Glyceraldehyde enantiomers

Figure 9.1 Enantiomers of glyceraldehyde. Enantiomers are mirror images of each other and are chemically similar but will cause a beam of plane polarized light to rotate in opposite directions. Glyceraldehyde has only one asymmetric centre ( ) and the designation of D or L is determined by the orientation of the O and OH groups about this carbon atom. For carbohydrates with more than one asymmetric carbon atom, the prefix d or l refers only to the configuration about the highest numbered asymmetric carbon atom, although in such enantiomers the configuration about all the asymmetric centres will also be reversed.

PROBLEM 24.13 Draw tetrahedral representations of the two glyceraldehyde enantiomers using wedged, dashed, and normal lines to show three-dimensionality. 

Triose structures. The two central structures for glyceraldehyde enantiomers are written according to the Mills convention (wedges out of the paper, dashed lines going in). The next structures are the full Fischer convention and the outer structures an abbreviated Fischer convention in which the hydrogen and central carbon atoms are implicit. 

SAMPLE SOLUTION (a) Redraw the Fischer projection so as to more clearly show the true spatial orientation of the groups. Next, reorient the molecule so that its relationship to the glyceraldehyde enantiomers in Figure 25.1 is apparent. 

FIGURE 1.12. Fischer projections of glyceraldehyde enantiomers (a) d-( + )-glyceraldehyde (b) L-(-)-glyceraldehyde. 

HC CH(0H) CH20H. optically active. D-glyceraldehyde is a colourless syrup. May be prepared by mild oxidation of glycerol or by hydrolysis of glyceraldehyde acetal (prepared by oxidation of acrolein acetol). DL-glyceraldehyde forms colourless dimers, m.p. IBS-S C. Converted to methylglyoxal by warm dilute sulphuric acid. The enantiomers... 

Adopting the enantiomers of glyceraldehyde as stereo chemical reference com pounds originated with proposals made in 1906 by M A Rosanoff a chemist at New York University... 

Glyceraldehyde can be considered to be the simplest chiral carbohydrate It is an aldotriose and because it contains one chirality center exists in two stereoisomeric forms the D and l enantiomers Moving up the scale m complexity next come the aldotetroses Examining their structures illustrates the application of the Fischer system to compounds that contain more than one chirality center... 

The original method had as a starting point the enantiomers of a standard compound, glyceraldehyde. 

Once the order is determined, the molecule is held so that the lowest group in the sequence is pointed away from the viewer. Then, if the other groups in the order listed are oriented clockwise, the molecule is designated (/ ), and if counterclockwise (5). For glyceraldehyde, the ( + ) enantiomer is (/ ) ... 

Detailed studies have been carried out on the stereoselectivity of a- and -substituted aldehydes toward the tartrate boronates.47 a-Benzyloxy and (3-benzyl oxy-a-methylpropionaldehyde gave approximately 4 1 diastereoselectivity with both the R, R- and S, S- enantiomers. The stereoselectivity is reagent (tartrate) controlled. The acetonide of glyceraldehydes showed higher stereoselectivity. 

Figure 10.4 The stereochemical relationship between amino acids and glycer-aldehyde. The designation of d or l to an amino acid refers to its absolute configuration relative to the structure of d- or L-glyceraldehyde respectively. The d and l forms of a particular compound are called enantiomers.

Most of these operations would yield the enantiomer of 22 in a conformation that is not the mirror image of the original one. However, the need for conformational adjustment is nothing unusual. It is equally necessary if we wish to proceed from the preferred conformation of (R)-glyceraldehyde to that of its enantiomer by exchanging the H and OH ligands. 

A practical, inexpensive one-step procedure was developed for the RhaD-catalyzed gram-scale synthesis of L-fructose. The requirement for DHAP as the donor substrate was circumvented by use of borate buffer, presumably by in situ formation of borate esters as a phosphate ester mimic. Racemic glyceraldehyde was also used, as the enzyme preferentially accepted the L-enantiomer as a substrate. The method can also be apphed to other products, including L-rhamnulose, and towards a two-step synthesis of L-iminocychtols. ... 

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