Glyceraldehyde absolute configurations

Glycal assembly method, 1002 (4- )-Glyceraldehyde. absolute configuration of, 980 (-)-Glyceraldehyde, configuration of, 300... 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

Although the D-form of glyceraldehyde was arbitrarily chosen as the dextrorotatory isomer without any knowledge of its absolute configuration, the choice was a fortuitous one. In 1951, with the aid of modern analytical methods, the D-configuration of the dextrorotatory isomer was unambiguously established. 

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.

In solution, enantiomers rotate the wave plane of linearly polarized light in opposite directions hence they are refered to as dextro"- or levo-rotatory , designated by the prefixes d or (+) and 1 or (-), respectively. The direction of rotation gives no clue concerning the spatial structure of enantiomers. The absolute configuration, as determined by certain rules, is described by the prefixes S and R. In some compounds, designation as the D- and L-form is possible by reference to the structure of D- and L-glyceraldehyde. 

Once the configuration of the glyceric acids was known (in relation to the glyceralde-hydes), it was then possible to relate other compounds to either of these, and each time a new compound was related, others could be related to it. In this way many thousands of compounds were related, indirectly, to D- or L-glyceraldehyde, and it was determined that 28, which has the d configuration, is the isomer that rotates the plane of polarized light to the left. Even compounds without asymmetric atoms, such as biphenyls and allenes, have been placed in the D or L series.71 When a compound has been placed in the d or l series, its absolute configuration is said to be known.72... 

At the time the choice of absolute configuration for glyceraldehyde was made, there was no way of knowing whether the configuration of (-l-)-glycer-aldehyde was in reality 3a or 3b. However, the choice had a 50% chance of... 

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