Pyrazolo pyridinium salts

Pyrazolo[l,5-a]pyridines can be prepared by cycloaddition of pyridinium A -imides (produced by iV-deprotonation of iV-amino-pyridinium salts with base ) with alkynes or A -amination of 2-alkynyl-pyridines. The cycloaddition of 3-benzyloxypyridinium AT-imide involves preferentially the more hindered C-2. ... 

In the base-induced conversion of pyridinium salts (199) to pyrazolo[l,5-a]pyridines, 4,4a-di-hydropyrido[l,2-t ][l,3,4]thiadiazine intermediates (200) could be identified by H NMR, but were not isolable. Their subsequent oxidation may afford 127t antiaromatic systems (201) which upon disrotatory cyclization to tricyclic thiiranes (202), followed by desulfurization or acyl migration, gave differently substituted pyrazolo[l,5-a]pyridines, (203) and (204), respectively (Scheme 14) <85BCJ1432,84H(22)2237>. A similar desulfurization reaction of this ring system has also been reported <73CPB2146>. 

Reactions of the pyridinium salts (245) with ethyl ethoxymethylenecya-noacetate give the pyrazolo[l,5-a]pyridine (247) via the intermediate (246), Scheme 55, and analogous reactions of the pyridinium salts (248) give the indolizines (249) [80JCR(M)0404, 80JCR(S)18],... 

Salts of 3-[3-(phenylpropynoyloxy)propyl]-N-aminopyridinium (43) (Equation (5)) undergo dipolar addition to form tricyclic pyrazolo[l,5-a]pyridines (44) <83H(20)20i, 85JSC(P1)379>. N-Methoxycarbonylmethyl-3-[3-(phenylpropynoyloxy)propy]pyridinium salts undergo a similar dipolar addition to give indolizine derivatives. 

H-Pyrazolo[3,4-b]pyridines 5, 269/355 suppl. 27 Pyrazolo[l,5-a]pyridinium salts, polyhalogeno-... 

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