Unexpected ring transformation to pyrrolo[3.2-b]pyridine derivatives. Fused azolium salts. 22.

2-Arylsulfanyl and 2-benzylsulfanylpyridinium N-arylimides (2) easily prepared from 3-aryltetrazolopyridinium salts (1) with aryl and benzylthiolates, respectively, reacted with various dipolarophiles yielding cycloadducts that underwent transformation to give tetrahydropyrrolo[3,2-b]pyridines (5, 6, and 8) in good yields. A similar rearrangement (formation of 15) was also observed in the case of parent derivatives being unsubstituted in position 2 (12). The abscence of any significant solvent effect, comparison of the sulfur and non-sulfur analogues, as well as the stereoselective nature of the observed ring transformation seem to support a sigmatropic mechanism. Structure elucidation of the products has been carried out by single-crystal X-ray diffraction and (1)H NMR experiments.

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.

Synthesis of Azinylvinylpyridazines: A General Note on the Isomerization of Hetaryldienamines.

Azinyldienamines underwent Diels Alder reaction of inverse electron demand with 1,2,4,5-tetrazine diester to give azinylvinylpyridazines. Comparison of the products obtained from mono-, di-, and triazinyldienamines revealed that, in some cases, isomerization of the olefinic side chain occurred which can be rationalized by the tautomeric conditions of the intermediates bearing these azine moieties. These experimental findings supported also by semiempirical calculations suggest the importance of the influence of the hetaryl group in such isomerizations.