Synthetic procedure to pyrido[2,1-f][1,2,4]triazinium salt and related compounds

N-aminopyridyl ketone salts were reacted with formamide to yield heteroaromatic pyrido[2,1-f][1,2,4]triazinium salts. Upon storage of these products in the presence of water, formation of covalent hydrates have been observed. Reaction of the same starting compound with urethane yielded 3-chloropyrido[2,1-f][1,2,4]triazinium salt which readily reacted with secondary amines to afford 3-amino derivatives. An analogous ring closure reaction of 2-formylaminomethyl- and formaminobenzylpyridine allowed the synthesis of the partially reduced 3,4-dihydropyrido[2,1-f][1,2,4]triazinium compounds. The cyclization procedure was also applied for the synthesis of the related pyrimido[2,1-f][1,2,4]triazinium salt.