Stereospecific and Stereoselective Reactions

In this section we discuss relationships between reactivity and stereochemistry. Many reactions can produce two or more stereoisomeric products. If a reaction shows a preference for one of the stereoisomers, it is stereoselective. Throughout the sections on individual reactions in Parts A and B, we discuss the stereoselectivity associated with particular reactions. In this chapter, we use a few examples to illustrate the fundamental concepts of stereoselectivity. 

Stereoselectivity is intimately related to the mechanism of the reaction. Some reactions are stereospecific, that is reactions in which stereoisomeric reactants each provide stereoisomeric products. For example, the Sjv2 substitution reaction results in an inversion of the configuration. It is a stereospecific reaction. The I -reactant gives the 5-product and the 5-reactant gives the / -product (assuming the priority order remains unchanged). 

As another example, epoxidation of -2-butene gives fran.y-2,3-dimethyloxirane, whereas Z-2-butene gives cw-2,3-dimethyloxirane. 

We discuss several examples to illustrate how reactant structure and mechanism can lead to stereoselectivity, including stereospecificity. We also consider enantioselective and enantiospecific reactions, which are reactions that favor one enantiomer of a reaction product. 

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If you have forgotten the difference, between stereoselective and stereospecific reactions, or between kinetic and thermcdynsm/c contrql, go -back and re-read Chapters 13 and 19— these concepts are very important for this chapter. 

Stepwise Mechanism Stereoselective and Stereospecific Reactions Theory of Concerted Reactions... 

It is important to use stereoselective and stereospecific reactions (where applicable), such as... 

Enzymes catalyze almost every metabolic reaction in extant cells. A few tmusually facile reactions, such as cyclization of L-glutamate 7-semialdehyde to form pyrroline-5-carboxylate in the proline biosynthesis pathway and decarboxylation of 2-amino-3-oxo-4-phosphonooxybutyrate in the pyridoxal phosphate (PLP) synthesis pathway, do not require acceleration to satisfy the demands of the cell. For all other reactions, catalysis is required because the rates of nonenzymatic reactions are very slow. Modern enzymes are marvelous catalysts. They accelerate reactions by up to 20 orders of magnitude, prevent side reactions of reactive intermediates, and catalyze stereoselective and stereospecific reactions. Further, they are often exquisitely regulated by small molecule ligands. 

Stereochemistry of Reactions Regioselective, Stereoselective, and Stereospecific Reactions... 

If you are unclear on the difference between stereoselective and stereospecific reactions, or... 

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