Chemoselective deprotection of alpha-indole and imidazole ribonucleosides.

A series of 2 ',3 '-isopropylidene and 5 '-trityl-protected alpha-indole and alpha/beta-benzimidazole and imidazole ribonucleosides were deprotected with different acids. Selectivity was achieved for 5 '-versus 2 ',3 '- deprotection by using formic acid in the alpha-indole ribonucleoside series. Treatment of alpha-indole ribonucleosides with a mixture of formic acid and ether at room temperature afforded 2 ',3 '-deprotected alpha-ribonucleosides, whereas treatment of the alpha-benzimidazole ribonucleosides with the same acid afforded the 5 '-deprotected ribonucleoside without any 2 ', 3 '-deprotected products. The structures of these ribonucleosides were elucidated with 2D (NOESY, COSY, and HMQC) NMR spectroscopy.