Synthesis of 9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine bearing aselectively removable protecting group

A facile and practical method to introduce fluorine at the up-side of the 2 '-carbon of nucleosides is described. 6-Chloropurine riboside 3 was convert ed to the 3'-O-benzoate 4a via a stannylene complex, then converted to the 3'-O-benzoyl-5'-O-tritylriboside 5a, In the presence of pyridine, migration of the 3'-benzoyl groups of 4a and 5a to 2'-OH was rather slow. Hence, 5a was reacted with diethylaminosulfur trifluoride (DAST) in CH,CI, in the pre sence of pyridine to give the 2'-deoxy-2'-fluoroarabinoside 6 in good yield . The 3'-O-benzoyl-5'-O-trityl protecting system was easy to deprotect sele ctively, Thus, treatment of 6 with ammonia in MeOH gave the 5'-O-trityl com pound 7, which was subjected to esterification with phenyl chlorothionoform ate, radical deoxygenation with tris(trimethylsilyl)silane and acid treatme nt to afford 9-(2,3-dideoxy-2-fluoro-beta-D-threo-pento-furanosyl)adenine ( FddA) 2, In addition, acid treatment of 7 gave 9-(2-deoxy-2-fluoro-beta-D-a rabinofuranosyl)adenine (FdaraA) 1.