Chemical synthesis of a tetrasaccharide related to the repeating unit of the O-polysaccharide isolated from Salmonella enterica O59

The chemical synthesis of the tetrasachharide repeating unit of the O-polysaccharide of Salmonella enterica O59 in the form of its p-methoxyphenyl glycoside is reported. The total synthesis of the tetrasaccharide has been achieved through concise protecting group manipulations and stereoselective glycosylations via thioglycoside activation using La(OTf)3 as a Lewis acid promoter in conjunction with N-iodosuccinimide. La(OTf)3 was found to be a better alternative to the more traditional Lewis acid promoters such as TfOH or TMSOTf.

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p-Methoxyphenyl 4-O-acetyl-2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-N-phthalimido-β-d-glucopyranosideC39H41O13N[α]D25=+127 (c 1.0, CHCl3)Source of chirality: d-glucosamine, l-rhamnose

p-Methoxyphenyl 2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-N-phthalimido-β-d-glucopyranosideC37H39O12N[α]D25=+132 (c 1.1, CHCl3)Source of chirality: d-glucosamine, l-rhamnose

p-Methoxyphenyl 3-O-acetyl-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranosyl-(1→4)-2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-N-phthalimido-β-d-glucopyranosideC52H54O17N4[α]D25=+142 (c 1.0, CHCl3)Source of chirality: d-glucosamine, l-rhamnose

p-Methoxyphenyl 2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranosyl-(1→4)-2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-2-deoxy-N-phthalimido-4,6-O-benzylidene-β-d-glucopyranosideC50H52O16N4[α]D25=+148 (c 1.0, CHCl3)Source of chirality: d-glucosamine, l-rhamnose

p-Methoxyphenyl 2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranosyl-(1→4)-2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-N-phthalimido-β-d-glucopyranosideC64H70O25N4[α]D25=+103 (c 0.9, CHCl3)Source of chirality: d-glucosamine, l-rhamnose, d-galactose

p-Methoxyphenyl 2,3,4,6-tetra-O-acetyl-β-d-galactopyranosyl-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranosyl-(1→4)-2,3-O-isopropylidene-α-l-rhamnopyranosyl-(1→3)-2-acetamido-4,6-O-benzylidene-2-deoxy-β-d-glucopyranosideC58H70O24N4[α]D25=+117 (c 0.9, CHCl3)Source of chirality: d-glucosamine, l-rhamnose, d-galactose

p-Methoxyphenyl β-d-galactopyranosyl-(1→3)-2-acetamido-2-deoxy-α-d-glucopyranosyl-(1→4)-α-l-rhamnopyranosyl-(1→3)-2-acetamido-2-deoxy-β-d-glucopyranosideC35H54O21N2[α]D25=+81 (c 0.8, H2O)Source of chirality: d-glucosamine, l-rhamnose, d-galactose