Abstract
The first chapter of this thesis addresses synthetic and mechanistic aspects of the (salen)Mn(III) complexes-catalyzed asymmetric epoxidation reaction. It was discovered that 4-phenyl pyridine N-oxide (4-PPNO) exerts a remarkable effect on the catalytic lifetimes of the (salen)Mn (III) catalysts. The additive effect was found to be especially dramatic in the epoxidation of electron deficient olefins. A mechanistic model is proposed to account for this effect. This discovery enabled us to expand the scope of the epoxidation to electron deficient olefins. An in-depth study of the (salen)Mn(III)-catalyzed asymmetric epoxidation of cis-cinnamate esters revealed that the steric properties of the ester group have a profound influence on enantioselectivity in the epoxidation reaction, with bulkier esters affording highest ee's. The sensitivity of the reaction selectivity to the steric properties of the cis-alkene was rationalized according to a "skewed" side-on approach of olefin to the metal-oxo intermediate. The synthetic applicability of the epoxidation of cinnamate esters was illustrated in the highly enantioselective synthesis of the taxol side chain and diltiazem.
The second chapter describes the development of a stereospecific and highly enantioselective epoxidation reaction using the readily available chloroperoxidase (CPO) from Caldariomyces fumago. The catalytic efficiency of CPO for the epoxidation was found to be improved dramatically by the slow addition of H$\sb2$O$\sb2$, the stoichiometric oxidant in the epoxidation, to the reaction mixture. The use of the co-solvent system of acetone/citrate buffer as reaction media also induced a significant improvement in the reactivity of CPO. An in-depth study of the substrate scope of the epoxidation revealed that an unusually broad range of substrates is accepted by CPO compared with previously reported enzymatic and microbial epoxidation methods. Most notably, excellent enantioselectivities have been obtained in epoxidations of several alkyl-substituted cis-disubstituted olefins, trisubstituted olefins and 1,1-disubstituted olefins.
The last chapter of the thesis describes the synthesis of a variety of chiral oxazolines from optically active cis- or trans-1-amino indan-2-ol. The oxazolines were examined for their effectiveness as ligands in several catalytic asymmetric reactions.