Studies On New Methods For The Synthesis Of Oligosaccharides

Development of new strategies for the assembly of oligosaccharides remains a challenge, and has received much attention in recent years. In this thesis, we investigated the ionic liquid supported synthesis of oligosaccharides, the Polyethylene glycol (PEG)-supported synthesis of natural products phenylpropanoids, and the oxidation of glycosyl sulfides to glycosyl sulfoxides using metalloporphyrins as catalysts. The results are summarized as below.1. A novel and efficient ionic liquid supported synthesis of oligosaccharides with a general protocol of coupling and purification has been developed. Glucopyranoside was immobilized to N-methylimidazole to give the ionic liquid supported glucopyranoside, which was deprotected, glycosylated and then cleaved from the support to afford six oligosaccharides in high yields (80-85%) and excellent purities (90-95%). A trisaccharide was also obtained by repeating the deprotection-glycosylation-cleavage operation. Furthermore, we also developed a method for the synthesis of oligosaccharides using benzyl ether as the linker. Using these procedures, the intermediates could be purified by simple washing. Our method represents an attractive alternative to the classical solid-and fluorous-phase synthesis strategies and combines the advantage of performing homogeneous chemistry on a relatively large scale while avoiding of large excesses of reagents.2. Phenylpropanoid glycosides are an interesting group of natural products that exist widely in herbal plants, and most of them possess strong biological activities. Herein, we investigated the possibility for the solution-phase synthesis of natural glycosides Osmanthuside B6 and Lugrandoside on PEG-support. However, our effort failed because of the poor loading capacity.3. Glycosyl sulfides and glycosyl sulfoxides are useful synthetic intermediates or synthons. Herein, we synthesized six metalloporphyrins linked with stable nitroxyl radials, and evaluated their catalytic activity for the oxidation of glycosyl sulfides to glycosyl sulfoxides. It was found that manganese porphyrins linked with stable nitroxyl radials could oxidize sulfides using NaOCl as the oxidant to give the corresponding sulfoxides in good to excellent yields (80-97%) with high chemoselectivity and diasteroselectivity (dr≥7:1 for glycosyl sulfoxides).