Stereoselective Reactions

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Chapter: Organic Chemistry : Stereochemical and Conformational Isomerism

Chiral compounds are very important substances. Many natural products, medicinal compounds, and biomolecules exist as a single, optically active stereoisomer.


STEREOSELECTIVE REACTIONS

Chiral compounds are very important substances. Many natural products, medicinal compounds, and biomolecules exist as a single, optically active stereoisomer. Furthermore the opposite enantiomer or diastereomer may not have any physio-logical activity or may, in fact, have a detrimental physiological effect. There is therefore great interest in reactions in which only one stereoisomeric form of a compound is produced by a particular synthetic sequence.

The simplest approach is to use a starting material of known absolute configu-ration and manipulate it to the final product using reactions whose stereochemical outcome at the chiral center is known and/or predictable. For example, the syn-thesis of unusual amino acids as single enantiomers can begin with a “normal” amino acid from the chiral pool. (The “chiral pool” is a large group of molecules which are readily available and whose absolute configurations are known with certainty. Often these molecules are or are derived from natural products which can be isolated from natural sources in enantiomerically pure form.)


Since the stereochemical changes in each reaction of the sequence are known, a particular amino acid starting material (R or S) will give a particular configu-ration in the product. In this strategy of asymmetric synthesis, all or part of the final molecular skeleton is derived from the chiral precursor. While simple, this strategy is limited by the size of the chiral pool and by the types of reactions which occur stereospecifically at tetrahedral centers.

It is much more common for reactions to produce new chiral centers from achiral starting materials. Consequently, if we are to use the whole arsenal of synthetic methods available to us and at the same time produce single stereoiso-mers, then we must be able to control (or at least understand) the stereochemistry of reactions occurring at achiral centers.

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