Stereoisomers of monosaccharides

In general, a molecule with n chiral centers can have 2 stereoisomers. Glyceraldehyde has 21 = 2 the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers. The stereoisomers of monosaccharides... 

Stereoisomers of monosaccharides exist because of the presence of chiral carbon atoms. They are classified as D- or L-depending on the arrangement of the atoms on the chiral carbon farthest from the aldehyde or ketone group. If the —OH on this carbon is to the right, the stereoisomer is of the D-family. If the —OH group is to the left, the stereoisomer is of the L-family. 

Figure 4.5.1 The cholesteric borate amide forms liquid crystals, which react stereose-lectively with stereoisomers of monosaccharides, e.g., the enantiomers of glucose. Double diboration may be involved in the recognition process (compare with Figs. 3.2.1, 3.5.3, and 3.5.4) (from James, Harada, Shimkin, 1993).

Enzymic methods for the quantitation of monosaccharides are employed when a higher degree of specificity is required than can be achieved by the majority of the chemical methods. They often enable the quantitation of one stereoisomer in the presence of others and can often differentiate between the a and 0 anomeric forms. 

A reaction that produces the same functional group at both ends of a monosaccharide halves the number of possible stereoisomers of the monosaccharide. 

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. 

Anomers are stereoisomers of a cyclic monosaccharide that differ in the position of the OH group at the hemiacetal carbon. 

P Anomer (Section 27.6) The stereoisomer of a cyclic monosaccharide in which the anomerie OH and the CH2OH groups are eis. In a D monosaccharide, the hydroxy group on the anomeric carbon is drawn up. 

Monosaccharide Stereoisomers Cyclic Structure of Monosaccharides Reactions of Monosaccharides Important Monosaccharides... 

Relate the absolute configuration of monosaccharide D or L stereoisomers to those of... 

The use of mass spectrometry in the structural analysis of carbohydrates, first reported in 1958 (114), was developed in detail by Kochetkov and Chizhov (115). They showed that, under electron impact, the acetylated and methyl ether derivatives of monosaccharides provided a wealth of structural information through analysis of typical fragmentation pathways of the initial molecular ion. This has proved of enormous utility in the structural elucidation of polysaccharides and complex oligosaccharides sequential permethylation, hydrolysis, reduction to the alditol, and acetylation, affords mixtures of peracetylated, partially methylated alditol acetates that can be separated and analyzed by use of a gas chromatograph coupled directly to a mass spectrometer (25). The mass spectra of stereoisomers are normally identical, while the gas chromatographic retention times readily permit differentiation of stereoisomers. 

A synthetic way to incorporate monosaccharides (o-glucose, D-galactose) and oligosaccharides (lactose, maltose, maltotriose) into [60]fullerene by thermolysis of per-O-acetyl glycosyl azides in the presence of fullerene (Scheme 23) was reported by Kobayashi [93]. This procedure, based on the cycloaddition reaction of alkyl azides with fullerenes [94], gave a mixture of two inseparable stereoisomers of N-b-glycopyranosyl azafulleroids in moderate 13-28% yields. 

Anomers Stereoisomes of cyclized monosaccharide molecules differing only in the configuration of the substituents on the carbonyl carbon (This carbon is a center of Chirality in the cyclized but not in the open-chain form of the molecule.)... 

The chirality of monosaccharides is related to the presence of asymmetric carhon in the molecule aldohexose contains four asymmetric carbons (see Table 1) and consequently 16 stereoisomers can be described. 

A A formula may represent more than two stereoisomers if it has more than one asymmetric carbon atom. The number of stereoisomers doubles for each additional asymmetric carbon atom. The monosaccharide glucose can be represented by the structural formula to the right. How many asymmetric carbon atoms does a glucose molecule have How many stereoisomers of this general structure are there ... 

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