A facile synthesis of [1,2,4]triazino[3,2-f]purines and [1,2,4]triazepino[3,2-f]purines

1,2,4]Triazino3,2-f]purines 3a-e and 1,2,4]triazepino3,2-f]purine 5 were synthesized by the reaction of 7,8-diamino-1,3-dimethylxanthine 1 with diketones such as glyoxal, diacetyl, dibenzoyl, pyruvic aldehyde dimethyl acetal, phenylglyoxal or acetylacetone in acetic acid in the presence of boric acid or polyphosphoric acid.