New Facile Tandem Route to Oxo- and Thioxo[1,2,4]triazolo[1 ,5-a]pyridinium Salts.

2-Arylsulfanyl- and benzylsulfanylpyridinium N-arylimides (2), easily available from tetrazolo[1,5-b]-pyridinium salts (1), participate in 1,3-dipolar cycloaddition with aryl isothiocyanates and aryl isocyanates to result in formation of fused thioxo- and oxo[1,2,4]triazolium salts (5 and 12), respectively. This transformation is interpreted as a regular 1,3-cycloaddition followed by spontaneous elimination of the aryl- or benzylsulfanyl group. Formation of these triazolium salts can be followed-under appropriate reaction conditions-by ring-opening reactions to afford some new triazolyldienes (6). Recognition of the intermediate participation of the thiolate anion along the pathway 1 - 5 allowed elaboration of a simple procedure to 5 implying a tandem reaction sequence. [ABSTRACT FROM AUTHOR]