Preparation of New Nitrogen-bridged Heterocycles. 11. A New Synthetic Method of Pyrazolo[1,5-<i>a</i>]pyridines from the Alkaline Treatment of 1-[(Acylmethylthio)methyleneamino]pyridinium Salts
Alkaline treatment of 1-[(acylmethylthio)methyleneamino]pyridinium halides gave unexpectedly 3-acyl- or 3-(acylthio)pyrazolo[1,5-a]pyridine derivatives in 35–87% yields. Some 4,4a-dihydropyrido[1,2-d][1,3,4]thiadiazine intermediates involved in these reactions could be detected by their nmr follows and three related 4a,8-dimethyl derivatives were isolated. Substituent effect and possible mechanisms