Analysis of the kinetics of isomerization of spiro oxindole alkaloids

The isomerization of the spiro oxindole alkaloids mitraphylline, isomitraphylline, pteropodine, isopteropodine, speciophylline and uncarine F in water has been studied at several temperatures and the rate coefficients have been determined. The effect of pH on the rate of reaction and the equilibrium composition has been investigated, The rate coefficients in water and in organic solvents correlate satisfactorily with the Dimroth-Reichardt solvent polarity parameter, The present results support the existence of a zwitterionic intermediate stabilized by polar solvents and show that protonation of the alkaloids inhibits the isomerization, The crystal structure of pteropodine has been determined.