Synthesis of 3′,5′‐Cyclic Phosphate and Thiophosphate Esters of 2′‐ C ‐Methyl Ribonucleosides

2′‐ C ‐Methylnucleosides are known to exhibit antiviral activity against Hepatitis C virus. Since the inhibitory activity depends on their intracellular conversion to 5′‐triphosphates, dosing as appropriately protected 5′‐phosphates or 5′‐phosphorothioates appears attractive. For this purpose, four potential pro‐drugs of 2′‐ C ‐methylguanosine, i.e. , 3′,5′‐cyclic phosphorothioate of 2′‐ C ‐methylguanosine and 2′‐ C , O 6 ‐dimethylguanosine, 1 and 2 , respectively, the S ‐[(pivaloyloxy)methyl] ester of 2′‐ C , O 6 ‐dimethylguanosine 3′,5′‐cyclic phosphorothioate and the O ‐methyl ester of 2′‐ C , O 6 ‐dimethylguanosine 3′,5′‐cyclic phosphate, 3 and 4 , respectively, have been prepared.