Octasaccharide

In the crucial step of the synthesis (Scheme 16), the disaccharide glycosyl donor 17 was used once again, in condensation with the acceptor 19 having two free OH groups, to produce the octasaccharide 20. This product was then converted in 7 steps via the 1-trichloroacetamido compound, analogous to the synthesis described in Scheme 15, into the final compound 21. 

Mutant YE-2 of Rhizobium meliloti excretes a mixture of soluble polysaccharides that include a complex succinoglycan having a branched octasaccharide repeat as well as a simple galactoglucan (22) having a linear disaccharide repeat.102 In contrast to the case of the succinoglycan, oriented fibers of the potassium salt of 22 have yielded good X-ray data and its three-dimensional structure has been established.39 The polymer forms a two-fold helix of pitch... 

Whether the chemical repeating motif is a simple disaccharide or up to a complex octasaccharide, the molecular morphologies of branched polysaccharides are quite intriguing. The structure of welan (43) is a pleasant surprise. Others... 

Hakomori methylation analysis, and preparation and GLC gf the PAAN derivatives (4), were used to determine the linkages. C NMR of partially hydrolysed samples confirmed the octasaccharide repeat unit, the beta linkages and the presence of the carboxylic acids. 

Seifert, J. Unverzagt, C., Synthesis of a core-fucosylated, biantetmary octasaccharide as a precursor for glycopeptides of complex N-glycans. Tetrahedron Lett. 1996,37, 6527-6530. 

Fig. 3. Top Resolution-enhanced 600 MHz H NMR spectrum of the octasaccharide repeating unit from Hafnia alvei strain 2 O-specific polysaccharide, (a)-(g) 500 MHz ID TOCSY subspectra with selective excitation of the respective anomeric proton resonances, (h) ID TOCSY subspectrum with the H3eq resonance of the neuraminic acid residue selectively excited. (Reproduced (adapted) with permission from Gamian et al. [25J. Copyright 1991...

Mannose tri-, tetra- and pentasaccharide together with GM1 octasaccharide were a gift from Dr. M.A. Chester (Lund, Sweden). At the time of writing, these standards are no longer available. Pentasaccharides (from human urine) and sialylated tetra-saccharides (from human milk) are available from Sigma however, they are cost-prohibitive for routine use. 

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