Synthesis of S-linked oligoxylans

The transition from a petroleum-dependent economy to one based on sustainablebio-resources will largely be founded on plant cell walls, as these are the largestsource of biomass on earth. However, the development of lignocellulosic biomassconversion to fine chemicals and polymers still remains a big challenge for thebiofuel industry. In particular, the enzymatic hydrolysis of lignocellulosicpolysaccharides is one of the limiting steps of the entire procedure and thereforethe enzymes involved in the degradation process must ideally be characterized andunderstood. This requires a detailed understanding of cell wall polysaccharidecomposition and architecture. Hemicelluloses are the second most abundantpolymers in lignocellulosic biomass. They include different types ofpolysaccharides like xyloglucans, xylans, mannans, glucomannans and β-(1→3,1→4)-glucans. Xylans are heteropolymers possessing a β-(1→4)-Dxylopyranosebackbone, which is branched by short carbohydrate chains. Thebranches include D-glucuronic acid and its methyl ether, L-arabinose and/or variousoligosaccharides like D-xylose, L-arabinose, D- or L-galactose and D-glucose. Thehydrolysis of these polysaccharides is catalyzed by several families of enzymes,collected under the name of Glycosyl Hydrolases (GHs). Among other methods,the use of enzyme inhibitors like thio-linked oligosaccharides has for a long timebeen a common tool to analyze and characterize these enzymes.In the present work the chemical synthesis of thio-analogs of xylo- andarabinoxyloglycans is presented. Furthermore, the selection of a reliable method forthe incorporation of thiolinkages in the synthesis of oligoxylans is alsoinvestigated.Therefore, different strategies for assembling S-linked-disaccharides have beenapproached both involving 1-thioglycoside donors and thioacceptors. Advantagesand disadvantages concerning the different methods are described and evaluated inrelation to the synthesis of linear and branched oligoxylans.