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 trifluoroacetic acid and/or 1,2-dichlorobenzene undergo valence bond isomerization to ring-opened reactive intermediates (e.g., 8 gave 13) which can participate in (i) electrophilic substitution as nitrenium cations to yield pyrazole- and indazole-fused new heterocycles (e.g., from 13, 14, and 15 are formed), (ii) pseudoelectrocyclization (e.g., intermediate 19c leads to the pyrazolo[3,4-b]pyrazine 21), or (iii) in nucleophilic addition as carbenium cations (e.g., 1 gave the methoxy-substituted adduct 22 when heated in methanol). Comparison of these and some recent results reveals that this ring opening 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 cyclizations") of neutral fused [1,2,3]triazoles and tetrazoles.
Béres
Mariann
M
Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungary.
Hajós
György
G
Riedl
Zsuzsanna
Z
Soós
Tibor
T
Timári
Géza
G
Messmer
András
A