Abstract
The synthesis of d- and l-glycero-α-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/trifluoromethanesulfonic anhydride couple, both the d- and l-glycero series exhibit excellent β-selectivity with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford β-selectivity. With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent β-selectivity was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobutyronitrile brought about clean fragmentation to the 6-deoxy-glycero-β-d-manno-heptopyranosides. This chemistry was applied to the stereocontrolled synthesis of methyl α-l-rhamno-pyranosyl-(1→3)-d-glycero-β-d-manno-heptopyranosyl-(1→3)-6-deoxy-glycero-β-d-manno-heptopyranosyl-(1→4)-α-l-rhamno-pyranoside, a component of the lipopolysaccharide from Plesimonas shigelloides.

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