1.3.4- Oxadiazol-2-ones, rearrangements

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). ... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of 3-formylamino-l,2,4-oxadiazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out7 The treatment has shown the participation of asymmetric transition states and non-concerted processes via symmetrical intermediates. A detailed ab initio and density functional study of the Boulton-Katritzky rearrangement of 4-nitrobenzofuroxan has indicated a one-step mechanism for the process. 

Complex products were also obtained from electron-poor dienes50. Photolysis of (t-Bu2Si)3 with tetrazine 8 gave a complex mixture of products, from which 9 and 10 could be isolated (equation 20). In 10, one of the CF3 groups has been completely degraded, with simultaneous rearrangement of the ring skeleton. The reaction with oxadiazole 11 also took an unexpected course (equation 21). 

The iron-catalyzed [3 + 2]-cycloaddition (Huisgen reaction) of nitriles and carbonyl compounds as reported by Itoh et al. is one of the rare examples reported where an iron reagent can be utilized for the synthesis of 1,2,4-oxadiazoles (Scheme 9.35) [93]. In this reaction, methyl ketones are nitrated at the a-position by Fe(N03)3 to generate an a-nitro ketone. This intermediate rearranges to an acyl cyanate, which reacts further with the nitrile to give the heterocyclic product 48 in good to excellent yields (R1 = Ph, R2 = CH3 95% yield). 

The rearrangement is the transformation of one of the acetamidofuroxan cycles into the 1,2,4-oxadiazole ring with the cleavage of the 0(1)-N(5) bond of the furoxan and the formation of a nitromethylene fragment [558] (Scheme 104). 

Secondary ketoenamines react with electrophilic diazenes to give products which undergo rearrangement to an adduct which affords benzimidazolinones after subsequent cyclization322. In the case of tertiary a-enaminones, l,3,4-oxadiazole-2-ones, as unstable intermediates of a [4 + 2]-cycloaddition, can be obtained323 (equation 241). 

The thermolysis of 2-methyl-5-phenyl-l,3,4-oxadiazol-2-(3H)-one 106 produces a nitrile imine intermediate 105 which rearranges to the carbodiimide 107. The same carbodi-imide is obtained in the thermolysis of 2-methyl-5-phenyltetrazole 104. 

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