Isomerization of isoxazole

A. Isomerization of Isoxazoles Bearing Electron-Donating Groups. 60... 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

MNDO results suggest that the activation energies are similar for the gas phase thermal isomerization of isoxazole to oxazole via either a nitrile ylide or a keteneimine, through an azirine intermediate. The first step is rate limiting, which is in good agreement with experimental results (90JPO611). 

Computational studies on the mechanisms of thermal isomerization of isoxazole and 5-methylisoxazole have been reported.162... 

The photochemically induced isomerization of isoxazole (279) to oxazole (281) is known to proceed via the intermediate keto-azirine (280). An M.O. study of the isomerization reactions of the azirine that are involved has been reported. Irradiation of (280) at >300 nm gives n tt excitation of the CO chromophore to 5i, intersystem crossing to Ti, and then C—N bond rupture, to form (279). Irradiation at 254 nm, however, gives n - n excitation of the C=N chromophore to S2, intersystem crossing to T, and then C—C bond rupture, to form (281). Such an isomerization reaction resulted in the photochemical conversion of isoxazolophane (282) into (283) (40%). Irradiation of (283) at 254 nm produced a quantitative yield of the expected oxazolophane. A new rearrangement was found, however, on irradiation of (283) at >300 nm, when the acyl-ketenimine (284) was formed, which could be converted into (285) by acid hydrolysis. 

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