Pyrazol-3,5-diones, tautomerism

Several years later other workers used the same method to confirm that pyrazol-3,5-diones 51a,b/5-hydroxypyrazol-3-ones 52a,b exist as tautomeric mixtures in solution. The reactions with diazomethane took place in a mixture of diethyl ether and methanol and afforded a mixture of the respective pyrazol-3,5-diones 53a,b and 5-methoxypyrazol-3-ones 54a,b. The suitability of these structurally fixed methyl derivatives as model compounds in the study of the tautomeric structures by NMR spectroscopy was investigated (80CB3910) (Scheme 16). 

Acylation and metallation studies on monomers, dimers, trimers, and tetramers containing linked thiophen, pyridine, pyrimidine, furan, benzofuran, benzothiophen, and indole moieties have been published/ Deuteriation, halogenation, and diazo-coupling reactions of 2-oxo- and 2-thioxo-l,2-dihy-dropyrimidinium salts have been studied and compared with results for 2,2-dialkyl-1,2-dihydropyrimidinium and 2,3-dihydro-1,4-diazepinium salts in order to demonstrate the effect of an adjacent 0x0- or thioxo-group on the properties of a 1,5-diazopentadienium system/ Vilsmeier formylation of, and tautomerism in, 2-hydroxypyrazolo[5,l-h]quinazolone and l-phenylpyrazolo[5,l-A]-quinazoline-2,9-dione have been studied/ The pyrazolo[3,4-c]pyrazole (71) has been methylated and acetylated, the major products being (72)/ ... 

---