Di- and tri-saccharide glycosyl donors for the synthesis of fragments of the O-specific antigen of Shigella dysenteriae type 1.

Methyl O -(2,4-di- O -benzoyl-3- O -bromoacetyl-α- l -rhamnopyranosyl)-(1 → 3)-2,4-di- O -benzoyl-α- l -rhamnopyranoside was treated with dichloromethyl methyl ether and ZnCl 2 to give O -(2,4-di- O -benzoyl-3- O -bromoacetyl-α- l -rhamnopyranosyl)-(1 → 3)-2,4-di- O -benzoyl-α- l -rhamnopyranosyl chloride. Similar treatment of methyl O -(3,4-,6-tri- O -acetyl-2-azido-2-deoxy-α- d -glycopyranosyl)-(1 → →3)-2,4-di- O -benzoyl-α- l -rhamnopyranoside ( 13 ) gave crystalline O -(3,4,6-tri- O -acetyl-2-azido-2-deoxy-α- d -glucopyranosyl)-(1 → 3)-2,4-di- O -benzoyl-α- l -rhamnopyranosyl chloride ( 14 ), which was also obtained by treatment of methyl O -(3,4,6-tri- O -acetyl-2-azido-2-deoxoy-α- d -glucopyranosyl)-(1 → 3)-2,4-di- O -benzoyl-1-thio-α, l -rhamnopyranoside ( 12 ) with chlorine. In contrast to the conversion 12 → 14 , which was stereospecific, the reaction of methyl O -(3,4,6-tri- O -acetyl-2-azido-2-deoxy-α- d -glucopyranosyl)-(1 → 3)-( O -2,4-di- O -benzoyl-α- l -rhamnopyranosyl)-(1 → 3)-2,4-di- O -benzoyl-1-thio-α- l -rhamnopyranoside with chlorine gave a mixture of the corresponding α-( 16 ) and β-( 17 ) glycosyl chlorides. Condensation of the mixed chlorides 16 and 17 with 1,3,4,6-tetra- O -acetyl-α- d -galactopyranose, followed by reduction-acetylation of the product, gave a fully protected derivative of the tetrasaccharide α- d -Glc p NAc-(1 → 3)-α- l -Rha p -(1 → 3)-α- l -Rha p -(1 → 2)-α- d -Gal p .