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

Abstract

The reaction of appropriate pteridin-4-ylhydrazines (3) with orthoesters has been used to prepare s-triazolo[4,3-c]pteridine (la) and some C-methylated derivatives. Most of these underwent a Dimroth-like rearrangment to yield the isomeric s-triazolo[1,5-c]pteridines (2) but the 3-ethyl derivative(lc) gave an unusually stable intermediate, N-[3-(5´-ethyl-1´,2´,4´-triazol-3´-yl)pyrazin-2-yl]- formamide (4a), which resisted re-closure. In an attempt to prepare the parent rearranged heterocycle (2a) independently, the last intermediate, N-acetoxy-N´-(pteridin-4-yl)formamidine (6d), failed to cyclize: instead, it lost acetic acid to give pteridin-4-ylcyanamide (6e). The main systems (1) and (2)are easily distinguished by their characteristic n.m.r. spectra but not by u.v. spectra.