The topicity of enantioselective reactions

The Re/Si system can also be used to specify enantiotopic groups or atoms. In this case the group from which the other three appear clockwise in order of decreasing priority is described as Re while the other, from which they appear anticlockwise is Si. The two examples (13) and (14) illustrate the application of this rule. 

The / u system of nomenclature, introduced by Seebach and Prelog to describe the relative configuration of diastereomers, can be extended to specify the relative topicity of diastereoseiective reactions. W This involves comparing the/ /5 mdiRe/Si specifications of the stereogenic units and/or enantiotopic faces or groups involved. The combinations (R, Re), (5, Si), (Re, Re) and (Si, Si) are denoted Ik (like) while (R, Si), (S, Re), (Re, Si) and (Si, Re) are denoted ul (unlike). To illustrate the use of this system we will again take the examples of section 1.11 which are representative of the two major types of diastereoseiective reaction. 

In the first case we begin by assigning the diastereotopic faces of the carbonyl group in (15) as Re and Si according to the procedure described in section 2.4 and neglecting for the moment the presence of the stereogenic centre. This initially makes the front face as shown 

For reactions in which two stereogenic units are formed simultaneously we first assign the/ e and 5/ faces of each prochiral reactant molecule. For this purpose it is important to choose the actual reacting species rather than a precursor. For the reaction of benzaldehyde with the anion of ethyl propionate this gives the results shown in (18) and (19). 

The relative topicity of any of the four possible modes of reaction can now be specified by comparing the Re/Si designations of the reacting faces and assigning the reaction as Ik or ul as before. Thus, for example, reaction between the Si face of (18) and the Re face of (19) as shown in (20) is denoted ul and gives rise to the major product (21). 

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