Process for the preparation of polysaccharides

The present invention relates to a process for the preparation of a polysaccharide composed of D-xylose units of formula (III) linked together via beta 1,4 glycosidic bonds wherein R1 is hydrogen or acetyl, R2 is hydrogen, acetyl or a 4-O-methyl glucuronic acid unit, wherein, when R2 is a 4-O-methyl glucuronic acid unit, the R1 group on the same saccharide unit is defined as G, wherein G is hydrogen or acetyl, and wherein the sugar unit at the reducing end of sun such polysaccharide is xylose, lyxose or xylulose, said process comprising the following steps: selective deacetylation of xylan extracted from beech wood; and isomerization of the selectively deacetylated xylan achieved in step or the following steps: isomerization of xylan extracted from beech wood; and selective deacetylation of isomerized xylan achieved in step. The process is useful for the preparation of pentosan polysulfate or pharmaceutically acceptable salts thereof for pharmaceutical use.