New examples of specific-base catalysis in monunuclear rearrangements of heterocycles found via a designed modification of the side-chain structure

To select suitable candidates for the occurrence of specific-base-catalysis in MRH of (Z)-hydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole we have designed a modification of the side-chain structure by linking a strong electron-withdrawing system to the hydrazono group. Thus we have synthesized the (Z)-semicarbazone (3d), the (Z)-phenylsemicarbazone (3e), and the (Z)-acetylhydrazone (3f) of the above oxadiazole and examined their kinetic behavior in dioxane/water in a large range of proton concentrations (pS(+) 4.0-14.5). In all the pS(+) range examined only a base-catalyzed process has been evidenced (no uncatalysed path occurs). The behavior at the largest pS(+) values (the reactivity tends to a limiting rate constant) and that at variable concentrations of buffer have furnished clear evidence for the occurrence of a specific-base-catalysed process, thus reaching the target proposed and confirming the prevision