L-Monosaccharides

L-Monosaccharides do exist in nature, serving a few relatively specialized roles. L-Galactose is a constituent of certain polysaccharides, and L-arabinose is a constituent of bacterial cell walls. 

The designations D and L refer to the configuration of the stereocenter farthest from the aldehyde or ketone group on a monosaccharide chain. When the monosaccharide chain is drawn as a Fischer projection, a D-monosaccharide has the -OH on this carbon on the right an L monosaccharide has it on the left. 

Fig. (46). Analysis of D and L monosaccharides and a and P terminal glycoside linkages in oligo and polysaccharides.

An extensive NMR study of various 1 4-linked disaccharides with D- and L- monosaccharides mixed in disaccharides has been reported [66]. Each group of disaccharides exhibits specific changes in chemical shifts due to glycosidations of the C-4 hydroxy group, which are different between the groups. The differences in chemical shifts were correlated to the preferred conformations of the disaccharides and used to predict the and NMR chemical shifts of 1 4-linked oligo- and polysaccharides. 

Preparation. L-Mannose can be prepared by chain elongation methods, starting from L-arabinose. Both L-glucose and L-mannose are obtained. The nitromethane synthesis leads to a mixture of the nitroalditols that are separated by fractional crystallization and converted individually into the L-monosaccharides.130... 

Or, possibly, even L-monosaccharides joined through -L-linkages. 

D-monosaccharide one that has the same configuration at its penultimate carbon as L-glyceraldehyde is called an L-monosaccharide. 

Which compounds are D-monosaccharides and which are L-monosaccharides (See Example 17.1)... 

L-Monosaccharide A monosaccharide that, when written as a Fischer projection, has the —OH on its penultimate carbon to the left. 

Draw Fischer projections for the four aldotetroses. Which are D-monosaccharides, which are L-monosaccharides, and which are enantiomers Refer to Table 25.1 and write the name of each aldotetrose. 

An L-monosaccharide has a configuration about its penultimate carbon atom that is the same as the configuration of L-glyceraldehyde, wifh the —OH group on the left in a Fischer projection. 

The multiple peak patterns obtained in the g.l.c. of the trimethylsilylated (-)-2-butyl glycosides or acetylated (-I-)-2-octyl glycosides of d- and l-monosaccharides have been used for the assignment of the absolute configuration of the sugars. 

Another unique cyclodextrin has been synthesized to give the first cyclodextrin with L-monosaccharide residues [66]. L-Rhamnose was converted into an a-1 ->4-linked L-rhamnohexasaccharide derivative (2,3-dodeca-o-benzyl-l-methyl-a-thio-L-rhamnohexaoside) that was cyclized using phenylselenyl triflate to form a-1 4-linked cyclo-L-rhamnohexaose (see Fig. 8.9 for the structure). 

The D- and L-monosaccharides have reversed, mirror image configurations at every chiral center. 

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