Chemistry Reference
In-Depth Information
Fig. 3 Hybridization in
syn
- and
anti
-pyramidalized
double bonds
syn
anti
carbon atoms C
1
,C
2
,C
3
,C
4
,C
4a
,C
9a
and C
5
,C
6
,C
7
,C
8
,C
8a
,C
10a
of the benzene
rings A and B, may be used (Fig.
1
).
2.2 Chirality
The non-planarity of the BAE conformations may introduce chirality [
28
]. The
conformations may be described by the following elements of chirality.
Central double bonds in twisted conformations of BAEs are helical and such
molecules are chiral [
41
]. The absolute configuration is defined by the helicity, which
is given by the sign of the pure ethylenic twist
(Eq. (
1
)), and may be indicated by
stereodescriptors
P
and
M
, following the Klyne-Prelog convention (Fig.
4
.) [
42
].
The helicity of the central ethylene group vanishes in planar conformations
ω
90
) are also
achiral (
D
2d
) unless they are appropriately substituted. It should be noted that the
helicity describes the inherent chirality of the twisted ethylene group backbone or
skeleton irrespective of possible substitution(s). On the other hand, the definition of
axial chirality requires at least one substituent on each end of the “axis” and
therefore does not apply to unsubstituted BAEs. However, axial chirality may be
applied in substituted BAEs. For example, in the above-mentioned case of orthog-
onally twisted conformations, introduction of one substituent on each tricyclic
moiety (or labeling of the atoms) results in a chiral structure; the absolute config-
uration may be defined using the sign of the torsion angle
0
or
180
). Orthogonally twisted conformations (
(
ω
¼
ω
¼
(C
9a
-C
9
-C
9
0
-C
9a
0
)(C
9a
and C
9a
0
are assumed to be on the substituted sides of the tricyclic moieties) based
on axial chirality. In this case the chirality axis passes through the carbon atoms C
9
and C
9
0
of the central double bond (see also Sect.
“Orthogonally Twisted Confor-
mations”
in Sect.
4.1.3
).
In substituted BAEs, folding of the tricyclic moiety introduces another, inde-
pendent element of chirality, reminiscent of a tripodal unit [
41
]. The assignment of
absolute configuration of a tripodal unit requires two rules, a sequence rule and a
conversion rule (assignment rule), as provided by the Cahn-Ingold-Prelog (CIP)
system [
41
,
43
]. In the present Chapter, the atom C
9
will be chosen to define the
center of the tripod and the atoms C
1
,C
8
, and C
9
0
, to define the position of the
'ligands.' (Using the positions of C
9a
,C
8a
, and C
9
0
would define the absolute
configuration of the pyramidalization.) The sequence order is C
1
τ
C
9
0
.
C
8
,
>
>
Assuming relative priorities X
H, this sequence is in accord with
the CIP rules, which allow ranking of the three tree-graph ligands of C
9
based on the
>
C and R
>
