Synthesis and anomeric stability of (1→6)-thiourea-linked pseudooligosaccharides

The synthesis of oligosaccharide mimics incorporating a thiourea intersaccharide bridge by coupling of mono- and disaccharide per-O-acetylated glycosyl isothiocyanates and methyl 6-amino-6-deoxy-α-d-glucopyranoside, is reported. An unexpected anomerization reaction was observed, however, for α- and β-d-mannopyranosylthioureido derivatives upon Zemplén deacetylation. Kinetics studies revealed that this anomerization process is base catalysed and strongly dependent upon reaction temperature and reaction time. The fully unprotected compounds could be obtained without epimerization by performing the last step at 0 °C. In all cases, the final pseudooligosaccharides were stable in water solution at room temperature in the absence of base.