Abstract
A stereoselective synthesis of a dimer, β- D-Fru f-(2→2)-α- D-Gal p-(1-PO 3H-3)-[β- D-Fru f-(2→2)]- D-Gal p, of the repeating unit of the K52 type CPS of E. coli is described. The β-fructofuranosyl residue was introduced in a DMTST-promoted coupling using a 1,4-TIPS-bridged thiofructofuranoside donor and a 2-OH TMSE galactoside acceptor affording exclusively the β-linked disaccharide. Protecting group manipulation of this disaccharide yielded both a 3-OH acceptor and a reducing disaccharide, which were linked via a phosphate diester bridge using H-phosphonate chemistry. The hemiacetal is present only as the α-anomer, why phosphonylation yielded stereoselectively the α-H-phosphonate monoester. Activation of the latter with pivaloyl chloride in the presence of the 3-OH disaccharide acceptor and subsequent I 2-oxidation gave the target dimer in good yield.
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