Ring Transformation of Heterocycles Adel M. Awadallah Chemistry Department, Faculty of Science, Islamic University of Gaza, P. O. Box108, Gaza, Palestine; Tel , Fax: ,

Ring Contraction

a) Transformation of 4-amino-4,5-oxadiazoles into the aromatic 1,3,4-oxadiazoles. Acylation of 4-amino-4,5-dihydro-1,2,4-oxadiazoles 1 in refluxing toluene or treatment of the acyl derivatives of these compounds 2 with trifluoroacetic anhydride brings up ring transformation to the aromatic 1,3,4-oxadiazoles 3 [1].

Similarly, 4-amino-4,5-dihydro-1,2,4-oxadiazoles 4 are transformed into the corresponding 2-arylamino-1,3,4-oxadiazoles (6, X = O) or thiadiazoles (6, X = S) via reaction with phenylisocyanate or phenylisothiocyanate, followed by brief treatment of the resulting adducts 5 with trifluoroacetic anhydride at ambient temperature [2].

b) Ring contraction of 1,2,3,4-tetrahydro-1,2,4,5-tetrazines into 1H-1,2,4-triazoles. The reaction of nitrile oxides 7 with methylhydrazones 8 in chloroform was found to constitute a convenient synthetic route to the novel 4,5-dihydro-6H-1,2,4,5-oxatriazines 9 [3].

Risitano and coworkers obtained triazoles 10 directly from the appropriate monomethylhydrazones of aryl aldehydes in refluxing ether for 2 hours [4].

1,2,3,4-tetrahydro-1,2,4,5-tetrazines 9 derived from methylhydrazones of aryl aldehydes underwent ring contaction, via elimination of H2O, to yield the respective 1H-1,2,4-triazoles 10. This transformation was envisaged to proceed via the ring-opened (E)-hydrazonoximes which then suffered dehydrative cyclization. The process is acid-catalyzed and was thermally induced. Apparently, the driving force for this transformation was linked to the aromaticity of the triazole product [5].

c) Transformation of dihydrotriazoles into 1-Aryl-3-acetyl-5- methyl-1H-1,2,4-triazole

d) Ring Expansion 1,2,4-triazoles into 1,2,3,6-oxatriazine Oximic 1,2,4-triazole ligands 15 were prepared from the reaction of triazoles 1 with hydroxylamine hydrochloride at room temperature. At higher temperatures, the reaction afforded, however, the novel ring transformation product 4-amino-2-(4-chlorophenyl)-5-methyl-2H-1,2,3,6- oxatriazine 16. The reaction of the ligands 15 with nickel(II) and palladium(II) acetates in ethanol at room temperature yielded the respective square planar complexes 18. X-ray structure determination of one of these complexes revealed that metallation led to unexpected ring transformation of the triazole ligand. It is probable that such ring transformation generated the imidazole-N-oxide intermediate 17 which coordinated to Ni(II) ion, and the 4N-donor set comprises both imidazole nitrogen and arylhydrazone nitrogen. [7]

3-Acetyl-1,2,4-triazole hydrazones (19) gave Square planar nickel(II) complexes (21) upon reaction with Ni(OAc)2 in EtOH at room temperature in a 2:1 mole ratio. [8]

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