Fischer projection, enantiomer nomenclature

There are 20 common a-amino acids that are found in aU living systems (Table 12.1), and each enjoys a more or less unique biosynthesis from fragments found in Chapter 11 and a more or less unique degradation back to those same fragments. Generally, as shown in Table 12.1, the amino acids have the L (in the Fischer projection nomenclature. Chapter 7) or S configuration, although the R enantiomers are also found—but rarely. 

When neither the enolate 29 nor the aldehyde contains stereogenic units, both reactants have enantiotopic faces and 30a and 30b are enantiomers. The same is true for the pair 31a and 31b. However, 30 and 31 form a pair of diastereomers. When an aldol addition leads to an excess of one of these diastereomers 30 or 31, it is said to exhibit simple diastereoselectivity. Several notations that assign descriptors to diastereomeric aldols are found in the literature. The classical erythro/threo nomenclature, which is based on Fischer projection formulas [62], will not be used in this chapter, because it can cause considerable confusion with branched carbon chains. Among the... 

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