ABSTRACT
As with some other completely unusual substrates, the N-acetylated b-glycosylamines of l-glucose (15) and l-xylose (16), b-galactosylation could be detected to give the disaccharide derivatives 17 and 18, respectively, in about 20% yield. It should be noted that the recognition of these acceptor substrates by the enzyme occurred to give the b,1-3-interglycosidically linked disaccharides 17 and 18 by use of b,1-4-galactosyltransferase (Fig. 13.3) as demonstrated by Nishida et al. (2000).In a more recent example Rech et al. (2011) demonstrated a combinatorial one-pot synthesis of poly-LacNAc glycans (20, 21) which play essential roles in carbohydrate-protein interactions and are of interest for, e.g., biomaterial surfaces (Fig. 13.4). By
employing two glycosyltransferases and uridine-diphospho-glucose 4′-epimerase LacNAc glycans up to six disaccharide (20, 21) units could be obtained for the first time starting from a LacNAc-linker-t-Boc (19) motive in high yields. 13.2.1.2 a,1-3/4-fucosyltransferaseThe concept of using non-natural donor substrates could be extended to several other glycosyltransferases. Taking as LacNAc acceptor glycoside the b-8-methoxycarbonyloctyl derivative 22 (Gokhale et al., 1990), transfer of the activated modified fucose components GDPl-Gal (23, R1= R2=OH), GDP-3d-l-Gal (24, R1=H, R2=OH), and GDP-3,6d2-l-Gal (GDP-3d-l-Fuc, 25, R1=R2=H) could be realized by Binch et al. (1998). Employing human milk a,1-3/4-fucosyltransferase, the corresponding modified Lewis X trisaccharide derivatives 26-28 could be isolated by Stangier et al. (1998) in 84-93% yield (Fig. 13.5). In a similar pathway sialyl Lewis X motives with a number of different fucose derivatives could be obtained by Wu and coworkers (2010).Human milk oligosaccharides, the gold standard for nourishment of early infants, play a key role in health and development (Bode, 2012; Boehm and Stahl, 2007). Gaining access to these oligosaccharides is of particular interest. Miyazaki et al. (2010) demonstrated the synthesis of the hexasaccharide lacto-N-difucohexaose I (33), which binds to Heliobacter pylori. Starting from lactose (29) the core structure, lacto-N-tetraose (30), was obtained employing b-1,3-GnT (UDP-a-d-GlcNAc) and b-1,3-galactosidase (Galb-oNP) (Fig. 13.6).
