Page 156 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

136 lyase. The reaction occurs by an anti elimination involving the amino group and the
3-pro-R hydrogen.
CHAPTER 2
Stereochemistry,
Conformation, H H R H S
and Stereoselectivity S –
CO 2 CO 2 H
NH 3 +

2.1.8. Resolution—The Separation of Enantiomers

Since all living cells and organisms involve reactions of enantiomerically pure
materials such as carbohydrates, proteins, and DNA, most naturally occurring chiral
compounds exist in enantiomerically pure form. Chemical reactions, however, often
produce racemic mixtures. This is always the case if only racemic and/or achiral
reactants, reagents, catalysts, and solvents are used. The products of chemical reactions
can be enantiomerically enriched or enantiopure only if chiral starting materials,
reagents, catalysts or solvents are used. (See Section 2.5 for a discussion of enantiose-
lective reactions.) Racemic mixtures can be separated into the two enantiomeric forms.
The process of separating a racemic mixture into its enantiomers is called resolution,
and it can be accomplished in several different ways.
Historically, the usual method was to use an existing enantiomerically pure
compound, often a naturally occurring material, as a resolving agent. When a racemic
mixture of A (R,S-A) reacts with a pure enantiomer (S-B), the two products are
diastereomeric, namely R,S-AB and S,S-AB. As diastereomers have differing physical
properties, they can be separated by such means as crystallization or chromatography.
When the diastereomers have been separated, the original reaction can be reversed
to obtain enantiomerically pure (or enriched) samples. The concept is summarized in
Scheme 2.2. Scheme 2.3 describes an actual resolution.

Scheme 2.2. Conceptual Representation of
Resolution through Separation of Diastere-
omeric Derivatives

R-A + S-A 1:1 racemic mixture
react with S-B

R-A-S-B + S-A-S-B
separate by physical methods

R-A-S-B S-A-S-B
reverse the
reaction

R-A S-A