Abstract
Polysaccharides are Nature’s most abundant biomaterials essential for plant cell wall construction and energy storage. Seemingly minor structural differences result in entirely different functions: cellulose, a β (1–4) linked glucose polymer, forms fibrils that can support large trees, while amylose, an α (1–4) linked glucose polymer forms soft hollow fibers used for energy storage. A detailed understanding of polysaccharide structures requires pure materials that cannot be isolated from natural sources. Automated Glycan Assembly provides quick access to trans-linked glycans analogues of cellulose, but the stereoselective installation of multiple cis-glycosidic linkages present in amylose has not been possible to date. Here, we identify thioglycoside building blocks with different protecting group patterns that, in concert with temperature and solvent control, achieve excellent stereoselectivity during the synthesis of linear and branched α-glucan polymers with up to 20 cis-glycosidic linkages. The molecules prepared with the new method will serve as probes to understand the biosynthesis and the structure of α-glucans.
Highlights
Polysaccharides are built from monosaccharide units connected through glycosidic bonds with different regio- and stereochemistry
Thioglycosides with temporary fluorenylmethoxycarbonyl (Fmoc) protecting groups have proven as useful building blocks for Automated Glycan Assembly (AGA) (Table 1A).[40]
Fmoc protection to mask the C4 hydroxyl group required for chain elongation was selected
Summary
Polysaccharides are built from monosaccharide units connected through glycosidic bonds with different regio- and stereochemistry. 4) glucan oligosaccharides with high cis-selectivity and good isolated yield.[49] In search for building blocks that maximize cisselectivity and yield during AGA, a comprehensive evaluation of different C3 and C6 ester groups and their influence on α (1−4) glucan formation was undertaken (see Table S1). Higher cis-selectivity was obtained with the more sterically hindered monosaccharide-acceptor compared to isopropanol, in particular when building blocks with bulky pivaloyl (Piv) 17 (entry 3 vs 17), triphenylacetyl 25 (entry 7 vs 21) protecting groups or disubstituted thioglycosides 27−30 (entries 9−12 vs 23−26) were used. Groups had been used in the solution phase synthesis to introduce cis-linked glucose residues, and an α-glucan decamer was prepared recently.[50,56] Overall, selectivities exceeding 10:1 (α/β) were achieved when all factors favoring cis-glycoside formation were combined using building blocks 27−30 carrying 3,6-diesters for AGA. Our results match literature reports that six continuous α (1−4) glucose residues are needed to form the repeating unit of helical structure, which can assemble with iodine complex nicely.[60,61]
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