摘要:
                                Cyclodesulfurative annulations of 5-[1-(3-substituted-thioureido)]-1-(beta-D-ribofuranosyl)imidazole-4-carboxamides (3a-h) with N,N'-dicyclohexylacarbodiimide are described.  Isoguanosine (4) and several 1-substituted isoguanosines 6a-f,h are readily prepared by a ring closure of the appropriate resulting 5-(3-substituted-1-ureido)-1-(beta-D-ribofuranosyl)imididazole-4-carbonitriles 5a-h.  Treatment of the appropriate 1-substituted isoguanosines under basic conditions has furnished the corresponding 1-substituted xanthosine 12.