The present invention relates to a novel method of chemically preparing functionalized derivatives of -(1,3)-glucans, enabling obtaining oligosaccharides which are free or which comprise specific groups such as, for example, sulphate, phosphate, methyl, in pre-defined positions. This method includes a reaction between a glycosyl donor of formula (Ia) or (Ib) and a glycosyl acceptor of formula (II), defined in the claims. Application: Preparation of biologically active compounds which can be used in the agricultural, cosmetic or pharmaceutical fields.