The synthesis of 2-azainosine and related derivatives by ring annulation of imidazole nucleosides

The synthesis of 7-(β-D-ribofuranosyl)imidazo[4,5-d]-v-triazin-4-one ( 6b , 2-azainosine) and 5-(β-D-ribofuranosyl)imidazo[4,5-d]-v-triazin-4-one ( 4b ) have been achieved for the first time by direct diazotization of AICA riboside ( 5b ) and iso-AICA riboside ( 3b ), respectively. The conditions required for cyclization of the model methyl bases, 3a and 5a , as well as the nucleosides 3b , 5b , and 7 are described.