Mannose trisaccharide

N-Alkyl Derivatives. - Isomeric mixtures of AT-(hexosid-2-yl)-pyrrolidine derivatives, e.g. 98, were formed along with some AT-demethylated amine 99 on treatment of the AT-oxide 100 with lithium diisopropylamide in the presence of various dienes, e.g. 2-methyl-1,4-butadiene. The synthesis of carbocyclic analogues of a mannose trisaccharide, linked via nitrogen atoms, is covered in Chapter 18. 

Xanthan gum is a long-chain polysaccharide composed of the sugars glucose, mannose, and glucuronic acid. The backbone is similar to cellulose, with added side chains of trisaccharides (three sugars in a chain). 

A polysaccharide such as xanthan gum is a chain of sugars. Some familiar polysaccharides are starch and cellulose. The backbone of xanthan gum is similar to cellulose, but the trisaccharide side chains of mannose and glucuronic acid make the molecule rigid, and allow it to form a right-handed helix. These features make it interact with... 

Xanthan (Figure 11) is a commercially important polysaccharide produced by the bacterium Xanthomonas campestris.187 188 The xanthan backbone consists of a P(l-4)-linked D-glucopyranose chain with a trisaccharide side chain attached at C3 to alternate glucose residues. These side chains consist of an acetylated mannose residue, a glucuronic acid residue, and a pyruvate ketal linked to a terminal mannose residue. The acetate and pyruvate content depend on the fermentation and isolation conditions used by the supplier. 

The quality of this library was examined by monosaccharide compositional analysis. A portion of the trisaccharide library was treated with aqueous trifluroacetic acid at 100°C for 4 h. Analysis of the resulting mixture of monosaccharides on a Dionex HPLC system with a PA1 column and pulsed amperometric detection (PAD) showed that galactose, glucose, and mannose were present in approximately the required ratio. 

Bacteria form and secrete a variety of heteropolysaccharides, several of which are of commercial value because of their useful gelling properties. Xanthan gum (formed by Xanthomonas campestris) has the basic cellulose structure but every second glucose residue carries an a-l,3-linked trisaccharide consisting of 6-0-acetylmannose, glucuronic acid, and mannose in the following repeating unit 131132... 

The synthesis of hexa- and nonasaccharides (8 ), the dimer and the trimer of the trisaccharide I may be presented as an example for the stepwise synthesis of the Salmonella newington polysaccharide which contains the repeating units I joined through the B-galactosyl 1— 6-mannose linkage (9 ). ... 

As an example of use of the chemico-enzymatic approach,the synthesis of the S. anatum polysaccharide (24) which consists of trisaccharide I units joined by eC-galactosyl-1— 6-mannose linkages, will be discussed. 

The first stage of the synthesis is the preparation of the substrate for enzymatic polymerization, the polyprenyl pyrophosphate-galactosy1-rhamnosy1-mannose XXI. Since the most convenient way to control the polycondensation reaction is the use of isotopic methods, a procedure for incorporation of tritium into trisaccharide I was developed (2 5). Labelled trisaccharide was then converted into the glycosyl phosphate XIX through interaction of its peracetate III with anhydrous phosphoric acid (26). Conditions were found under which the reaction is accompanied by minimal destruction and yields the Ot-phosphate of the trisaccharide. 

Three branched trisaccharides (18, 19, and 20) in which the mannose residue was glycosylated by each of the three 3,6-dideoxyhexose isomers, were synthesized from a common disaccharide precursor (21). The latter was constructed from 8-methoxyl-carbonyloctanol 3-O-benzoyl-4,6-0-cyclohexylidene-a-D-mannopyranoside 22 and tetra-O-benzyl-a-D-galactopyranosylchloride (23). Transesterification of disaccharide 24 leads to the selectively protected disaccharide 21 which was reacted with the ap-... 

After criticism [142] of the interglycosidic NOEs observed in the previously described study a very precise approach towards the conformational analysis of thea-(l-3) bond was performed by NOE measurements of specifically deuterated compounds [143], The conformation was determined earlier by several interglycosidic NOEs to protons that have their resonances in an area of high spectral overlap. The synthesis of C-deuterated di- and trisaccharides made an unequivocal assignment of the enhanced signals possible. Thus, the enhancements of H2, H3, and H4 of the P-D-mannose upon irradiation of Hl. as well as the enhancement of HS. upon irradiation of HS. man are indicative of the rigid conformation at the glycosidic bond and confirmed the previous study. 

The octasaceharide which carries an additional A -acetylglucosamine at the reducing end in comparison to the heptasaccharide was prepared by an independent synthesis. This compound shows a different conformational behavior than the heptasaccharide [122]. The minimum energy conformation, as calculated by the GESA program, shows the trisaccharide at the 6-position of the P-mannose bent back towards the reducing end. This particular effect is in qualitative agreement with the observation of the conformational difference between the tri- and the pentasaccharide described above. The NMR analysis of this octasaceharide confirms the calculated structure. One important fact for the correct prediction of the conformational data was the simultaneous treatment of all independent parameters in the optimization procedure (Table 8). 

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