Chemistry Reference
In-Depth Information
9'
9
X
R
1
8
9
R
X
1
8
9'
9
'
8
8
1
1
9'
M
P
R
S
Fig. 4 Chiral elements in BAEs and the corresponding stereodescriptors
for absolute
configuration
atoms three and four bonds from the center; e.g., for a BAE substituted in the
2-position: C
9a
{C[
X
(C), C(C,H)], C[C(C,
R)
, H]}
C
8a
{C[
X
(C), C(C,H)], C[C(C,
>
C
9
0
{C[
C
(Y,C), C(C,H)], C[C(Y,C), C(C,H)]} [
41
]. The assignment of
the absolute configuration,
R
or
S
, for substituted folded tricyclic moieties is
illustrated in Fig.
4
. Note that unsubstituted folded moieties are achiral, but
nevertheless may be assigned handedness in a labeled atom sense (see below).
Propeller twisted tricyclic moieties are intrinsically chiral, even when
unsubstituted. The corresponding helicity may be defined using the sign of a torsion
angle across the non-bonding C
4a
-C
10a
axis, e.g.,
H
), H]}
>
(C
9a
-C
4a
-C
10a
-C
8a
). However,
the direction of the propeller twist of tricyclic moieties usually depends on the twist
of the central double bond and thus is not an independent element of chirality in
overcrowded bistricyclic aromatic enes (notable exceptions are perchlorobifluor-
enylidene [
44
] and bis-dibenzo[
a,i
]fluorenyllidene [
45
]). Thus, it is usually unnec-
essary to designate the propeller twist separately.
The stereodescriptors
E
/
Z
specify the relative positions of the two substituents.
The stereodescriptors
syn
- and
anti
- indicate the relative direction of folding of the
two moieties. The stereodescriptors
R
/
S
denote the direction of folding of tricyclic
moieties. Thus, assigning the absolute configuration
R
/
S
of both moieties includes
some redundancy when combined with the
E
/
Z
- and the
anti
-/
syn
-folding classifi-
cations. Only the eight combinations shown in Fig.
5
are possible. In the combina-
tions
E-anti
and
Z-syn
, the folding of the moieties is necessarily heterochiral, i.e.
,
RS
0
or
SR
0
, while in the combinations
Z-anti
and
E-syn
folding is homochiral, i.e.,
RR
0
or
SS
0
.
τ
2.3 X-Ray Structures
In bistricyclic aromatic enes, a variety of non-planar conformations have been
revealed [
1
-
3
,
28
]. The molecular architecture gives rise to four basic conforma-
tions: twisted (t),
anti
-folded (a),
syn
-folded (s), and folded (f). Each conformation
represents a compromise between
-delocalization (favoring planarity) and
non-planarity due to intramolecular overcrowding. Most of
π
the non-planar
