Synthesis of a tetrasaccharide fragment of mycobacterial arabinogalactan

The arabinogalactan of mycobacteria contains both monosaccharides in the furanose ring form, which are absent in mammals. We report here the first synthesis of the tetrasaccharide fragment alpha-D-Araf-(1-->5)-beta-D-Galf-(1-->5)-beta-D-Galf-(1-->6)-D-Galf, conveniently derivatized for further elongation. The strategy relied on the use of suitably substituted D-galactono-1,4-lactones as precursors for the galactofuranose units. Reduction of lactone tetrasaccharide 9 with disiamylborane afforded the tetrasaccharide synthon 1. The tetrasaccharide contains the linker unit of the arabinan to the galactan.