1.2.5- Oxadiazoles, 3-hydroxy-, tautomerism

The 1,2,5-oxadiazole ring is a stable system and annular-group tautomerism is not favored. Although three tautomeric forms can be drawn for 3-hydroxyfurazans (Scheme 3) IR and NMR data for chloroform solutions show only the presence of the hydroxy compound. Ring-chain tautomerism is an important feature of furoxan chemistry and the equilibration between the isomeric furoxans is discussed in detail later in this chapter (Section 4.05.5.2.1). 

Tautomerism involving substituents is not favored and even the 3-hydroxy compounds for which lactam tautomers can be proposed (Scheme 3) react exclusively in the conjugated from. Thus N-ethyl-l,2,5-oxadiazol-3(2/ -one (49) is prepared via a 3-silyloxy derivative rather than by direct alkylation of lactam (50) <93T5905>. 

In addition, ab initio calculations for the unknown imsubstituted sydnone molecule (1, R = R = H) and the tautomeric species 2H-l,2,3-oxadiazol-5-one (442) and 5-hydroxy-1,2,3-oxadiazoIe (443) have been reported. The meso-ionic tautomer (1) is calculated to be less stable... 

Diese Eigenscbaft ist industriell von besonderem Interesse fur die Synthese von thcrmostabilen Polymeren. 2-Oxo-2,3-dihydro- und 2-Thiono-2,3-dihydro-l,3,4-oxadiazole sind nur formal als Hetarene aufzufassen, da sie nicht als tautomere 2-Hydroxy- bzw. 2-Mercapio-l, 3,4-oxa-diazole vorliegen. 

In 5-hydroxy-3-phenyl-l,2,4-oxadiazole (97a) the keto forms (97b, c) predominate due to the proximity of the electronegative oxygens at the sp2 carbon. Of these three tautomers (97c) is favored by IR, UV and NMR spectral data (76BSB35). The tautomer (98b) is more important in the 5-phenyl isomer in solution. Nevertheless the position of tautomeric equilibrium in the two cases is dependent on solvent. For example, in the solid (98b, c) predominates but in acetone and other oxygen solvents (98a) allows for an effective hydrogen bonded dimer (99). 

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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