A-fucosylation

The efficiency of this approach could be demonstrated in a-fucosylation with donor 32a and the acceptors 19C and 33A-D (Table XXXIII). Thus, with the help of this inverse procedure, the versatile building blocks for syntheses of the Lea, Le, Ley, and H antigen determinants are readily accessible (176). Presumably, this procedure may become of general importance when reactive glycosylating agents are employed. Alternatively, the reactivity of the fucosyl donor could be decreased, as has been recently proven very successfully (177). 

Scheme 6.6 a-Fucosylation of a disaccharide acceptor with complete regio/stereoselctivity. 

An improved method for the synthesis of a-fucosyl (l- 3)-3-thio-glycoside (65 c) involved the nucleophilic displacement of a cyclic sulfamidate derived from allosamine (67). The regioselective opening of (67) with (30b) led to (65c) in good yield (Scheme 21) [51]. 

H. Kunz and C. Unverzagt, Protecting group-dependent stability of intersaccharide bonds— synthesis of a fucosyl-chitobiose glycopeptide, Angew. Chem. lnt. Ed. Engl. 27 1697 (1988). 

Kunz, H. and Unverzagt, C. (1988) Protecting-group-dependant stability of intersaccharide bonds-Synthesis of a fucosyl-chitobiose glycopeptide. Angew. Chem. Int. Ed. 27, 1697-1699. 

The efficacy of the in situ anomerisation procedure was demonstrated by the condensation of a fucosyl bromide with a glycosyl acceptor in the presence of tetra-n-butyl ammonium bromide to give a trisaccharide mainly as the a anomer (Scheme 4.6). 

So far, we have never observed, by n.m.r. spectroscopy, oligosaccharides or glycopeptides bearing Fuc and NeuAc both linked to the same N-acetyllactosamine branch this is in agreement with the biosynthetic principle of mutual exclusion existing in transferring (1— 3)-1 inked a-fucosyl and (2— 6)-linked a-sialyl groups to the same N-acetyl lactosamine branch, as formulated by Hill and coworkers.88... 

Svensson SCT, Thiem J. Purification of a-fucosidase by C-glycosylic affinity chromatography, and the enzymic synthesis of a-fucosyl disaccharides Carbohydr. Res. 1990 200 391-402. 

SCHEME 15.10 a-Fucosylation of serine and threonine building blocks with thioglycoside 95, which is protected with non-participating methoxybenzyl groups (Mpm), or with peracetylated 98 which, via anomerisation, gives the thermodynamically more stable a-fucosides 97 and 99. 

Peters, S, Lowary, T L, Hindsgaul, O, Meldal, M, Bock, K, Sohd-phase synthesis of a fucosylated glycopeptide of human factor IX with a fucose-a-(l-0)-serine linkage, J. Chem. Soc. Perkin Trans. 1, 3017-3022, 1995. 

Mer, G, Hietter, H, Lefevre, J-F, Stabilization of proteins by glycosylation examined by NMR analysis of a fucosylated proteinase inhibitor. Nature Struct. Biol., 3, 45-53, 1996. 

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