A new stereocontrolled access to β-d-mannopyranosides and 2-acetamido-2-deoxy-β-d-mannopyranosides starting from β-d-galactopyranosides

A new stereocontrolled synthesis of β-d-mannopyranosides was defined relying on a high yielding sequence based on the following three key steps: (a) a stereospecific inversion at C-2 of β-d-galactopyranosides by an oxidation–reduction procedure; (b) a regiocontrolled formation of 4-deoxy-β-d-threo-hex-3-enopyranosides; (c) a regio- and stereocontrolled hydroboration–oxidation of the above enol ethers. The flexibility of this new method was demonstrated by its extension to the synthesis of 2-acetamido-2-deoxy-β-d-mannopyranosides and of an orthogonally protected β-d-mannopyranoside scaffold and, finally, by the transformation of lactose into the two biologically relevant disaccharides with primary structure β-d-Manp-(1→4)-d-Glc and β-d-ManNAcp-(1→4)-d-Glc.