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. Chirality 27:155–122, 2015. © 2014 Wiley Periodicals, Inc