SYNTHESIS OF TRISACCHARIDE AND TETRASACCHARIDE FRAGMENTS OF THE SHIGELLA-DYSENTERIAE TYPE-1 O-ANTIGEN DEOXYGENATED AND FLUORINATED AT POSITION-3 OF THE METHYL ALPHA-D-GALACTOPYRANOSIDE TERMINUS

The blockwise synthesis of methyl alpha tri- and tetrasaccharide analo gs of the biochemical repeating unit of the Shigella dysenteriae type 1 O-polysaccharide is described. Modifications include deoxygenation a nd deoxyfluorination at position 3 of the galactopyranoside residue. M ethyl -O-benzylidene-3-deoxy-alpha-D-xylo-hexopyranoside (8) and methy l ylidene-3-deoxy-3-fluoro-alpha-D-galactopyranoside (9) were condense d with )-(1-->3)-2,4-di-O-benzoyl-alpha-L-rhamnopyranosyl chloride to give, after deprotection, the target trisaccharide methyl anosyl-(1--> 2)-3-deoxy-alpha-D-xylo-hexopyranoside and the corresponding fluorinat ed oligosaccharide. For the tetrasaccharide synthesis, the glycosyl ac cepters 8 and 9 were condensed with the temporarily protected )-(1-->3 )-2,4-di-O-benzoyl-alpha-L-rhamnopyranosyl chloride. Removal of the ch loroacetyl group was followed by condensation of the resulting selecti vely deblocked trisaccharides with ri-O-acetyl-2-azido-2-deoxy-alpha-D -glucopyranosyl chloride. Reduction and deprotection then gave the fre e methyl 2-acetamido-2-deoxy-alpha-D-glucopyranosyl-(1-->3) anosyl-(1- ->2)-3-deoxy-alpha-D-xylo-hexopyranoside and the fluorinated analog. ( C) 1998 Elsevier Science Ltd. All rights reserved.