A high yielding chemical synthesis of sialyl Lewis x tetrasaccharide and Lewis x trisaccharide; Examples of regio- and stereodifferentiated glycosylations

Virtually complete regioselective galactosylation of the diol acceptor p-me thoxyphenyl 6-O-benzyl-2-deoxy-2-tetrachlorophthalimido-beta-n-glucopyranos ide (8) with the donor phenyl 2,3,4-tri-O-acetyl-6-O-benzyl-1-thio-beta-D-g alactopyranoside (11) gave the lactosamine derivative 14, which was fucosyl ated with the donor 15 to give the Le(x) trisaccharide glycoside 2 after de protection. Regioselective sialylation of the partially protected Le(x) tri saccharide triol 24 with the sialyl donor 25 gave, after deprotection, the SLe(x) tetrasaccharide glycoside 1. The overall yields of 2 and 1 from the monosaccharide starting materials 8, 11, 15, and 25 were 56% and 29%, respe ctively. In contrast to the virtually complete regio- and stereoselective g alactosylation of 8, fucosylation with the benzyl-protected donor 15 gave t he corresponding 1-->3- and 1-->4-linked disaccharides in a ratio of 3.6:1 (highly stereo- but not regioselective glycosylation), whereas fucosylation with acetyl-protected donor 18 gave a 2.2:1 beta/alpha-mixture of 4-O-link ed disaccharides (highly regio- but not stereoselective glycosylation).