5-Azido-5-deoxy-1,2-O-isopropylidene-α-D-xylofuranose was used to obtain two reversed nucleoside analogues with either the 5-aminoimidazol-4-carboxamide or 5-amino-1,2,3-triazol-4-carboxamide groups attached, through the N1 site, to the C5′ site of the sugar. When deprotected these two compounds cyclised spontaneously and regiospecifically to form a bond between the exocyclic nitrogen and the anomeric carbon of the sugar. These reversed cyclonucleoside analogues are respectively members of the 2,5-epoxyimidazo[1,5-a][1,3]diazocine and 5,8-epoxy[1,2,3]triazolo[1,5-a][1,3]diazocine systems, novel ring systems with therapeutic potential. The shape of the cyclised imidazole compound and its immediate precursor has been studied by molecular modelling.