Synthesis of sialyl-alpha-(2 -> 3)-neolactotetraose derivatives containing different sialic acids: Molecular probes for elucidation of substrate specificity of human alpha 1,3-fucosyltransferases

A series of sialyl-alpha-(2-->3)-neolactotetraose derivatives containing N-acetyl-(NeuAc), N-glycolyl- (NeuGc) and N-butanoylneuraminic acid, and 3-deoxy-D-glycero-D-galacto-2-nonulosonic acid (KDN) have systematically been synthesized as molecular probes for elucidation of substrate specificity of human alpha 1,3-fucosyltransferases (Fuc-TVII and Fuc-TVI). 2-Methyl-(3,4, 6-tri-O-acetyl-1,2-dideoxy-alpha-D-glucopylano)-[2',1' :4,5]-2-oxazoline (1) was coupled with 2-(trimethyl-silyl)-ethyl (2,4,6-tri-O-benzyl-beta-D-galactopyranosyl) -(1 --> 4)-2,3,6-tri-O-benzyl-beta-D-glucopyranoside (2) to give a trisaccharide 3 which, upon successive O-deacetylation, benzylidenation and reductive opening of the benzylidene group, afforded a common glycosyl acceptor 5. Glycosylation of 5 with sialyl-alpha-(2-->3)-galactose donors 6-8, 19 and 21 gave the corresponding pentasaccharides 22-25, which were converted to a series of sialyl-alpha-(2-->3)-neolactotetraose derivatives 30-33. In the competitive enzyme assay, the NeuGc derivative 32 showed the most potent activity for Fuc-TVII, while the KDN derivative 31 was less active than the standard NeuAc derivative 30. In contrast, the N-butanoylation of neuraminic acid enhanced the activity for Fuc-TVI.