Isoxazoles, photorearrangements

The photorearrangements of isoxazoles are well known reactions. Thus... 

Relatively few studies of photorearrangements in five- and six-membered nitrogen-containing heterocycles have been reported. Isoxazole-to-oxazole photorearrangements have been employed in the syntheses of 2-(2-aryl-5-oxazolyl)benzoates, and nitrogen-oxygen bond homolysis appears to be the first step (as shown in Scheme 5) in the photorearrangement of the fused... 

The well documented photorearrangement of isoxazoles to oxazoles has been investigated theoretically, and it is suggested that excitation of the aclyazirine... 

Further studies of the photorearrangement of five-membered heterocycles have been described. The isoxazole-oxazole rearrangement has previously been shown... 

One case where an azirine intermediate is believed not to arise in the isox-azole photorearrangement has been reported by Padwa and coworkers. When 4-benzoyl-5-methyl-3-phenyl-isoxazole (158) or 4-acetyl-3,5-diphenyliso-xazole (159) was thermolyzed (230°C), a 2/f-azirine is presumably formed which subsequently opens to the nitrile ylide 160. Subsequent closure of 160 affords the isomeric oxazoles 161 and 162 in a ratio of 3 4. 

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