Glyceraldehyde configuration

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

The small capital letter prefix refers to configuration, related to n glyceraldehyde, and not to the direction of optical rotation. The sign of optical rotation is expressed as (+) and (—) or as d and I or by the words dextro and Uuvo. Th is we have n.( —)-fructose and ,-(+).arabinose. 

The particular aldotetrose just shown is called d erythrose The prefix d tells us that the configuration at the highest numbered chirality center is analogous to that of d (+) glyceraldehyde Its mirror image is l erythrose... 

Highest numbered chirality center has configuration analogous to that of D glyceraldehyde -... 

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

From this, it is clear that D-glyceraldehyde is (i )-glyceraldehyde, and L-alanine is (S)-alanine (see figure). Interestingly, the u-car-bon configuration of all the L-amino acids except for cysteine , (S). Cysteine, by virtue of its thiol group, is in fact (i )-cysteine. 

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. 

Figure 9.9 Assigning configuration to (a) (-)-glyceraldehyde and (b) ( r)-alanine. Both happen to have the Sconfiguration, although one is levorotatory and the other is dextrorotatory.

Molecules like lactic acid, alanine, and glyceraldehyde are relatively simple because each has only one chirality center and only two stereoisomers. The situation becomes more complex, however, with molecules that have more than one chirality center. As a general rule, a molecule with n chirality centers can have up to 2n stereoisomers (although it may have fewer, as we ll see shortly). Take the amino acid threonine (2-amino-3-hydroxybutanoic acid), for example. Since threonine has two chirality centers (C2 and C3), there are four possible stereoisomers, as shown in Figure 9.10. Check for yourself that the R,S configurations are correct. 

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