CONFIGURATIONAL ISOMERS
93
C C C
C C C
Ph
H
Ph
H
C
C
C
Ph
H
Ph
H
chirality in allenes
two enantiomeric forms
mirror
≡
C C C
Ph
H
H
Ph
rotate
structure
90º
overlap of p orbitals to generate
π bonds means groups are held at
right angles to each other
The third example of chirality without a chiral centre
is provided by spiro compounds, which we shall meet
later when we consider the stereochemistry of polycyclic
systems (see Section 3.5.1), but at this stage it is worth
noting that they provide a third example of chirality
without a chiral centre. Spiro compounds contain two
ring systems that have one carbon in common, and
it is easy to see this carbon could be chiral if four
different groupings are present. A nice natural example,
the antibiotic griseofulvin, is shown here.
spiro compounds
O
OMe
MeO
Cl
O
O OMe
griseofulvin
* chiral centre
*
spiro
rings share one atom
NH
N
H
HN
N
H
mirror
NH H
N
≡
two enantiomeric forms
this has a chiral centre
this has no chiral centre
rotate
structure
90º
However, it is also possible to visualize spiro
compounds with groupings that are not all different,
where enantiomeric forms exist because mirror image
compounds are not superimposable. The diamine
shown is chiral, in that the mirror image forms are
not superimposable, even though only two types of
substituent are attached to the spiro centre. Both rings
in this compound will have the chair conformation,
but it is not easy to draw these because one ring will
always be viewed face on. The solution is to ensure
the spiro centre is not on the left or right tip of either
ring.
HN
HN
NH
H
N
rotate
structure
it is difficult to show the chair
conformation for both rings
NH
NH
the solution is to ensure
the spiro centre is not on
the tip of either ring
93
C C C
C C C
Ph
H
Ph
H
C
C
C
Ph
H
Ph
H
chirality in allenes
two enantiomeric forms
mirror
≡
C C C
Ph
H
H
Ph
rotate
structure
90º
overlap of p orbitals to generate
π bonds means groups are held at
right angles to each other
The third example of chirality without a chiral centre
is provided by spiro compounds, which we shall meet
later when we consider the stereochemistry of polycyclic
systems (see Section 3.5.1), but at this stage it is worth
noting that they provide a third example of chirality
without a chiral centre. Spiro compounds contain two
ring systems that have one carbon in common, and
it is easy to see this carbon could be chiral if four
different groupings are present. A nice natural example,
the antibiotic griseofulvin, is shown here.
spiro compounds
O
OMe
MeO
Cl
O
O OMe
griseofulvin
* chiral centre
*
spiro
rings share one atom
NH
N
H
HN
N
H
mirror
NH H
N
≡
two enantiomeric forms
this has a chiral centre
this has no chiral centre
rotate
structure
90º
However, it is also possible to visualize spiro
compounds with groupings that are not all different,
where enantiomeric forms exist because mirror image
compounds are not superimposable. The diamine
shown is chiral, in that the mirror image forms are
not superimposable, even though only two types of
substituent are attached to the spiro centre. Both rings
in this compound will have the chair conformation,
but it is not easy to draw these because one ring will
always be viewed face on. The solution is to ensure
the spiro centre is not on the left or right tip of either
ring.
HN
HN
NH
H
N
rotate
structure
it is difficult to show the chair
conformation for both rings
NH
NH
the solution is to ensure
the spiro centre is not on
the tip of either ring