Chemoselective synthesis of oligosaccharides of 2-deoxy-2-aminosugars.

Along with the application of the S-benzoxazolyl glycosides to the high-yielding synthesis of disaccharides of the 2-amino-2-deoxy series, chemoselective assembly of oligosaccharides containing multiple residues of 2-amino-2-deoxyglycoses is reported. This modified armed-disarmed approach is relying on the observation that 2-N-trichloroethoxycarbonyl derivatives of S-benzoxazolyl glycosides are significantly more reactive than their 2-N-phthaloyl counterparts in MeOTf-promoted glycosylations. This allowed efficient chemoselective synthesis of 1,2-trans-linked oligosaccharides, the disarmed reducing end of which can be activated for immediate second step glycosidation in the presence of a more powerful activator, AgOTf. As a result of this two-step activation, trans-trans-patterned trisaccharides could be assembled in a highly efficient manner. This result differs from the classic armed-disarmed technique, according to which usually cis-trans-patterned oligosaccharides are generated.