Enantiomers, monosaccharides

M Stefansson, M Novotny. Electrophoretic resolution of monosaccharide enantiomers in borate-oligosaccharide complexation media. J Am Chem Soc 115 11573-11580, 1993. 

FIG. 18 Discrimination of monosaccharide enantiomers by gel formation of their esters with 15 in chloroform. Gel-solution phase transition temperatures versus the mol9<- of ester are shown. (From Ref. 97.)... 

Stefansson M, Novotny M (1993) Electrophoretic resolution of monosaccharide enantiomers in borate-oUgosaccharide complexation media. J Am Chem Soc 115 11573-11580 Kano K, Tamiya Y, Hashimoto S (1992) Binding forces in complexation of p-alkylphenols with P-cyclodextrin and methylated P-cyclodextiins. J Incl Phenom Mol Recognit Chem 13 287-293... 

Several branched monosaccharides have trivial names, some established by long usage. Examples are given below, together with systematic names for the (cyclic or acyclic) forms illustrated. (See also the alphabetical listing of trivial names in the Appendix.) Enantiomers of the sugars listed should be named systematically. 

Dermatan sulfate, also termed chondroitin sulfate B, a related glycosaminoglycan constituent of connective tissue, was known to be composed of galactosamine and a uronic acid, originally believed to be glucuronic acid but then claimed to be iduronic acid based largely on color reactions and paper chromatography. However, the d or L-enantiomer status of the latter monosaccharide was not clear. Jeanloz and Stoffyn unequivocally characterized the monosaccharide as L-iduronic acid by consecutive desulfation, reduction, and hydrolysis of the polysaccharide, followed by isolation of the crystalline 2,3,4-tri-0-acetyl-l,6-anhydro-/ -L-idopyranose, which was shown to be identical to an authentic specimen synthesized from 1,2-0-isopropylidene-/ -L-idofuranose.34... 

An interesting approach to both enantiomers of biologically active cyclo-phellitol, based on the latent symmetry concept, from the same common starting material D-xylose was proposed by Kireev et. al. Functionalization of this monosaccharide at the C-l provided compound 64, while similar processes initiated from the end (C5) afforded the ent-64 (Fig. 23).36 Proper functionalization of these intermediates led to both enantiomers of cyclophellitol. 

All carbohydrates can exist in either of these two forms and the prefix of D or L only refers to the configuration around the highest numbered asymmetric carbon atom. Enantiomers have the same name (e.g. D-glucose and L-glucose) and are chemically similar compounds but have different optical properties. The majority of naturally occurring monosaccharides, whether they be aldoses or ketoses, are of the D configuration. 

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms (pp. 17-19). The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers (Fig. 7-2). 

Compounds with the same chemical formula are called isomers. If two monosaccharide iso mers differ in configuration around one specific carbon atom (with the exception of the car bonyl carbon) they are defined as epimers of each other. If a pair of sugars are mirror images of each other (enantiomers), the two members of the pair are designated as D- and L-sugars. 

C.-H. Lin, T. Sugai, R. L. Halcomb, Y. Ichikawa, and C. H. Wong, Unusual stereoselectivity in sialic acid aldolase-catalyzed aldol condensations Synthesis of both enantiomers of high-carbon monosaccharides, J. Am. Chem. Soc. 774 10138 (1992). 

The IUPAC-IUBMB short form symbolism18 2-Carb-38.5 is used here and elsewhere in this chapter for designation of oligosaccharide structures. Parent monosaccharide residues are the d enantiomers except for IdoA, which is the l enantiomer. Uronic acid residues modified by 44-unsaturation are prefixed by A4tS, and the associated anomeric symbol relates to the configuration at the 5-position of that residue before 4,5-elimination. The abbreviation HexA is used to denote either d-G1cA or L-IdoA. 

Initially, the de novo synthesis of enantiomerically pure carbohydrates [110] and glycolipids [111] using transition metal complexes and chiral auxiliaries afforded only modest success. Ultimately, it was the use of enantiomerically pure aldehydes, such as the R and S enantiomers of 2-(phenylseleno)propionaldehyde, to convey facial selectivity upon the LACDAC reaction that enabled the synthesis of optically pure glycals. Syntheses of several complex monosaccharides such as the main sialic acid-type N-acetylneuraminic acid (Neu5Ac) and rac-3-deoxy-ma o-2-octulosonic acid (KDO) were accomplished with this technology [112, 113], The LACDAC... 

Of theoretical and practical value has been the use of anti-carbohydrate antibodies to determine the d or l configurations of monosaccharides. Antibodies generated with the d enantiomer of the sugar in the antigen are inhibited by the d enantiomer of the monosaccharide, but not by the l. The reverse holds for antibodies generated with the l enantiomer of the monosaccharide component in the antigen. Some representative... 

Do not rotate a Fischer projection formula in the plane of the page, because you might inadvertently convert a compound into its enantiomer. When using Fischer projections it is usually best to convert them to structures with wedges and dashes, and then manipulate them. Although a Fischer projection formula can be used for the stereogenic center in any compound, it is most commonly used for monosaccharides. 

The number of possible stereoisomers of a monosaccharide increases exponentially with the number of stereogenic centers present. An aldohexose has four stereogenic centers, and so it has 2 = 16 possible stereoisomers, or eight pairs of enantiomers. 

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