Chitin, ap olymer composed of b(1-4)-linked Nacetyl-glucosamine monomers,a nd its partially deacetylated analogue chitosan,a re abundantb iopolymersw ith outstandingm echanical as well as elastic properties. Their degradation products,c hitooligosaccharides (COS), can trigger the innate immune response in humans and plants.B othm aterial and biological propertiesa re dependent on polymer length, acetylation,a sw ell as the pH. Withoutw ell-defined samples, ac omplete molecular descriptiono ft hese factors is still missing. Automated glycan assembly (AGA)e nabled rapid access to synthetic well-defined COS. Chitin-cellulose hybrid oligomers were prepared as importantt ools for as ystematic structural analysis. Intramolecular interactions, identified by molecular dynamics simulations and NMR analysis,u nderscore the importanceo ft he chitosana mino group for the stabilization of specific geometries.

The sequence, length and substitution of a polysaccharide influence its physical and biological properties. Thus, sequence controlled polysaccharides are important targets to establish structure–properties correlations. Polymerization techniques and enzymatic methods have been optimized to obtain samples with well-defined substitution patterns and narrow molecular weight distribution. Chemical synthesis has granted access to polysaccharides with full control over the length. Here, we review the progress towards the synthesis of well-defined polysaccharides. For each class of polysaccharides, we discuss the available synthetic approaches and their current limitations.

The first total synthesis of the branched oligosaccharide OSE-1 of Mycobacterium gordonae (strain 990) is reported. An intramolecular aglycon delivery approach was used for constructing the desymmetrized 1,1'-α,α-linked trehalose moiety. A [3+2] glycosylation of the trisaccharide donor and trehalose acceptor furnished the right hand side pentasaccharide. Regioselective O3 glycosylation of L-rhamnosyl 2,3-diol allowed expedient synthesis of the left hand side tetrasaccharide. The nonasaccharide was assembled in a highly convergent fashion through a [4+5] glycosylation.

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Total synthesis of three important trehalose containing tetrasaccharides isolated from Mycobacterium smegmatis is reported for the first time, using regioselective opening of benzylidene acetals and stereoselective glycosylations as key steps. The 1,2-cis stereoselectivity in the glycosylation reactions was achieved using anchimeric assistance from a remote participating group, steric effects and solvent participation. The synthetic strategy can also be utilized for the assembly of structurally related oligosaccharides from M. tuberculosis.