17/-Pyrazolo -1,2,4-triazoles tautomerism

Dipole moments and total energies for pyrazolo[l,5-h]-s-triazole tautomeric forms 97a-97c were calculated using CNDO/2 and CNDO/S (76T341) the results predict the stability of tautomers in a 97a > 97b > 97c sequence. So far, no ab initio calculations or synthesis of 97 have been reported. For pyrazolo[3,2-c]-s-triazole 98 (76T341), the calculated dipole moments, electronic absorption, and proton chemical shifts are in a good... 

In the reactions with phosphonio-a-methoxycarbonyl-alkanides, the products of type 261 derived from 1,3-cycloaddition can rearrange to the tautomeric lif-pyrazolo-triazole (87MI2). The reaction of 3-diazopyra-zoles and 3-diazoindazole with acyl-substituted phosphonium ylides led to pyrazolo-triazine and indazolo-triazine derivatives 266 instead of the expected triazole compounds (8IJHC675). In this case, the ylides, which can exist as phosphonium enolates, possess nucleophilic and electrophilic centers in a /8-relationship, giving [7 + 2] or [11 -I- 2]cycloaddition reactions. With dimethylsulfonio-a-aroyl-methanides, very complex, temperature-dependent mixtures were obtained, in some cases with sulfur retention (87MI3). 

For azapentalenes of type A (e.g. 80,73b 52, and 28160), rule ii should favor the a structure shown in each case. Unfortunately, the position of equilibrium in these compounds has not been measured. A rare case where the annular tautomerism of a type A azapentalene has been investigated is the pyrazolo[3,4-d]-i -triazole 90. Although results from dipole moment measurements were inconclusive, it is likely that 90a and 90c predominate at equilibrium.367... 

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