Regioselective Glycodesilylation of Silylated Glycosides as a Useful Tool for the Preparation of Oligosaccharides

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Abstract
A series of fully or partially protected alkyl and aryl 4,6-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-d-glycopyranosides and 1-thio-β-d-glucopyranosides, respectively, were glycosylated by acetylated α-d-glucopyranosyl-, galactopyranosyl- and cellobiosyl fluoride under Lewis acid-catalysis to give the corresponding β-(1→6)-linked di- and trisaccharides, respectively, in moderate to high yield. With benzylated glucopyranosyl fluoride, anomeric mixtures of disaccharides were obtained depending on the solvent that was used for the glycosylation step. The initially formed oligosaccharides having a 1-fluoro-1,1,3,3-tetraisopropyl-1,3-disiloxane-3-yl substituent at position 4 were converted by treatment with tetrabutylammonium fluoride into the corresponding 4-OH compounds which are suitable as glycosyl acceptors for further extension of the sugar chain. Selective glycodesilylation of methyl 2-O-benzoyl-6-O-dimethylthexylsilyl-3,4-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-α-d-glucopyranoside at position 6 without affecting positions 3 and 4 was possible under similar conditions. 4,6-O-(1,1,3,3-tetraisopropyl-1,3-disiloxane-1,3-diyl)-protected d-glycopyranosyl donors (fluoride, chloride, trichloroacetimidate and ethyl 1-thio-glycoside) of glucose and mannose were also prepared and used for the construction of the corresponding silylated disaccharides. Regioselective ring opening of the silylated glycosides by pyridine-polyhydrogen fluoride gave useful glycosyl acceptors that were subsequently coupled with 2,3,4,6-tetra-O-acetyl-α-d-glucopyranosyl trichloroacetimidate.

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