114
STEREOCHEMISTRY
Box 3.19 (continued)
H
H
H
A ring aromatic
HO
H
H
H
OH
estradiol
HO
OH
H
D
A
B
C
More dramatic changes are made to the shape of the steroid skeleton if ring fusions become cis rather than
trans. The most important examples involve the A–B and C–D ring fusions. It is not difficult to work out how
the modified skeleton looks after these changes. The approach is to start from the all-trans system and to delete
the appropriate ring, though retaining the bonds to the unchanged part as a guide to putting in the new ring. This
provides us with three of the bonds in the new ring, and it is just necessary to fill in the rest, using earlier decalin
or hydrindane templates.
H
H
H
H
H
H
H
H
H
H
A–B cis
H
H
H
H
C–D cis
use residual bonds to
form basis of new rings
cleave off appropriate ring,
leaving residual bonds
all-trans
A B
C D
A B
C D
A
C D
B
A B
C D
The approach is used to show the shape of cholic acid, one of the bile acids secreted into the gut to emulsify
fats and encourage digestion. Cholic acid is characterized by a cis fusion of rings A and B.
H
H
H
H
A–B cis
HO
H
H
H
CO 2 H
HO
OH
H
5
cholic acid
H
CO 2 H
H
H
OH
OH
H
OH
H
A B
C D
Digitoxigenin has cis fusions for both A–B and C–D rings. Glycosides of digitoxigenin are the powerful
heart drugs found in the foxglove, Digitalis purpurea. Note how a cis ring fusion changes the more-or-less flat
molecule of cholestane into a molecule with a significant ‘bend’ in its shape; digitoxigenin has two such ‘bends’.
These features are important in the binding of steroids to their receptors, and partially explain why we observe
quite different biological activities from compounds containing a common structural skeleton.
STEREOCHEMISTRY
Box 3.19 (continued)
H
H
H
A ring aromatic
HO
H
H
H
OH
estradiol
HO
OH
H
D
A
B
C
More dramatic changes are made to the shape of the steroid skeleton if ring fusions become cis rather than
trans. The most important examples involve the A–B and C–D ring fusions. It is not difficult to work out how
the modified skeleton looks after these changes. The approach is to start from the all-trans system and to delete
the appropriate ring, though retaining the bonds to the unchanged part as a guide to putting in the new ring. This
provides us with three of the bonds in the new ring, and it is just necessary to fill in the rest, using earlier decalin
or hydrindane templates.
H
H
H
H
H
H
H
H
H
H
A–B cis
H
H
H
H
C–D cis
use residual bonds to
form basis of new rings
cleave off appropriate ring,
leaving residual bonds
all-trans
A B
C D
A B
C D
A
C D
B
A B
C D
The approach is used to show the shape of cholic acid, one of the bile acids secreted into the gut to emulsify
fats and encourage digestion. Cholic acid is characterized by a cis fusion of rings A and B.
H
H
H
H
A–B cis
HO
H
H
H
CO 2 H
HO
OH
H
5
cholic acid
H
CO 2 H
H
H
OH
OH
H
OH
H
A B
C D
Digitoxigenin has cis fusions for both A–B and C–D rings. Glycosides of digitoxigenin are the powerful
heart drugs found in the foxglove, Digitalis purpurea. Note how a cis ring fusion changes the more-or-less flat
molecule of cholestane into a molecule with a significant ‘bend’ in its shape; digitoxigenin has two such ‘bends’.
These features are important in the binding of steroids to their receptors, and partially explain why we observe
quite different biological activities from compounds containing a common structural skeleton.