Abstract
M.Sc. (Chemistry)
The aim of this study was to investigate the oxidation of aromatic compounds and benzylidene acetals with dimethyldioxinne. Dimethyldioxirane, either by Insitu preparation or in the isolated form, has shown remarkable reactivity. This oxidant is effective in oxygen transfer, readily prepared from commercial materials and reacts under mild conditions.
Quinones are natural materials with biological activity and some are key intermediates in the synthesis of medicines. A well-known method for the preparation of quinones, is the oxidation of phenols and aromatic ethers. The oxidation of simple phenol and anisole derivatives by dimethyldioxirane yielded complex mixtures. More hindered aromatic compounds were investigated, since the oxidation of these compounds proceeded in a more controlled manner. In this study the phenols were found to be more reactive than the corresponding aromatic ethers.
para-Quinones were formed preferentially, whereas ortho-quinones were observed in one or two cases. Products formed in these oxidations depend on the substitution pattern of the substrate. The oxidation of phenols and aromatic compounds formed complex mixtures and does not have many synthetic applications. Mechanisms are proposed for the formation of the different products. Benzylidene acetals are important protecting groups in the chemistry of carbohydrates. Cleavage of these benzylidene acetals gave either the free diol or benzyl ether or benzoyl ester, depending on the reagent used. The reaction of dimethyldioxirane with benzylidene acetals gave the benzoylesters.
With dioxanes, e.g. 4,6-O-benzylidene sugar derivatives, there are no difference in stereoelectronic effects in the cleavage of C-4 and C-6. The major product has a 6-benzoate group, probably due to the greater stability of the primary benzoate. In dioxolanes both reactivity and regioselectivity of the reactions are influenced by stereoeleetronic effects. It has been found that the cleavage of dioxolanes took place preferably in a conformation where the oxygen is antiperiplanar towards the leaving group.
It has been shown that dimethyldioxinme can be used with success in the deprotection of benzylidene acetals in the chemistry of carbohydrates.