Regioselectivity in 1,5-electrocyclization of N-[as-triazin-3-yl]nitrilimines. Synthesis of s-triazolo[4,3-b]-as-triazin-7(8H)-ones

6-Benzyl-3-(arylmethylidenehydrazino)-as-triazin-5(4H)-ones underwent regioselective cyclization upon treatment with either bromine in acetic acid containing sodium acetate or with ferric chloride in refluxing ethanol to give the respective s-triazolo[4,3-b]-as-triazin-7(8H)-ones in overall good yields. The regioselectivity in the studied reactions was elucidated by comparison of 13C NMR spectra of and their methyl derivatives with those of their regioisomers prepared by independent methods.