The present work deals with studies on the oxidation of three Rauwolfia alkaloids having yohimbane skeleton, namely, reserpine, rescinnamine and ajmalicine, in various organic solvents using different oxidizing agents, i.e nitric acid, N-bromosuccinimide and benzoquinone.
The oxidation of Rauwolfia alkaloids yirlds 3,4-dehydro products which have been identified as 3,4-dehydroreserpine, 3,4-dehydrorescinnamine and 3,4-dehydroajmalicine by chromatographic and spectroscopic techniques. The 3-dehydro products of these alkaloids have been determined spectrophotometrically at 390,388 and 354 nm respectively in all the solvents and the data subjected to kinetic analysis. The apparent first-order rate constants for the reactions at various concentrations of the oxidizing agents have been determined and the second-order (bimolecular) rate constants (k2) for the reaction between the oxidizing species and the alkaloid have been calculated.
Using the kinetic data for the various reactions, an attempt has been made to correlate the rate of the reaction with the individual solvent parameters such as the dielectric constant, viscosity and pKa by plotting log k2 against 1/8, k2 against 1/viscosity and log k2 against pKa, respectively. For all the oxidatlion reactions involving the three alkaloids, log k2 is linear function of 1/8 in the acidic, neutral and alcoholic solvents. The second-order rate constants for the reactions vary from 0.444 dm3 mol-1s-1 to 120.909 dm3mol-1s-1 in these solvents. However, the magnitude of the rate constants in acidic solvents is about 5-10 fold higher than those of the alcoholic and neutral solvents for nitric acid induced oxidation of the alkaloids. This could probably be due to the reaction of the oxidizing agent with the protonated species in acidic media. The rates of oxidation are directly proportional to the viscosity of the medium in acidic solvents while inversely proportional to the viscosity in alcoholic and neutral solvents. The change in the slope (negative) in acidic solvents is difficult to explain on the basis of the anomalous behavior of the solvents with respect to the viscosity change. A decrease in the reaction rates in alcoholic solvents may result form the influence of hydrogen bonding in these solvents. Thelog k is also a linear function of the pKa of acidic solvents indicating therole of acidity function in determining the solvating power of the solvent and hence the reaction rates.
Reaction schemes for the oxidation of the alkaloidal species ae proposed. In acidic medium the reaction appears to proceed through a complex formation between the alkaloid and the oxidizing species while in the neutral and alcoholic solvents the reaction occurs through theinteraction between the neutral alkaloid and the oxidizing species. The kinetic data suggest the participation of a polar intermediate in the formation of the oxidation products.
The influence of the solvent on the oxidation of the Rauwolfia alkaloids may be rationalised on the basis of the cumulative effect of various solvent parameters including dielectric constant, viscosity and pKa, all of which may exert their influence to a variable extent depending upon the nature of the reactants and the products. Hydrogen bonding also appears to play a role in the reaction in alcoholic solvents.
Some synthetic studies on B-carboline alkaloids with special reference to yohimbane skeleton have also been carried out and presented in the last chapter of the thesis. The derivatives are based upon Vilsmeier and Friedal Craft’s reaction and their structures have been determined by various spectroscopic techniques.