Block synthesis of tetra- and hexasaccharides (beta-D-glycero-D-manno-Hep p-(1-->4)-[alpha-l-Rha p-(1-->3)-beta-D-glycero-D-manno-Hep p-(1-->4)]n-alpha-L-Rha p-OMe (n = 1 and 2)) corresponding to multiple repeat units of the glycan from the surface-layer glycoprotein from Bacillus thermoaerophilus.

A fully stereocontrolled block synthesis of the title tetra- and hexasaccharides has been achieved taking advantage of the ability of the 4,6-O-benzylidene acetal to control the stereochemistry of the beta-D-glycero-D-mannoheptopyranoside unit and of a 2,3-O-diphenylmethylene acetal to install the alpha-L-rhamnopyranosidic linkages. Comparison of the spectral data for the hexasaccharide with that of the natural isolate confirms the structure of this very unusual and structurally challenging glycan.