A Brønsted 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.