Direct Stereocontrolled Synthesis of 3-Amino-3-deoxy-β-Mannopyranosides: Importance of the Nitrogen Protecting Group on Stereoselectivity

The highly stereocontrolled synthesis of the 3-amino-3-deoxy-β-mannopyranosides is achieved by means of thioglycoside donors protected with a 4,6-O-benzylidene or alkylidene acetal and a benzylidene imine group. Among the various nitrogen protecting groups investigated only the Schiff's base was found to give high β-selectivity. N-Phthalimido and N-acetamido protected donors were found to be highly α-selective, whereas 3-azido-3-deoxy glycosyl donors gave intermediate selectivity. The reasons for the protecting group dependency are discussed in terms of the change in the O2−C2−C3−N3 torsional interaction on conversion of the covalent glycosyl triflates to the transient oxacarbenium ions.