CONFIGURATIONAL ISOMERS
115
H
H
H
OH
A–B cis, C–D cis
HO
OH
O
O
H
digitoxigenin
H
H
5
14
O
O
H
HO
H
A B
C D
Most natural steroids have the stereochemical features seen in cholesterol, though, as we have seen, there may
be some variations, particularly with respect to ring fusions affecting the A and D rings. Note that trans fusion at
the hydrindane C–D ring junction is energetically less favourable than a cis fusion (see Section 3.5.2), but most
natural steroid systems actually have this trans fusion.
O
O
H
H
H
H
H
HO
H
H
H
O
O
H
H
HO
diosgenin
diosgenin
H
H
H
H
∆
5
-unsaturation
H
H
H
H
E
F
B
A
C D
We have met diosgenin as an example of a natural spiro compound (see Box 3.17), and further examination
of the structure shows the
5 double bond as in cholesterol, a second five-membered ring cis-fused onto the
five-membered ring D, as well as the spiro fusion of a six-membered ring. Before this structure dismays you,
take it slowly and logically. It should not be too difficult to end up with the stereodrawing shown here.
Box 3.20
The shape of penicillins
Penicillins are the most widely used of the clinical antibiotics. They contain in their structures an unusual fused
ring system in which a four-membered β-lactam ring is fused onto a five-membered thiazolidine. Both rings are
heterocyclic, and one of the ring fusion atoms is nitrogen. These heteroatoms do not alter our understanding of
molecular shape, since we can consider that they also have an essentially tetrahedral array of bonds or lone pair
electrons (see Section 2.6.3).
We have seen that, in cyclobutane and cyclopentane, a lower energy conformation is attained if the rings are not
planar (see Section 3.3.2). If one fuses a five-membered ring onto a four-membered ring, models demonstrate that
it is only possible to have a cis fusion in such a structure, and that conformational freedom in the four-membered
ring disappears if we are to achieve this bonding; the four-membered ring reverts to a more planar shape. It is
still possible to have the five-membered ring non-planar, thereby reducing eclipsed interactions.
115
H
H
H
OH
A–B cis, C–D cis
HO
OH
O
O
H
digitoxigenin
H
H
5
14
O
O
H
HO
H
A B
C D
Most natural steroids have the stereochemical features seen in cholesterol, though, as we have seen, there may
be some variations, particularly with respect to ring fusions affecting the A and D rings. Note that trans fusion at
the hydrindane C–D ring junction is energetically less favourable than a cis fusion (see Section 3.5.2), but most
natural steroid systems actually have this trans fusion.
O
O
H
H
H
H
H
HO
H
H
H
O
O
H
H
HO
diosgenin
diosgenin
H
H
H
H
∆
5
-unsaturation
H
H
H
H
E
F
B
A
C D
We have met diosgenin as an example of a natural spiro compound (see Box 3.17), and further examination
of the structure shows the
5 double bond as in cholesterol, a second five-membered ring cis-fused onto the
five-membered ring D, as well as the spiro fusion of a six-membered ring. Before this structure dismays you,
take it slowly and logically. It should not be too difficult to end up with the stereodrawing shown here.
Box 3.20
The shape of penicillins
Penicillins are the most widely used of the clinical antibiotics. They contain in their structures an unusual fused
ring system in which a four-membered β-lactam ring is fused onto a five-membered thiazolidine. Both rings are
heterocyclic, and one of the ring fusion atoms is nitrogen. These heteroatoms do not alter our understanding of
molecular shape, since we can consider that they also have an essentially tetrahedral array of bonds or lone pair
electrons (see Section 2.6.3).
We have seen that, in cyclobutane and cyclopentane, a lower energy conformation is attained if the rings are not
planar (see Section 3.3.2). If one fuses a five-membered ring onto a four-membered ring, models demonstrate that
it is only possible to have a cis fusion in such a structure, and that conformational freedom in the four-membered
ring disappears if we are to achieve this bonding; the four-membered ring reverts to a more planar shape. It is
still possible to have the five-membered ring non-planar, thereby reducing eclipsed interactions.