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-alpha-L-rhamnopyranosyl)-(1-->3)- 2,4-di-O-benzoyl-alpha-L-rhamnopyranoside was treated with dichloromethyl methyl ether and ZnCl2 to give O-(2,4-di-O-benzoyl-3-O-bromoacetyl-alpha-L-rhamnopyranosyl)-(1-->3)2,4- di-O- benzoyl-alpha-L-rhamnopyranosyl chloride. Similar treatment of methyl O-(3,4,6-tri-O-acetyl-2-azido-2-deoxy-alpha-D- glucopyranosyl)-(1-->3)-2,4-di-O-benzoyl-alpha-L-rhamnopyranoside (13) gave crystalline O-(3,4,6-tir-O-acetyl-2-azido-2-deoxy-alpha-D-glucopyranosyl)-(1-- >3)-2,4-di-O- benzoyl-alpha-L-rhamnopyranosyl chloride (14), which was also obtained by treatment of methyl O-(3,4,6-tri-O-acetyl-2-azido-2-deoxo-alpha-D- glucopyranosyl)-(1-->3)-2,4-di-O-benzoyl-1-thio-alpha-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-alpha-D-glucopyranosyl)-(1-->3)-(O-2,4-di-O-benzoyl-alpha-L- rhamnopyranosyl)-(1-->3)-2,4-di-O-benzoyl-1-thio-alpha-L-rhamnopyrano sid e with chlorine gave a mixture of the corresponding alpha- (16) and beta- (17) glycosyl chlorides. Condensation of the mixed chlorides 16 and 17 with 1,3,4,6-tetra-O-acetyl-alpha-D-galactopyranose, followed by reduction-acetylation of the product, gave a fully protected derivative of the tetrasaccharide alpha-D-Glc pNAc-(1-->3)-alpha-L-Rhap-(1-->3)-alpha-L-Rhap-(1-->2) -alpha-D-Galp.