Compounds with more than one stereogenic unit

3 Compounds with more than one stereogenic unit 

The first of these, introduced by Seebach and Prelog in 1982,W involves comparison of the configuration of the stereogenic units present, once they have been assigned as R/S. The combinations (/ ,/ ) and (5,5) are denoted by / (like) and (/ , 5) and (5, R) by u (unlike). Thus, for example, compound (3) which has the configuration (25,3R) is a M diastereomer and (4), with the (5,5) configuration, is /. 

It is important to remember that / and u only specify the relative configuration and both (-)-threonine (3), which is (25, 3/ ), and its 

Finally in this section it is worth considering one further example, that of tartaric acid, which illustrates several important points. Since the compound has two stereogenic centres we might expect four possible stereoisomers (5-8). In fact we can see that (7) and (8) are identical since they differ only by a rotation of 180° about a horizontal 

Even though they are achiral, meso compounds are of some value as substrates in asymmetric synthesis, since, upon enantioselective reaction, they give chiral products with two contiguous stereogenic centres (see sections 6.5.2 and 6.5.3). 

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