A new and efficient approach to the synthesis of 6-amidino-2-oxopurines

The reaction of 5-amino-4-cyanoformimidoylimidazoles 1a and 1b with tosyl i socyanate proved to be a mild and efficient method for the synthesis of the corresponding 6-amidino-2-oxopurines 5. These compounds, which were isolat ed in almost quantitative yield, rearrange in the presence of acetic acid-D MF to give a pyrimido[5,4-d]pyrimidin-2-one 6. The structure of compound 6 was confirmed by X-ray crystallography. The pathway for both reactions is d iscussed. Studies on the reactivity of tosyl isocyanate with imidazoles 2, 7, 8 and 16, obtained from 1 by selective acylation of the amino or imino n itrogen atoms, enabled clarification of the mechanism for purine formation.