Definition of Stereoselective Synthesis

The stereoselective process is not necessarily a synthesis, i.e., a reaction in which bonds are formed indeed, stereoselectivity can also be observed when some bonds of a molecule are broken to produce two or more species. 

More specifically, the stereoselective synthesis of an enantiomerically enriched compound requires that the two enantiomers of the product (that therefore must be a molecule contain- 

1 Since Mislow s original proposal was made [4], the definitions of stereoselective synthesis and of stereoselectivity have remained virtually unchanged and are largely agreed upon see for instance the definition of these terms reported in ref. [5], 

The efficiency of a stereoselective process, determined by the ratio between the concentrations of the obtained stereoisomers, is defined as stereoselectivity or stereoselection. This ultimately depends on the difference in the free energy of activation (AAG ) of the reaction pathways leading to the stereoisomeric products obviously, the higher this difference, the more stereoselective the process. 

It is common practice to speak of diastereoselective synthesis when the products are dia-stereoisomers, and of enantioselective synthesis when they are enantiomers. In both cases, however, the stereoisomers are obtained in unequal amounts because of the diastereoisomeric relationships between the competitive transition states (TS) leading to different stereoisomers. Consider for instance the reactions reported in Fig. 1. 

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