The condensation of 2-bromo-5,6-dichlorobenzimidazole (I) with 1,2,3,5-tetra-O-acetyl-L-ribofuranose (II) by means of N,O-bis(trimethylsilyl)acetamide (BTSA) in refluxing acetonitrile gives 2-bromo-5,6-dichloro-1-(2,3,5-tri-O-acetyl-beta-L-ribofuranosyl) benzimidazole (III), which is then treated with an excess of isopropylamine in hot ethanol.