More definitions32.8 More definitions
- Any non-racemic chiral substance is calledscalemic
- A chiral substance isenantiopureorhomochiralwhen only one of two possible enan-
tiomers is present. - A chiral substance isenantioenrichedorheterochiralwhen an excess of one enan-
tiomer is present but not to the exclusion of the other. - Enantiomeric excessoreeis a measure for how much of one enantiomer is present
compared to the other. For example, in a sample with 40% ee in R, the remaining 60%
is racemic with 30% of R and 30% of S, so that the total amount of R is 70%.
32.9 Enantiopure preparations
Several strategies exist for the preparation of enantiopure compounds. The first method
is the separation of a racemic mixture into its isomers. Louis Pasteur in his pioneering
work was able to isolate the isomers of tartaric acid because they crystallize from solu-
tion as crystals with differing symmetry. A less common and more recently discovered
method is by enantiomer self-disproportionation, which is an advanced technique involving
the separation of a primarily racemic fraction from a nearly enantiopure fraction via column
chromatography.
In a non-symmetric environment (such as a biological environment) enantiomers may react
at different speeds with other substances. This is the basis forchiral synthesis, which pre-
serves a molecule’s desired chirality by reacting it with or catalyzing it with chiral molecules
capable of maintaining the product’s chirality in the desired conformation (using certain
chiral molecules to help it keep its configuration). Other methods also exist and are used
by organic chemists to synthesize only (or maybe onlymostly) the desired enantiomer in a
given reaction.
32.10 Enantiopure medications
Advances in industrial chemical processes have allowed pharmaceutical manufacturers to
take drugs that were originally marketed in racemic form and divide them into individual
enantiomers, each of which may have unique properties. For some drugs, such as zopiclone,
only one enantiomer (eszopiclone) is active; the FDA has allowed such once-generic drugs
to be patented and marketed under another name. In other cases, such as ibuprofen,
both enantiomers produce the same effects. Steroid receptor sites also show stereoisomer
specificity.
Examples of racemic mixtures and enantiomers that have been marketed include:
- Ofloxacin(Floxin) andLevofloxacin(Levaquin)
- Bupivacaine(Marcaine) andRopivacaine(Naropin)
- Methylphenidate(Ritalin) andDexmethylphenidate(Focalin)
- Cetirizine(Zyrtec) andLevocetirizine(Xyzal)
- Albuterol(Ventolin) andLevalbuterol(Xopenex)