1.2.4- Oxadiazoles Thermal rearrangement

Aminoethyl)-l,2,4-oxadiazoles (606 R = alkyl or aryl) readily isomerize to 3-acylamino-2-pyrazolines (607) on heating. Treatment of the salt (608) with triethylamine yields the transient ylide (609), which decomposes to methyl-phenylcarbodi-imide (610). 2,3-Diphenyl-l,2,4-oxadiazolin-5-one (611) rearranges photochemically to a mixture of the oxadiazolinones (612) and (613) pyrolysis gives 2-phenylbenzimidazole with extrusion of carbon dioxide. The oxadiazole (614) rearranges thermally to the indazole (615). ... 

N-C-N side chain was used by Cusmano and co-workers in the preparation of triazolo[l,5-a]quinolines and triazolo[l,5-a]pyridines. The base-catalyzed thermal rearrangement of 1,2,5-oxadiazole 22 afforded the desired triazolo[l,5-a]quinoline (E)-23 in good yield as a single stereoisomer. The expected Z-oxime products readily isomerize to the respective -oximes under the reaction conditions. When the phenyl-substituted 1,2,5-oxadiazole 24 was subjected to the reaction conditions, however, complete isomerization of the products was not observed and a mixture of stereoisomers (E)-25 and (Z)-25 was obtained. 

The thermally induced rearrangements in the furoxan series have also been found. In particular, the transformation of 3-R-substituted 4-(3-ethoxycarbonylthioureido)-l,2,5-oxadiazole 2-oxides into derivatives of 5-amino-3-(a-nitroalkyl)-l,2,4-thiadiazole and into (5-amino-l,2,4-thiadiazol-3-yl)nitroformaldehyde arylhydrazones has been reported (Equation 8) <2003MC188>. 

Ammonia and primary amines react with dibenzoylfuroxan to give 3-amino-4-nitrosoph-enylisoxazole (54), which may rearrange thermally to 3-amino-4-benzoylfurazans. Earlier suggestions that the products are 3-acyl-5-amino-l,2,4-oxadiazoles have been discounted (69JHC317). A mechanism involving nitrile oxide and glyoxime intermediates has been proposed (Scheme 11) and it is apparent that other diacylfuroxans behave similarly. 

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