| Cholesterol plays a key role in membrane dynamics and regulating pathogenic processes. It has been shown to modulate glycolipid conformations, which is important for controlling the recognition processes of raft associated proteins and it can mask carbohydrates. Some bacteria like Helicobacter pylori utilize cholesterol by obtaining it from the host to make cholesteryl-containing glucosides that allow the pathogen to evade the immune system. H. pylori infects over 50% of the world's population and can lead to gastric ulcers and cancer if left untreated. Studying of these bacteria will help determine the interplay involved with cholesterol extrusion from the host, the production of cholesteryl glucosides, and their role in pathogenicity.;H. pylori produces three main cholesterol containing glucosides, (1) alpha-cholesteryl glucoside (alphaCG), (2) tetradecanoyl C-6 hydroxyl acylated-alpha-cholesteryl glucoside (alphaCAG) and (3) alpha-cholesteryl phosphatidyl glucoside (alphaCPG), which together make up 25% of the total lipid content in the bacterial membrane. Prior to these synthetic efforts, biological studies have relied mainly upon natural sources, which were mixtures of these glycolipids, making it difficult to determine the independent roles of each substituent. These synthetic studies have utilized glycosyl iodide chemistry to develop a stereoselective methodology for the synthesis of cholesteryl glucosides and analogues.;H. pylori glucosides and analogues can be generated efficiently using glycosyl iodides generated by reaction with iodotrimethyl silane (TMSI) and microwave glycosylation conditions. This study has provided the characterization of glycosyl-beta-iodides and use of these reactive species to afford alpha-linked glycosidated products with nucleophiles via presumably direct SN2-displacement of the beta-iodide. The new reaction pathway with cholesterol produces alphaCG, which then was enzymatically, regioselectively acylated at the C6 hydroxyl with a tetradecanoyl group by a lipase to afford alphaCAG quantitatively. The development of a convergent synthesis streamlined the synthesis of alphaCAG by use of the same lipase to install the tetradecanoyl group onto the C6-hydroxyl of free glucose, followed by the use of a one-pot glycosylation/deprotection procedure. These new synthetic procedures were applied to glucose, galactose, and lactose and have provided sufficient quantities of well-defined cholesteryl glycosides as standards for biological studies of H. pylori. These glycosides have provided a starting point for identifying other important cholesteryl glucosides and the enzymes used for their biosynthesis in H. pylori.. |
