No description available.

Bochum, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Bochum, Univ., Diss., 2001. / Computerdatei im Fernzugriff.

Bochum, Universiẗat, Diss., 2001.

No description available.

The aim of our project is to build new catalysts for the asymmetric epoxidation of alkenes using ð-interactions as fundamental factors for the control of the facial selectivity. It was decided to employ cinchona alkaloid derivatives as the basic core of our catalysts. We envisage that the alkene would interact selectively with the aromatic rings of the catalyst to give the corresponding epoxide in good enantiomeric excess. Quinuclidine derived cataysts of simplified structures were synthesised to find the best conditions for the experiments using chiral cinchona derivatives. An important result to be taken into account in the development of the chiral catalysts was the influence of the counterion on the conversion rate. The triflate gave the highest epoxidation rates for trans-â-methylstyrene. The use of a mixture MeOH/ DMM/ H2O led to a two fold increase in reaction rate and is recommended to increase the van der Waals interactions between the aromatic rings of the catalyst and the alkene. New catalyst amd a new epoxidation protocol for the epoxidation of alkenes mediated by dioxiranes have been disscussed .Encouraging results have been obtained from this work.

Chapter I contains a brief introduction to the applications of dioxirane chemistry and outlines the mechanistic investigations carried out to date. Chapter 2 describes investigations into the dimethyldioxirane oxidation of para-substituted N,N-dimethylanilines in acetone. The N-oxides were found to be the only products. Relative rates were determined and compared with those of reactions with methyl iodide and other oxidants. The dimethyldioxirane reactions followed the Hammett relationship with a p value of -1.0. The reaction rates are strongly accelerated in the presence of water and the overall reaction mechanism is electrophilic in nature and does not involve free radical species or electron transfer. Chapter 3 looks at the regioselectivity of dimethyldioxirane when used to oxidise several polyfunctionalised nitrogenous drugs with a view to developing a system for use in oxidative degradation studies. The regioselectivity of dimethyldioxirane in the oxidation of polyhydroxy steroids, namely a series of bile acid methyl esters, is discussed in chapter 4. No evidence for preferential oxidation of axial over equatorial hydroxyls or vice versa was seen. Instead the least hindered hydroxyl at C3 was oxidised preferentially with some oxidation also occurring at C6 and C7. Hydroxyls at the sterically hindered C 12 were not oxidised. This provides further evidence for the proposed butterfly transition state. Finally, chapter 5 discusses the use of novel trifluoromethyl aryl ketones as promoters for Oxone®mediated epoxidations. 4-(trifluoroacetyl)benzoic acid was used succesfully and can be readily isolated for re-use by simple base extraction.