Chemoselective activation of nucleoside 3'-O-methylphosphonothioates with 1,3,5-triazinyl morpholinium salts.

Chemoselective and stereospecific O-activation of 2'-deoxynucleoside 3'-O-methylphosphonothioates 5 with N-methyl-N-4,6-dimethoxy-1,3,5-triazin-2-yl morpholinium salts results in formation with retention of configuration of 5'-O-DMT-2'-deoxynucleoside 3'-O-(4,6 dimethoxy-1,3,5-triazin-2-yl methylphosphonothioates (7). Active esters 7 are convenient intermediates for hydrolytic interconversion of RP-5 into SP-5 and can be used as monomers for stereoselective synthesis of dinucleoside (3',5')-methyl phosphonothioates.