[Main Page] [Discussion of T4 stereochemistry]
Hello,
Several recent messages really deal with perspective drawing and
their combination with stereo bonds.
The answer depends on what we are discussing: the specific case or a general use of perspective drawings.
Obviously, perspective drawings are widely used, quite clear and
useful for presentation. For example, pagodane
Such drawings will be used but we need to mention that they are
hardly recognizable in the sense of configuration by computer and by
people with not so good perspective
imagination.
Basically, perspective drawing is a simple projection of 3D structure on the plain. So, it doesn't imply (in general) any absolute configuration. With additional conventions or drawing elements it can represent an absolute configuration.
It could be: 1. an assumption that lower part of a cycle is closer to a viewer But people do not so strictly follow this assumption and some drawings may need another orientation of a structure
2. bond crossings (broken remote bonds) Not so often used and too difficult to analyze as soon as one crossing influences the configuration of an atom that far from it. So, it's impossible to understand the configuration of some center taking into account this atom and its neighbors only.
3. bond widths, sometimes together with wedges It can lead to mistakes and contradictions as soon as a perspective drawing is not laying in the plain and an addition of substituents with wedges is not so strict. But such combined drawings can help in computer analysis of perspective drawings.
Have you seen such presentation of bicyclo[2.2.1]heptane? I've too
often.
Quite beautiful, but really incorrect due to a wrong configuration of
the closer bridgehead atom.
Another one is correct but looks not so clear (but there could
be problems with analysis of configuration of disubstituted bridge
atom for unsymmetrical case (if we take into account only sharp ends)).
For the previously mentioned structure with shown bond crossing or
not
or 
the situation is quite simple (for human!!!) as soon as: - it is a small cycle, so both hydrogen at bridgeheads can be directed out of cycle only
- conformations of all single bonds are almost fixed
- the structure has no absolute stereo (symmetry plain through A-C-B)
But in general case all is far less clear. Cycles can be larger; there could be more cycles and substituents.
It seems we can formulate several recommendations. Some of them are quite obvious, some are not.
1. First of all, it seems that we can agree that perspective drawings
can be used for cyclic systems only. For example, being drawn very
close to [2.2.1] bicycle the
following acyclic structure hardly can be recognized as D-ribose.
2. We may state that perspective drawings without any supplementary
features (bond crossings, bold bonds and wedges) do not imply any
absolute configuration.
And maybe even relative (?).
3. Also it is important to mention that, being clear for human,
perspective drawings without stereo bonds cannot be interpreted
strictly by computers in the sense of
configuration if they do not contain stereo bonds to define
configurations. So, such drawings hardly can be used in any computer-based programs to
interpret (search,
name, take into account) configurations. (Such algorithms can be created but the result in general case could
be not so reliable)
4. And the last statement. To be sure in computer
interpretation of perspective drawing with stereo bonds, the drawing
must have stereo bonds to provide independent analysis
of each center and its neighbors. So, each defined center needs at
least one stereo bond.
For example,
(numbers are added for clarity)
The structure above has four correctly defined centers:
, 
, 
and 
Obviously combination of perspective with stereo bonds is more
complex to draw and may lead to additional misinterpretations. For
example the following drawing is not correct:
It defines incorrect configuration of A-C-B center.
Now about the mentioned flat drawing of bicyclic systems with wedges.
This drawing strictly and correctly defines the configuration of bridgeheads and C,D-disubstituted center.
The question may be the configuration of A,B-disubstituted center. We have the following fragment:
If we support analysis of two ends of wedge it will define the
configuration strictly and it is the same as:
It's quite correct (but in 3D space is equivalent to TBPY-4)
If we take into account sharp ends only this center we'll need an
additional wedge(s).
and AB-center will be defined correctly as:
And a small addition about above statement # 4 (all centers need at least one stereo bond drawn correctly).
We have the following perspective drawing of adamantane with
wedges.
The red center is drawn incorrectly as soon it has the following
definition:
Better drawing would be:
(as soon as all centers are drawn correctly).
It is not so important for adamantane itself, but for chiral
adamantane derivatives and other cases of perspective drawings with
stereo bonds it would be more strict drawing.
It's only an opinion. Any comments are welcome. --
Best regards,
Andrey
