From Chondroitin Polymer to Size‐Defined Hyaluronan Oligosaccharides

Efficient and stereocontrolled synthesis of size‐defined hyaluronan oligomers is described for the first time starting from natural chondroitin polymer. Semisynthesis from chondroitin polymer afforded a disaccharide fragment that was used as starting material for the efficient preparation of a protected hyaluronic acid disaccharide building block. Selective inversion of configuration at C‐4 of the D ‐galactosamine in the chondroitin disaccharide was one of the key steps necessary to afford a hyaluronic disaccharide skeleton. Stereoselective glycosylation, efficient reduction of the N ‐trichloroacteyl group into the corresponding N ‐acetyl group and deprotection afforded hyaluronan oligomers in good yield.