Chemical syntheses are reported for GDP-fucose (S), GDP-3-deoxy-fucose (6), and GDP-arabinose (7), the demethyl analog of 5. All three sugar nucleotides were found to act as donor substrates for an a(1+4) fucosyltransferase isolated from human milk when PDG~~(~+~)PDG~~NAC-O(CH~)~COOM~ (1) was used as the acceptor. The rate of transfer of sugar residues to 1 was measured using a coupled spectrophotometric assay and was found to be 100% (S), 2.3% (6), and 5.9% (7). The product Lea-active oligosaccharide analogs were identified by both an enzyme-linked immunosorbent assay (ELISA) and by 'H NMR spectroscopy.Key words: glycosyltransferase, oligosaccharide synthesis, sugar-nucleotide analog, ELISA assay, fucosyltransferase.UDAY B. GOKHALE, OLE HINDSGAUL et MONICA M. PALCIC. Can. J. Chem. 68, 1063(1990.On a rCalisC la synthkse du GDP-fucose (S), du GDP-3-desoxy-fuxose (6) et du GDP-arabinose (7) Introduction broad use in the study of carbohydrate binding by antibodies, The use of enzyme assisted oligosaccharide synthesis is gaining wide popularity since it allows the rapid and specific homologation of relatively small, readily chemically synthesized mono-and disaccharides into larger and morecomplex structures (1). This homologation is most frequently accomplished using specific glycosyltransferases that transfer sugars from the appropriate sugar-nucleotide donors, all of which are commercially available, to suitable oligosaccharide acceptors. Accessibility to the required mammalian glycosyltransferases, most of which must still be isolated from natural sources, has been the single major impediment to the general applicability of this combined chemical-enzymatic approach to oligosaccharide synthesis. The availability of the required enzymes is, however, rapidly increasing as three of these enzymes are now commercially available and at least six have recently been cloned (2).Analogs of the naturally occumng oligosaccharides, particularly deoxygenated oligosaccharides, are invaluable probes in studies on the molecular recognition of complex carbohydrates by protein combining sites (3, 4). The effect of specifically deoxygenating individual hydroxyl groups of oligosaccharide ligands, for example, provides direct evidence for the involvement of those hydroxyl groups in hydrogen bonding with the protein. While the utility of oligosaccharide analogs is generally recognized, the difficulty of preparing the required structures by multistep chemical synthesis remains a major obstacle to their