Indolizines, triazolo[4,3-a]pyridines, benzimidazo[1,2-d]oxadiazoles, and pyrazolo[1,5-c]triazoles via nitrogen and sulfur ylides

The pyridinium salts 2a,b reacted with dimethyl acetylenedicarboxylate (DMAD) to give the indolizine derivatives 6a,b. Pyridinium salts 2a,b also reacted with pyrazole-5-diazonium salt to afford the hydrazonoyl bromides 8a,b, which on treatment with aqueous ethanolic sodium carbonate furnished the 8aH-1,2,4-triazolo[4,3-a]pyridine 10. When sulfonium bromide 11 was treated with nitrous acid and with pyrazole-5-diazonium salt, it afforded the new hydroximoyl and hydrazonoyl halides 12 and 17, respectively. Compound 12 reacted with 2-methylthiobenzimidazole to furnish benzimidazo[1,2-d]-1,2,4-oxadiazole derivative 14. Treatment of either 12 with 3-phenyl-5-aminopyrazole or 17 with triethylamine resulted in the formation of the same product: pyrazolo[1,5-c]-1,2,4-triazole derivative 16. (C) 2004 Wiley Periodicals, Inc.