Compelling evidence for a stepwise mechanism of the alkaline cyclisation of uridine 3 '-phosphate esters

A Bronsted graph with a convex break at pK(a) (Lg) = 12.58 provides compelling evidence for an intermediate in the alkaline cyclisation of uridine 3'-phosphate esters. The transient pentacoordinated oxyphosphorane dianion intermediate collapses to reactant and cyclic uridine 2', 3'-monophosphate faster than it can pseudo-rotate and isomerise to the 2'-isomer.