Synthesis of a small library of linear and branched beta-(1-3)-D-glucans; Octasaccharide to hexadecasaccharide
Description
This research project focused on the synthesis of a small library of linear and branched (1→3-beta-D-. beta-Glucans, isolated from various microorganisms, are polysaccharides composed of anhydro-D-glucose units joined together by beta-glycosidic linkages. In addition to linear beta-(1→3)-linkages, glucans can also be branched with side chains of varying length via beta-(1→6)-glycosidic bond along the backbone. beta-Glucans are known as biological response modifiers (BRMs) and they have attracted great attention in recent years as non-specific immune stimulants with low toxicity. However, the results of physiological studies can sometimes give contradictory results mainly due to purification difficulties and the lack of structurally well-defined, pure glucans. Therefore, a synthetic strategy for the production of structurally well-defined glucans is still in great need Recently, we utilized the C-2 acyl protecting group 4-acetoxy-2,2-dimethylbutanoate (ADMB), as the neighboring participation group in making beta-glucosyl linkages with high diastereoselectivity. Using a stepwise synthetic strategy with a disaccharide acceptor as the reducing terminus, reiterative glycosylation with a disaccharide donor (linear or branched with C-2 ADMB ester) and selective deprotection of the C-3 silyl ether, we have successfully prepared a small library of linear and branched glucans. During the course of this work, we have obtained a single crystal of a fully protected linear hexasaccharide. By the X-ray structure of the hexasaccharide, we are able to explain the abnormalities of the 1H NMR. Also, we have employed a convergent block-wise approach to synthesize a linear hexadecasaccharide by using an 8 + 8 glycosylation