Linear synthesis of the hexasaccharide related to the repeating unit of the O-antigen from Shigella flexneri serotype 1d (I: 7,8)

• Use of H2SO4-silica as the promoter for NIS-mediated activation of thioglycosides.
• Rational use of chloroacetate group for temporary protection.
• Considerable improvement for the total synthesis of the target oligosaccharide.

Total synthesis of the hexasaccharide repeating unit of the O-antigen from Shigella flexneri serotype 1d (I: 7,8), α-D-Glcp-(1→3)-α-L-Rhap-(1→2)-α-L-Rhap-(1→3)-α-L-Rhap-(1→3)-[α-D-Glcp-(1→4)]-β-D-GlcpNAc, is reported by following a linear strategy. The target hexasaccharide was synthesized by sequential glycosylations of suitably protected monosaccharide derivatives prepared from commercially available monosaccharides through rational protecting group manipulations. Stereoselective glycosylations were accomplished by the activation of thioglycoside using N-iodosuccinimide and H2SO4-silica. The use of H2SO4-silica in place of traditional promoters like TfOH or TMSOTf was proved to be a better option for the NIS-mediated thioglycoside activation.

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