The reaction of aspirin with base

Aspirin anion appears to exist only fleetingly, rearranging via acetyl transfer to the ortho carboxylate group, as indicated by IR, UV and NMR. The resulting mixed anhydride cyclises to the more stable bicyclic orthoacetate isomer, a process facilitated by time and increasing pH. Mechanistic possibilities are discussed to explain these intriguing observations.

Deprotonation of aspirin leads to a mixture of mixed anhydride (II) and cyclic orthoacetate (III), as indicated by IR, UV and NMR. Aspirin anion(I) itself appears to have only a fleeting existence.