Synthesis of tri-, penta-, and heptasaccharides, functionalized with orthogonally N-protected amino residues at the reducing and non-reducing ends

The synthesis of four bifunctionalized orthogonally N-protected oligosaccharides derived from lactose and mannose, intended as cross-linking derivatives, is described. The aminosugar at the non-reducing end is derivatized with an N-Boc-protected glycine moiety, and further connected to either a mannose (1→6) disaccharide or (1→3) lactose units (one, two or three) resulting in tri-, penta-, or heptasaccharides. All of the synthesized oligosaccharides have an N-benzyloxycarbonyl-aminoethyl residue at the reducing end. The orthogonal N-Boc/N-Cbz protection group pattern enables further conjugation/derivatization and results in a hydrophilic cross-linking molecule. It was found that the order of the final synthetic steps was crucial to avoid acyl migration. A suitable amide coupling protocol has been applied to introduce the N-Boc-protected glycine moiety in alcoholic solvent. The synthesized oligosaccharides will provide a model system to investigate the influence of length, structure and flexibility.