116
STEREOCHEMISTRY
H
N
N
O
S
H
O
benzylpenicillin
(penicillin G)
S
O
H 2 N
HO 2 C
H
N
H
H
can only be cis-fused
four-membered ring is planar
five-membered ring is non-planar
ring fusion forces N into one configuration
N
S
O
H 2 N
H
2
3
4
5
7
1
6
6-aminopenicillanic acid
CO 2 H
CO 2 H
The cis fusion in which one of the fusion atoms is
nitrogen merely indicates that the nitrogen lone pair
electrons occupy the remaining part of the tetrahedral
array. It does, however, mean that inversion at the
nitrogen atom (see Section 3.4.1) is not possible,
since that would hypothetically result in formation
of the impossible trans-fused system. The ring
fusion has thus frozen the nitrogen atom into one
configuration.
Fusion of a four-membered ring onto a sixmembered ring is also only possible with a cis
fusion; cephalosporins provide excellent examples
of such compounds, and the comments made above
for penicillins are equally valid for these compounds.
O
H
N
N
O
S
CO 2 H
H 2 N
CO 2 H
H
O
O
cephalosporin C
3.5.3 Bridged ring systems
In bridged ring compounds, rings share more than two
atoms, and the bridge can consist of one or more atoms.
We have already met an example in bornane (see
Section 3.3.2), which we used as an illustration of how a
cyclohexane ring can be forced into a boat conformation
to achieve the necessary bonding.
cyclohexane ring
forced into boat
conformation
bornane
≡
If we inspect the ring system of bornane, omitting the
methyl groups, we can see that there are actually several
bridges of different lengths spanning the bridgehead
atoms, depending upon which atoms are considered.
This is used in nomenclature, as illustrated below,
including in square brackets all the bridges, listed in
decreasing lengths. Numbering, when necessary, always
starts from a bridgehead atom. A closer inspection of the
shape of bicyclo[2,2,2]octane (best with a model), which
has two-carbon bridges, shows that each ring system has
the boat conformation.
one-atom bridge
bridgehead
bridgehead
bicyclo[3,1,1]heptane bicyclo[2,2,1]heptane
bicyclo[2,2,2]octane
two-atom bridge
two-atom bridge
bicyclo[2,2,2]octane
≡
all rings boat
STEREOCHEMISTRY
H
N
N
O
S
H
O
benzylpenicillin
(penicillin G)
S
O
H 2 N
HO 2 C
H
N
H
H
can only be cis-fused
four-membered ring is planar
five-membered ring is non-planar
ring fusion forces N into one configuration
N
S
O
H 2 N
H
2
3
4
5
7
1
6
6-aminopenicillanic acid
CO 2 H
CO 2 H
The cis fusion in which one of the fusion atoms is
nitrogen merely indicates that the nitrogen lone pair
electrons occupy the remaining part of the tetrahedral
array. It does, however, mean that inversion at the
nitrogen atom (see Section 3.4.1) is not possible,
since that would hypothetically result in formation
of the impossible trans-fused system. The ring
fusion has thus frozen the nitrogen atom into one
configuration.
Fusion of a four-membered ring onto a sixmembered ring is also only possible with a cis
fusion; cephalosporins provide excellent examples
of such compounds, and the comments made above
for penicillins are equally valid for these compounds.
O
H
N
N
O
S
CO 2 H
H 2 N
CO 2 H
H
O
O
cephalosporin C
3.5.3 Bridged ring systems
In bridged ring compounds, rings share more than two
atoms, and the bridge can consist of one or more atoms.
We have already met an example in bornane (see
Section 3.3.2), which we used as an illustration of how a
cyclohexane ring can be forced into a boat conformation
to achieve the necessary bonding.
cyclohexane ring
forced into boat
conformation
bornane
≡
If we inspect the ring system of bornane, omitting the
methyl groups, we can see that there are actually several
bridges of different lengths spanning the bridgehead
atoms, depending upon which atoms are considered.
This is used in nomenclature, as illustrated below,
including in square brackets all the bridges, listed in
decreasing lengths. Numbering, when necessary, always
starts from a bridgehead atom. A closer inspection of the
shape of bicyclo[2,2,2]octane (best with a model), which
has two-carbon bridges, shows that each ring system has
the boat conformation.
one-atom bridge
bridgehead
bridgehead
bicyclo[3,1,1]heptane bicyclo[2,2,1]heptane
bicyclo[2,2,2]octane
two-atom bridge
two-atom bridge
bicyclo[2,2,2]octane
≡
all rings boat