Monosaccharide stereoisomers

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

The monosaccharide glucose contains four asymmetric carbons carbons 2, 3, 4 and 5. A general formula for calculating the maximum number of stereoisomers is given by 2n, where n is the number of asymmetric carbons. For this compound, since n = 4, the maximum number of stereoisomers is 24 = 16. 

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. 

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... 

When a monosaccharide forms a cyclic hemiacetal (or hemiketal), the carbonyl carbon becomes a stereocenter. Thus, cyclization leads to formation of two possible stereoisomers. These isomers are called anomers, and the former carbonyl carbon is called the anomeric carbon. The isomer with the anomeric OH (shown in blue) pointed down is the alpha anomer. The isomer with the anomeric OH (shown in blue) pointed up is the beta anomer. (Figure 12.14)... 

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. 

Lubineau reported a series of research works on the preparation of C-branched monosaccharides and C-disaccharides under indium promoted Barbier-type allylation in aqueous media [49]. In the case of substrate 97, the reaction, which took place in H20/Et0H (1 2) at 50 °C, gave unique stereoisomer 98 with complete regio- and diastereoselectivity. From... 

Some other molecules generated in sugar pyrolysis, while maintaining the initial monosaccharide cycle, do not show any stereoisomers. This is the case for 1,4-dideoxy-D-glycero-hex-1-enopyranos-3-ulose (1-hydroxymethyl-5-hydroxy-2,3-dihydropyranone) and for 1,6-anhydro-3,4-dideoxy-D-glycero-hex-3-enopyranos-2-ulose (levoglucosenone) (showing C-H bonds) ... 

One approach utilized in carbohydrate pyrolysis is Py-MS. Py-MS using El type ionization does not differentiate well between anhydrosugar stereoisomers, and its utilization to obtain structural information on carbohydrates is rather limited. However, there are reports [10a] where the type of bond between monosaccharide units can be differentiated using Py-EI MS. As an example, the Py-EI MS results are shown for a (1 -> 2)-p-glucan and for a (1 -> 3)-p-glucan (laminaran). 

Carbohydrates play an important role in many research and industrial domains. The huge number of stereoisomers, the immense combination possibilities of carbohydrate monomers in oligosaccharides, and the lack of chromophores are the major problems in the analysis of carbohydrates. Capillary electrophoresis (CE), in its various modes of operation, has been developed as a very useful tool in the analysis of carbohydrate species such as monosaccharides and oligosaccharides, glycoproteins, and glycopeptides. 

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