Chemistry Reference
In-Depth Information
With biphenyls, allenes, and
spiro
compounds, groups
are held at right angles by a rigid system, and this feature
allows the existence of non-superimposable mirror image
stereoisomers, i.e. enantiomers. It is useful to think of this
arrangement as analogous to a simple chiral centre, where
the tetrahedral array also holds pairs of groups at right
angles. In contrast to tetrahedral carbon, it is not even
necessary for all the groups to be different to achieve
chirality, as can be seen in the examples above.
A
A
D
D
E
B
B
with biphenyls, allenes, and
spiro
compounds,
groups are held at right angles by a rigid system;
the arrangement produces non-superimposable
mirror images and is thus analogous to a
chiral centre
Box 3.12
Torsional asymmetry: gossypol
The concept of torsional asymmetry is not just an interesting abstract idea. Some years ago, fertility in some Chinese
rural communities was found to be below normal levels, and this was traced back to the presence of
gossypol
in
dietary cottonseed oil. Gossypol acts as a male contraceptive, altering sperm maturation, spermatozoid motility,
and inactivation of sperm enzymes necessary for fertilization. Extensive trials in China have shown the antifertility
effect is reversible after stopping the treatment, and it has potential, therefore, as a contraceptive for men.
OHC
OH
HO
CHO
HO
OH
OH
HO
HO
OH
OH
HO
CHO
HO
OHC
OH
(+)-gossypol
(-)-gossypol
Gossypol is chiral due to restricted rotation, and only the (
−
)-isomer is pharmacologically active as an
infertility agent. The (
)-isomer has been found to be responsible for some toxic symptoms. Most species of
cotton (
Gossypium
) produce both enantiomers of gossypol in unequal amounts, with the (
+
+
)-enantiomer normally
predominating over the (
)-gossypol from this type of
mixture - the racemate complexes with acetic acid, whereas the separate enantiomers do not. The racemic form
can then be resolved (see Section 3.4.8) to give the useful biologically active (
−
)-isomer. It has proved possible to separate racemic (
±
−
)-isomer.
mirror
mirror
3.4.7 Prochirality
Enantiotopic groups
A
A
A
A
A
A
E
D
E
B
B
B
B
D
D
We have defined chirality in terms of 'handedness', such
that mirror image stereoisomers are not superimposable.
In the case of tetrahedral carbon, chirality is a conse-
quence of having four different groups attached to it.
If two or more groups were the same, then the com-
pound would be termed achiral (see Section 3.4.1). Now
we introduce another term,
prochiral
. Achiral molecules
that can become chiral by one simple change are called
prochiral. The simplest example we could include under
this definition would be an achiral molecule in which
two groups are the same. The two like groups are termed
enantiotopic
, in that separate replacement of each would
generate enantiomers.
Molecules that are
superimposable on their
mirror images are
achiral
chiral centre
achiral molecules, that can become chiral
by one simple change are called
prochiral
;
the A groups are termed
enantiotopic
A
A
E
or
A
A
E
enantiomers
B
B
B
