KINETICS AND MECHANISM OF THE ALKALINE-HYDROLYSIS OF SECURININE

The hydroxide ion-catalyzed hydrolysis of securinine involves the ring opening of the lactone moiety. The rate of hydrolysis is insensitive to the ionic strength. The observed pseudo-first-order rate constants reveal a decrease of approximately 4-fold due to the increase in the MeCN content from 4 to 50% (v/v) in mixed aqueous solvent. The temperature dependence of the rate of hydrolysis follows the Eyring equation, which yields Delta H-* and Delta S-* as 11.0 kcal mol(-1) and -34.5 cal deg(-1) mol(-l), respectively. The hydroxy carboxylate product of the alkaline hydrolysis of securinine is shown to undergo cyclization in acidic medium to yield securinine. The observed pseudo-first-order rate constants for cyclization increase linearly with an increase in [H+]. The change in the content of MeCN from 3.8 to 47.2% (viv) in mixed aqueous solvents does not show an effect on the rate of the cyclization reaction. The most plausible mechanisms for alkaline hydrolysis and acid cyclization reactions are also discussed.