Tetrasaccharide repeating unit block synthesis

Figure 2. Block synthesis of a tetrasaccharide repeating unit of the Y O-antigen.

Selective benzoylation (and allylation) of methyl (and benzyl 4-O-benzyl-a-L-rhamnopyranoside (4) has been studied by phase-transfer catalysis and by activation via the tributyl stannate. The former reaction gave the 2,4-dibenzyl ether (5) in high yield, whereas the latter process gave mainly the 3,4-diether (6). An alternative means of obtaining the 2,4-diether (5) involved selective tritylation to give (7) followed by benzylation and removal of the trityl group. These dibenzyl ethers have been employed in the synthesis of the tetrasaccharide repeating unit of the immunodeterminant of Klebsiella serotype K36 polysaccharide. Similar results have been recorded for the selective benzylation of other 4,6-blocked a-mannosides. The syntheses of 3-O-benzyl-D-ribose, 2-0-benzyl-D-ribose, and l,6-anhydro-2,3,2, 4-tetra-0-benzyl-jS-lactose have been recorded. The latter was employed in the synthesis of lacto-A -tetraose. ... 

---