Five-membered ring systems oxadiazoles

Likewise, trifluoromethyl-substituted nitrile imines [172] and nitnle oxides [173,174, 7 75] have been used to synthesize tnfluoromethyl substituted five-membered ring systems of the pyrazole, isomzole, isoxazohne, and 1,2,4-oxadiazole... 

In contrast to 6-substituted-bipyridine systems such as 49, attachment of a five-membered heterocycle to the 2-position of 1,10-phenanthroline, which is a structural modification similar to the replacement of one of the terminal rings of terpyridine, does generally bring the ligand field into the crossover region and spin transitions have been observed for such systems when the heterocycle is thiazole 51 [74], imidazoline [75], triazole [76], pyrazole [77] and oxadiazole [78]. 

Oxadiazoles are unknown. The importance of the 1,2,3-oxadiaza ring system lies in the stabilization offered in the derivatives of higher oxidation state known as sydnones (1) and sydnonimines (2). Fusion with an aromatic ring does not stabilize the five-membered... 

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