ADDITION-REACTIONS OF GLYCAL ESTERS - ACCESS TO GLYCOSYL DONORS OF KDO, D-GLYCERO-D-TALO-2-OCTULOSONIC AND D-GLYCERO-D-GALACTO-2-OCTULOSONIC ACID RESIDUES

Addition reactions of O-acetylated glycal esters of Kdo mono-, alpha-( 2-->8)- and alpha(2-->4)- linked Kdo disaccharide derivatives 1a-c wit h NIS in acetic acid afforded good yields of trans-diaxial as well as minor amounts of trans-diequatorial and cis-configured 2-O-acetyl-3-de oxy-3-iodo derivatives, which were efficiently reduced with Bu(3)SnH/A IBN to give the corresponding per-O-acetylated Kdo methyl ester deriva tives. Similar reactions of 1a with NBS or NCS furnished the trans-dia xial 2-O-acetyl-3-bromo-3-deoxy- as well as 3-chloro-3-deoxy derivativ es as the main products. Reaction of 1a with NBS in aqueous MeCN provi ded the 2,3-trans-bromohydrin derivative 11c, which upon treatment wit h DBU in MeCN gave the elimination product 11 and the alpha-2,3-anhydr o derivative 12 as a suitable donor of glycosides with D-glycero-D-tal o- or D-glycero-D-galacto configuration, respectively.