1,2,5-Thiadiazoles, 3-hydroxy-, tautomerism

Pyrazblin-5-one, 3-alkyl-(l,2,4-thiadiazol-5-yl)-reactions, 6, 483 2-Pyrazolin-5-one, 3-amino-tautomerism, 5, 215 2-Pyrazolin-5-one, 4,4-diazido-rearrangement, 5, 720 2-Pyrazolin-5-one, 3-hydroxy-tautomerism, 5, 215 2-Pyrazolin-5-one, 3-methyl-1 -phenyl-reactions, 5, 252... 

Hydroxy-l,2,4-thiadiazoles can exist in 3-tautomeric forms (Scheme 1). Chemical evidence suggests that the OH form 4 predominates however, UV data suggest that the lactam form 5 is the major tautomer in ethanol <1996CHEC-II(4)307>. 

Thiadiazoles were first described in 1882 by Fischer and further developed by Busch and his coworkers. The advent of sulfur drugs and the later discovery of mesoionic compounds greatly accelerated the rate of progress in the field of thiadiazoles. Thiadiazoles carrying mercapto, hydroxy and amino substituents can exist in many tautomeric forms and this property is being intensively studied using modern instrumental methods. 

C. Prototropic Tautomerism of Amino-, Hydroxy-.Vfercapto-l,3,4-thiadiazoles. ... 

The position of equilibrium in the tautomers of 3-hydroxy-1,2,4-thiadiazoles (Scheme 9) is not conclusively known. The existing chemical evidence suggests that the OH form (13) predominates. However, a UV spectral study was interpreted to suggest that the lactam NH form (14a) contributed substantially to the tautomeric equilibrium in ethanol <76AHC(S1)266, p. 377). 

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