5-Hydroxy-1,2,4-oxadiazoles, structure

The last ones isomerize on standing into N-alkyl derivatives. A mixture of both 0 and N ethyl isomers has been obtained by heating the silver salt of 3-methyl-5-hydroxy oxadiazole with ethyl iodide. The structure of both isomers has been established by comparing their IR spectra only N-ethyl oxadiazole shows a CO absorption band at 5-68 [jl. 

X-Ray elucidation provided a final proof for the structure elucidation of 8-(4-chlorophenyl)-8-hydroxy-5-methyl-8/f-[l,4]thiazino[3,4-H[l,2,4]oxadiazol-3-one 49, which was obtained as a product of ring-transformation reaction <1997J(P2)2407>. This analysis revealed that the oxadiazole ring is planar, whereas the thiazine ring is in a distorted half-chair conformation with a displacement asymmetry parameter AC2 (S—G(3)) = 0.031. The structure analysis represents the first X-ray elucidation of a [l,4]thiazine ring fused to a [l,2,4]triazole moiety. 

Nuclear magnetic resonance studies on meso-ionic l,2,4-triazol-3-ones (200) were used to examine their relationship to the alternative l,3,4-oxadiazol-2-imine structure (153). The effect of solvent polarity upon the ultraviolet spectrum of anhydro-3-hydroxy-1,4-diphenyl-1,2,4-triazolium hydroxide (200, R = = Ph, R = H) has been discussed... 

Hydroxy- (8a), 2-mercapto- (8b) and 2-amino-l,3,4-oxadiazoles (8c) are in equilibrium with the tautomeric oxadiazolines (9a), (9b) and (9c) respectively. Evidence from UV (72CJC3079) and IR (Section 4.23.2.2.3) spectra supports structure (9a) for A2-l,3,4-oxadiazolin-5-ones and structure (9b) for A2-l,3,4-oxadiazoline-5-thiones. The UV and IR spectra, fluorescence and p/iT values of 2-amino-l,3,4-oxadiazoles indicate that the amine tautomer (8c or d) rather than the imine tautomer (9c or d) predominates (69BSF870, 874, 64CR(258)4579>. 

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