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Consider the three molecules shown here.
Based on what we learned in the last webcast,
you should be able to distinguish
the faces of these alkenes
as enantiotopic, homotopic, and diastereotopic.
An interesting example
of where topic relationships between faces
becomes important is the enzyme-catalyzed example
that you see at the bottom of this slide.
An achiral nucleophile and an achiral electrophile
are combined in the presence of a chiral enzyme,
an aldolase, to produce an aldol product.
And notice that only a single diastereomer
is formed from this reaction.
This means that two things happened.
First of all, the enzyme was able to distinguish
the enantiotopic faces of the electrophile.
And secondly,
the enzyme was able to also distinguish
the enantiotopic faces of the nucleophile.
That leads to the formation
of only a single diastereomer
because there's a very specific approach
of the nucleophile to the electrophile,
and only two very specific faces
of the nucleophile and electrophile
come into contact.
So the immediate question here is
how was the enzyme able to accomplish
this remarkable distinguishing
of the two enantiotopic faces
of each of these molecules.
We can get a handle on this
if we remember from the last lesson
that enantiotopic faces can be distinguished
by a chiral probe.
Enzymes are certainly chiral probes
as they're made up of chiral amino acids
as we'll learn a little later on,
and the chiral probe is able to distinguish
the enantiotopic faces of the nucleophile,
the nucleophilic enolate, and the electrophile.
If the enzyme were achiral, however,
we would not see that same kind of distinguishing,
which is why if we just mixed up the nucleophile
and electrophile from the last slide,
for instance, in a flask,
we would see a variety
of the four possible stereoisomers
that could form.
The way an enzyme
or really any chiral object does this
is by possessing particular groups
in particular positions
such that there's an optimal orientation
of the chiral object for binding.
So, for instance, if you consider this substrate here
and Dr. Moore's hand as the enzyme,
if we note that
his thumb is an ideal binding point
for the carbonyl oxygen,
then only one orientation of the molecule
fits the orientation of his hand
assuming that orientation is fixed.
So only this orientation
is favorable for the binding of the substrate.
The carbonyl oxygen in that case
is in close proximity to the thumb.
If we turn the molecule over,
or if we turn it over in a different direction,
in both of those cases, the carbonyl oxygen
is not oriented well to be close to the thumb.
As a result, this binding is non-ideal,
and no reaction will occur of the substrate.
Notice in both of these bottom cases,
the exposed face
out here is different
from the exposed face in this case.
We have one enantiotopic face exposed
in the top case,
but in the bottom case,
we have the opposite enantiotopic face
available for reaction.
Because the binding is non-ideal, however,
we won't see a reaction
when that enantiotopic face is available.
We use the descriptors Re and Si
to describe different enantiotopic faces.
The way we do this is by orienting the molecules
such that we're looking down on the face of interest,
then examining the priority
of the three groups that define the plane
that we're looking at.
If the three groups are oriented
in a clockwise fashion
from highest to lowest priority,
then we're looking at the Re face.
If the groups are oriented
in a counterclockwise fashion
from highest to lowest priority,
then we're looking at the Si face.
Turning the molecules over
allows us to look at the opposite face.