Pyrazolo pyridinium

The mechanism of attack of 1,3-dipolar reagents on fluoroalkenes can be considered to be either stepwise or concerted. Heteroaromatic N-imines react by a stepwise 1,3 addition to perfluoroalkenes and -alkynes to give fluorinated pyrazolo[l,5-a]pyridines [82JCS(P1)1593]. Pyridinium /-butoxycarbonylmethylide with fluoroalkenes gave pyrrolo[l,2-a]pyri-dines [86JCS(P 1) 1769] and indolizines (22) are obtained with pyridinium phenacylide [91JFC(51)407]. 

Dipolar cycloaddition of 5-aminofuro[3,2-f]pyridinium tosylate 140 and the alkynyl ester 141 yields furo[3,2-f]pyrazolo[l,5- ]pyridine 142 in moderate yield (Equation 33) <1999CCC539>. 

Cycloaddition reactions of alkynes and furo[3,2-r ]pyridinium A -imides lead to furo[3,2-c]pyrazolo[l,5- ]pyridines, 50, in low yields <1999CCC539>. 

Condensed 4-nitropyrazoles have been produced from pyridinium N-imines and l,l-bis(methylmercapto)-2-nitroethylene 2-methylthio-l-nitropyrazolo[l,5-a]pyridines (54) (40-60%) are so obtained.49 After the electrocyclic transformation, the intermediate 53 does not lose the elements of nitrous acid, but rather undergoes dehydrogenation to yield the observed products (Scheme 11). Isoquinolinium (V-imine gives 2-methylthio-l-nitro-pyrazolo[5,1 -a]isoquino ine similarly.49... 

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. ... 

Bei 1-Pyrazolo-2,4,6-trimethyl-pyridinium-tetrafluoroborat kann am Pyrazol-Ring eine weitere Quater-nierung durchgefuhrt werden1880. 

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-... 

Jiao et al. developed a Cul- and AgCOg-cocatalyzed synthesis of pyrazolo[l,5- ] pyridine derivatives via dehydrogenative annulations of pyridinium ylide and terminal alkynes (Scheme 8.68). Ag2C03 might assist the formation of copper acetylide from alkyne with Cu(I) catalyst. The C(sp)-H bond of alkyne... 

Earlier work has demonstrated that pyridinium 7V-ylides and keten thioacetals and related compounds yield allyl ylides and indolizines/ " The extension of this reaction to pyridinium JV-imines provides a useful synthesis of pyrazolo-[l,5-a]pyridines (95), as shown in Scheme 21. ... 

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