Leading Question The Use of 1,2-epoxide derivatives of glycals in glycoside synthesis The use of 1,2-anhydrosugars as donors of glycosyl groups in glycoside synthesis has been studied for quite some time. The viability of this epoxide approach to creating polysaccharides was established with the discovery that a variety of glycols react with smoothly with 3,3-dimethyldioxirane (DMDO). This reaction generates a 1,2-epoxide glycal derivative that can then react with glycosyl acceptors. When the epoxide reacts with an acceptor such as a glucal, the product is a disaccharide in high yield. After protection of the newly generated hydroxyl group, the epoxidation and glycosylation procedure can be reiterated to create a trisaccharide. The best glycosyl donors are derivatives of 4,6-benzylidene glycals, particularly because of the high stereoselectivity they provide. In such a way, 1,2-epoxides can be used to construct β-glycosides. Construction of an α-glycoside form a 1,2-epoxides requires a different synthetic process. Reaction of a 1,2-epoxide with stannylether, promoted by AgBF4, followed by acetylation, leads to the α-glycoside. A potentially significant advantage of the epoxide coupling reaction is the fact that the reaction affords a unique, unprotected 2’-hydroxyl, which can be exploited in a number of ways. For example, the unprotected hydroxyl can be used in the synthesis of branched sugars. |
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