CONFIGURATIONAL ISOMERS
97
O
H 3 C
CO 2 H
pyruvic acid
NADH
lactate
dehydrogenase
H 3 C
CO 2 H
(S)-(+)-lactic acid
H
HO
stereospecific reduction;
hydride delivered to front
face (Re)
We can see from the diagram that hydride must be
delivered from the front face as shown, but it makes
sense to have a more precise descriptor for faces than
front or back. Once again, the Cahn–Ingold–Prelog
system can help us out. We assign priorities to the three
groups attached to the planar carbon.
O
H 3 C
CO 2 H
pyruvic acid
priority 1
priority 2
priority 3
clockwise: Re
O
HO 2 C
CH 3
pyruvic acid
priority 1
priority 3
priority 2
anticlockwise: Si
We then consider the descending sequence and decide
whether this is clockwise or anticlockwise; the face
that provides a clockwise sequence is then labelled Re
and the face that provides an anticlockwise sequence
is labelled Si. These are simply variants on R and
S, in fact the first two letters of rectus and sinister.
Note that there is no correlation between Re or Si
and the chirality R or S of the tetrahedral product
formed.
It can now be seen that, in the enzymic reduction of
pyruvic acid to lactic acid, hydride is delivered to the
Re face of the pyruvic acid.
C O
HO 2 C
H 3 C
Re face
Si face
(S)-(+)-lactic acid
C O
HO 2 C
H 3 C
H
H
OH
H
HO 2 C
H 3 C
stereospecific reduction;
hydride delivered to Re face
pyruvic acid
top face
bottom face
A molecule such as pyruvic acid is said to have two
enantiotopic faces. Attack of a reagent onto the Re face
yields one enantiomer, whereas attack onto the Si face
will produce the other enantiomer.
The Re and Si descriptors are similarly applied to
the carbon atoms making up C=C bonds. This gets
a little more complex, in that a C=C bond generates
four faces to be considered, two at each carbon. It is
necessary to systematically deduce the descriptor for
each, as shown below.
HO 2 C
H 3 C
C
C
H
CH 3
H 3 C
CO 2 H
H 3 C
H
priority 1
priority 2
priority 3
H 3 C
CO 2 H
H 3 C
H
H 3 C
CO 2 H
H 3 C
H
priority 1
priority 2
priority 3
Re
Si
Re
Si
Si
Re
HO 2 C
H 3 C
C
C
CH 3
H
Re
Si Si
Re
23
2
3
3
2
2
3
each sp 2 carbon has two faces
•
•
97
O
H 3 C
CO 2 H
pyruvic acid
NADH
lactate
dehydrogenase
H 3 C
CO 2 H
(S)-(+)-lactic acid
H
HO
stereospecific reduction;
hydride delivered to front
face (Re)
We can see from the diagram that hydride must be
delivered from the front face as shown, but it makes
sense to have a more precise descriptor for faces than
front or back. Once again, the Cahn–Ingold–Prelog
system can help us out. We assign priorities to the three
groups attached to the planar carbon.
O
H 3 C
CO 2 H
pyruvic acid
priority 1
priority 2
priority 3
clockwise: Re
O
HO 2 C
CH 3
pyruvic acid
priority 1
priority 3
priority 2
anticlockwise: Si
We then consider the descending sequence and decide
whether this is clockwise or anticlockwise; the face
that provides a clockwise sequence is then labelled Re
and the face that provides an anticlockwise sequence
is labelled Si. These are simply variants on R and
S, in fact the first two letters of rectus and sinister.
Note that there is no correlation between Re or Si
and the chirality R or S of the tetrahedral product
formed.
It can now be seen that, in the enzymic reduction of
pyruvic acid to lactic acid, hydride is delivered to the
Re face of the pyruvic acid.
C O
HO 2 C
H 3 C
Re face
Si face
(S)-(+)-lactic acid
C O
HO 2 C
H 3 C
H
H
OH
H
HO 2 C
H 3 C
stereospecific reduction;
hydride delivered to Re face
pyruvic acid
top face
bottom face
A molecule such as pyruvic acid is said to have two
enantiotopic faces. Attack of a reagent onto the Re face
yields one enantiomer, whereas attack onto the Si face
will produce the other enantiomer.
The Re and Si descriptors are similarly applied to
the carbon atoms making up C=C bonds. This gets
a little more complex, in that a C=C bond generates
four faces to be considered, two at each carbon. It is
necessary to systematically deduce the descriptor for
each, as shown below.
HO 2 C
H 3 C
C
C
H
CH 3
H 3 C
CO 2 H
H 3 C
H
priority 1
priority 2
priority 3
H 3 C
CO 2 H
H 3 C
H
H 3 C
CO 2 H
H 3 C
H
priority 1
priority 2
priority 3
Re
Si
Re
Si
Si
Re
HO 2 C
H 3 C
C
C
CH 3
H
Re
Si Si
Re
23
2
3
3
2
2
3
each sp 2 carbon has two faces
•
•