Chemistry Reference
In-Depth Information
drug was withdrawn from the market as soon as evidence of the birth
defects became known and for many years thalidomide disappeared from
the Pharmacopoeia. Recently, however, thalidomide has undergone some-
thing of a renaissance and is now the drug of choice for erythema nodosum
leprosum, a very severe inflammatory condition associated with leprosy.
The drug is used only in male patients or in female patients who are not of
child-bearing age.
Thalidomide is also undergoing trials as an adjuvant to cancer treat-
ment, where the inhibition of angiogenesis may be employed to damage a
tumour's blood supply and, hence, starve the tumour of oxygen and nutri-
ents. There are also reports that the drug may possess immunomodulatory
activity and may be of benefit in the treatment of autoimmune diseases such
as Crohn disease. It will be interesting to see whether thalidomide, after all
the damage and misery it has caused, can become a useful and beneficial
drug in the future.
Geometrical isomerism
Compounds that possess a multiple bond do not rotate easily about the
bond. This gives rise to a type of isomerism called
geometrical
(or
cis
-
trans
)
isomerism. If the substituents around the double bond are similar and both
are on the same side of the double bond, the term
cis
is used to describe the
molecule. If the same groups are on opposite sides of the double bond, the
term
trans
is used to describe the configuration, as illustrated in Figure 4.15.
Cl
Cl
Cl
H
C
C
C
C
H
H
H
Cl
cis
-Dichloroethene
trans
-Dichloroethene
Figure 4.15
Examples of cis and trans isomerism.
The
cis
-
trans
convention is perfectly adequate for the description
of simple compounds; however, for more complex examples a system
based on the Cahn-Ingold-Prelog rules has been developed. The groups
surrounding the double bond are assigned a Cahn-Ingold-Prelog priority
depending on the atomic numbers of the substituents. The configuration in
which the high-priority substituents are on the same side of the double
bond is called the (
Z
) isomer (from the German
zusammen
meaning
together). The alternative configuration, with the high-priority groups on
