Unexpected loss of stereoselectivity in ring-opening reaction of 2-alkoxy-2-thio-1,3,2-oxathiaphospholanes with a pyrophosphate anion.

A reaction of DBU promoted ring opening in nucleoside-3'-O- and nucleoside-5'-O-(2-thio-4,4-pentamethylene-1,3,2-oxathiaphospholane) monomers with a pyrophosphate or a methylenediphosphonate anion proceeds with substantial loss of stereoselectivity. Depending on the absolute configuration of the phosphorus atom, so far widely accepted the stereoretentive mechanism of condensation is accompanied by a stereoinvertive one, most likely employing an intramolecular ligand-ligand exchange in an uncharged intermediate.