Efficient synthesis of β-(1,6)-linked oligosaccharides through microwave-assisted glycosylation

A microwave-assisted glycosylation method was developed for efficient synthesis of oligosaccharides. Di-functional AB monomers, 2,3,4-tri-O-acetyl- α-d-galactopyranosyl bromide (3a) and 2,3,4-tri-O-acetyl-α-d- glucopyranosyl bromide (3b) were designed and synthesized as weakly reactive monomers to avoid unwanted glycosylation or degradation during preparation and storage. The glycosylations of these monomers gave low conversions and low molecular weight oligosaccharides at rt, reflux, and under low microwave energy irradiation. However, the glycosylation became very effective when high microwave energy was applied, giving 100% conversion and producing oligosaccharides with M_n = 4.76 kDa for 3a and M_n = 4.05 kDa for 3b. The acetylated oligosaccharides were further subjected to deprotection for structural analysis, which indicated the oligosaccharides contain predominantly linear β-(1,6)-glycosyl linkages.