Triazolopteridines. I. Simple s-Triazolo[4,3-c]pteridines and their derived [1,5-c] isomers

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Abstract
The reaction of appropriate pteridin-4-ylhydrazines with orthoesters was used to prepare s-triazolo[4,3-c]pteridine and some C-methylated derivatives. Most of these underwent a Dimroth-like rearrangment to yield the isomeric s-triazolo[1,5-c]pteridines but the 3-ethyl derivative gave an unusually stable intermediate, N-[3(5''-ethyl-1'',2'',4''-triazol-3''-yl)pyrazin-2-yl]-formamide, which resisted re-closure. To prepare the parent rearranged heterocycle independently, N-acetoxy-N''-(pteridin-4-yl)formamidine failed to cyclize: instead, it lost acetic acid to give pteridin-4-ylcyanamide. The main systems are easily distinguished by their characteristic NMR spectra but not by UV spectra.