Valence bond isomerization of fused [1,2,3]triazolium salts with bridgehead nitrogen atom. Fused azolium salts. 19

[1,2,3]Triazolo[1,5-a]quinolinium (7), [1,2,3]triazolo[1,5-b]isoquinolinium (8), and [1,2,3]triazolo[1,5-a]pyrazinium salts (9) when heated in trifluo roacetic acid and/or 1,2-dichlorobenzene undergo valence bond isomerization to ring-opened reactive intermediates (e.g., 8 gave 13) which can particip ate in (i) electrophilic substitution as nitrenium cations to yield pyrazol e- and indazole-fused new heterocycles (e.g., from 13, 14, and 15 are forme d), (ii) pseudoelectrocyclization (e.g., intermediate 19c leads to the pyra zolo[3,4-b]pyrazine 21), or (iii) in nucleophilic addition as carbenium cat ions (e.g., 1 gave the methoxy-substituted adduct 22 when heated in methano l). Comparison of these and some recent results reveals that this ring open ing of fused [1,2,3]triazolium salts is a general phenomenon and is closely related to the well-known retro-electrocyclizations (called "1,5-dipolar c yclizations") of neutral fused [1,2,3]triazoles and tetrazoles.