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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