Rhamnose 2-0- L

L-Rhamnose (hydrated). L-Rliamnose (anhydrous). lOo) 125 + 9 190 9 pentabenzoate, 71) Penta-acetate, 91)... 

A commonplace variation on the general pattern seen m carbohydrate structure is the replacement of one or more of the hydroxyl substituents by some other atom or group In deoxy sugars the hydroxyl group is replaced by hydrogen Two examples of deoxy sugars are 2 deoxy d nbose and l rhamnose... 

The hydroxyl at C 2 m D nbose is absent m 2 deoxy d nbose In Chapter 28 we shall see how derivatives of 2 deoxy d nbose called deoxynbonucleotides are the funda mental building blocks of deoxyribonucleic acid (DNA) the material responsible for stor mg genetic information L Rhamnose is a compound isolated from a number of plants Its carbon chain terminates m a methyl rather than a CH2OH group... 

FIGURE 7.12 Several deoxy sugars and ouabain, which contains a-L-rhamnose (Rlia). Hydrogen atoms highlighted in red are deoxy positions. 

In 1947, L-rhamnose was first recognized by Stacey as a constituent of Pneumococcus Type II specific polysaccharide. This finding was confirmed, in 1952, by Kabat et al. and in 1955 again by Stacey when 2,4- and 2,5-di-O-methyl-L-rhamnose were synthesized and the former was shown to be identical with a di-O-methylrhamnose, obtained by hydrolysis of the methylated polysaccharide. This result indicated a pyranose ring structure for the rhamnose units in the polysaccharide. Announcement of the identification of D-arabinofuranose as a constituent of a polysaccharide from M. tuberculosis aroused considerable interest. The L-enantiomer had been found extensively in polysaccharides, but reports of the natural occurrence of D-arabinose had been comparatively rare. To have available reference compounds for comparison with degradation products of polysaccharides, syntheses of derivatives (particularly methyl ethers) of both d- and L-arabinose were reported in 1947. 

Ionophoresis of Carbohydrates. Part VII. 2,5-Di-O-methyl-L-rhamnose Its Ionophoresis and Conversion into 6-Deoxy-2 5-di-0-methyl-L-altrose, A. B. Foster, J. Lehmann, and M. Stacey, /. Chem. Soc., (1961)4649-4653. 

Pectin belongs to a family of plant polysaccharides in which the polymer backbone consists of (1— 4)-linked a-D-galacturonic acid repeating-units. Often, (1— 2)-linked a-L-rhamnose residues interrupt the regular polygalacturonate sequence. The high viscosity and gelling properties of pectins are exploited by the food and pharmaceutical industries. X-Ray studies on sodium pectate, calcium pectate, pectic acid, and pectinic acid (methyl ester of pectic acid) have disclosed their structural details. 

Figure 10.26 Short enzymatic synthesis of L-fucose and hydrophobic analogs, and of L-rhamnose, by aldolization-ketol isomerization, including kinetic resolution of racemic hydroxyaldehyde precursors.

Deoxy-L-mannose (L-rhamnose) is common, but D-rhamnose has only been found in some 0-antigens, as in that of Pseudomonas cepacia IMV 3181, which is a homopolysaccharide containing both a- and y -pyranosyl residues. 

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