Chemistry Reference
In-Depth Information
f
-
C
s
(
xz
)
ft
RP
-
C
1
Fig. 20 Three-dimensional projection of the folded conformations of heteromerous and
substituted heteromerous BAEs including their symmetry operators
the symmetry to
C
1
. The conformations ft
P
-
C
1
and ft
M
-
C
1
of homomerous and
heteromerous BAEs are chiral. Monosubstitution introduces another element of
chirality leading to two diastereomeric pairs of enantiomers: ft
RP
-
C
1
and ft
SM
-
C
1
vs
ft
RM
-
C
1
and ft
SP
-
C
1
. Adding another substituent on the second moiety results in
four diastereomeric pairs of enantiomers: ft
E-RPR
0
-
C
1
and ft
E-SMS
0
-
C
1
, ft
E-RPS
0
-
C
1
and ft
E-SMR
0
-
C
1
, ft
Z-RPR
0
-
C
1
and ft
Z-SMS
0
-
C
1
, and ft
Z-RPS
0
-
C
1
and ft
Z-SMR
0
-
C
1
. Four
structures are
E
-isomers and four are
Z
-isomers. Due to the relationships of
E
,
Z
-
isomerism,
anti
- and
syn
-folding, and the absolute configuration of the substituted
moieties (
R
/
S
), the first and the last mentioned enantiomeric pair have
syn
-folded
character, while the second and third enantiomeric pair have
anti
-folded character
(cf. Fig.
5
).
In a global approach to the analysis of the relations between point group
symmetry and energy hyper surfaces, Mezey has proved five theorems which
allow the prediction of at least one conformation with stationary energy (zero
gradients) by comparing the symmetry elements present (or absent) in a closed
set of conformations with those of the boundary of the set [
280
]. In the case of the
bistricyclic aromatic enes it may be shown that there is at least one planar and one
orthogonally twisted stationary point with the respective highest possible symmetry
(Table
4
).
4.2 Symmetry and Dynamic Stereochemistry of BAEs
In the following, the mechanisms for conformational isomerizations of BAEs will
be analyzed in terms of local minima, and transition states. The transition states
represent pathways for interconversion between the minima. This analysis leads to
the representation of the dynamic stereochemistry of a molecule by the reaction
graph, which is a very elegant tool to analyze experimental dynamic stereochem-
istry and theoretical predictions based on computations.
