Chemistry Reference
In-Depth Information
Cholesterol
has a double bond in ring B at the A - B ring fusion, so this distorts the rings by demanding
that the arrangement around the double bond is planar. It is not possible to depict this perfectly in a typical
two-dimensional representation.
H
H
H
D
C
H
A
B
H
H
HO
5
H
6
H
HO
H
5
is a neat way of indicating
that there is a double bond at position 5
Note:
∆
5
-unsaturation
cholesterol
∆
The natural progestogen hormone
progesterone
also has a double bond at the A - B ring fusion, but this time
in ring A, so a similar distortion in ring A is required.
O
O
H
H
D
C
H
B
A
H
H
5
4
H
O
H
O
progesterone
4
-unsaturation
∆
The fungal sterol
ergosterol
has double bonds at positions 5 and 7, both in the B ring, which consequently
should become essentially planar. The picture shown is a rough approximation. The antifungal effect of polyene
antibiotics, such as amphotericin and nystatin (see Box 7.14), depends upon their ability to bind strongly to
ergosterol in fungal membranes. They do not bind significantly to cholesterol in mammalian cells, so this provides
selective toxicity. The binding to ergosterol is very much influenced by the changes in shape conferred by the
extra double bond in ring B.
H
H
D
C
H
8
B
A
H
HO
H
H
H
5
7
HO
H
6
ergosterol
5,7
-unsaturation
∆
In oestrogens, such as
estradiol
, the A ring is aromatic. Consequently, this ring is planar and distorts ring
B accordingly; again, it is difficult to draw this perfectly. The stereochemical outcome makes oestrogens seem
rather more flattened than the original all-
trans
arrangement in cholestane.
