of nitrones was discovered almost simultaneously with the peracid procedure (58JOC65l . The t-butyl nitrones of benzaldehyde and 4-nitrobenzaldehyde yielded about 90% of oxaziridines (254) and (255) on UV irradiation. 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

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A photochemical diaziridine synthesis, analogous to the photoisomerization of nitrones, was worked out at suitable azomethine imines. By the action of sunlight, 104 is isomerized to the unstable diaziridine 105 in nearly 80% yield. 

As a reversal to the photoisomerization of nitrones (see p 33), oxaziridines can be converted into nitrones by thermolysis. The required temperature depends on the type of oxaziridine substituents. 

The three types of three-membered ring are also accessible by photoisomerization of open chain 1,3-dipoles nitrones, azomethinimines and linear diazo compounds respectively. All three-membered rings prepared prior to 1967 were included in a comprehensive review . 

Photoisomerization of nonconjugated nitrones, in particular derivatives of 3-imidazoline-3-oxide and pyrroline-N -oxide, appears to be irreversible (Scheme 2.83) (10, 452). 

A detailed analysis of photoisomerization of the nitrone group in the nitroxyl radical 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-l-oxyl, based on double electron-nuclear resonance methods, has shown that in the absence of oxygen the photochemical reaction occurs without affecting the radical center. The... 

E,Z-Photoisomerization and the Accompanying Reactions of Oximes, Oxime Ethers, Nitrones,... 

Koyano and Tanaka found that E-to-Z- and Z-to- -photoisomerization takes place upon the irradiation of a solution of a-cyano-a,iV-diphenylnitrone in the presence of a triplet sensitizer, while the irradiation of E- or Z-forms of the nitrone in the absence of a sensitizer resulted in the formation of the corresponding oxaziridine. These results indicated that photochemical —> Z and Z —> isomerization take place from the triplet-excited nitrone, while the photochemical formation of the oxaziridine takes place from a singlet-excited nitrone (Scheme 8). 

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