Chemistry Reference
In-Depth Information
Disubstitution (one substituent on each tricyclic moiety) gives rise to
E
- and
Z
-
stereoisomers. It should be noted that the systematic nomenclature gives substitu-
ents on both moieties the lowest possible number, irrespective of their
E
-or
Z
-
relationship. Stereoisomers are distinguished by prefixes (
E
)- and (
Z
)-. Thus atoms
1 and 1
0
are
cis
in
Z
-isomers, but
trans
in
E
-isomers. In order to maintain a
consistent atom-labeling scheme for unsubstituted BAEs, as well as for
E
- and
for
Z
-disubstituted BAEs, this rule will be disregarded in the discussions of the
conformations and dynamic stereochemistry below. The labeled general structure
shown in Fig.
1
will be considered as the rigid molecular framework (see above)
and the
cis
relationship of positions 1 and 1
0
, etc. in the reference version of the
framework will be maintained regardless of the substitution pattern and
E
,
Z
-isom-
erizations.
E
-isomers thus have their substituents, e.g., at carbon atoms 2 and 7
0
.
Furthermore, the orientation of the molecular framework (before the application of
permutations or permutation-inversions) will always be kept fixed as in Fig.
1
with
positions 1 and 1
0
on the left-hand-side (negative
y
-axis direction) and the unprimed
moiety with bridge X at the top (positive
z
-axis direction). Thus, an alternative
orientation of the molecule in space (rotated by 180
about the
z
-axis) may be
defined by placing substituents at positions 7 and 2
0
.
4.1.2 Conformational Types
The following classification scheme of the types of conformations of overcrowded
bistricyclic aromatic enes may be adopted:
Planar conformations, p
Orthogonally twisted conformations, t
⊥
Twisted conformations, t
anti
-Folded conformations, a
syn
-Folded conformations, s
Folded conformations with a planar (or nearly planar) second moiety, f
Twisted/
anti
-folded conformations, ta
Twisted/
syn
-folded conformations, ts
Twisted/folded conformations, tf
anti
-Folded/twisted conformations, at
anti
-Folded conformations with unequal degrees of folding, au
syn
-Folded/twisted conformations, st
syn
-Folded conformations with unequal degrees of folding, su
Folded/twisted conformations, ft
Planar central ethylene group with propeller twisted tricyclic moieties, pt
The basic conformational types are schematically illustrated in Fig.
12
. Here the
circle and its center point represent the double bond in a Newman-like projection.
The lines represent the tricyclic moieties by indicating the positions 1, 8, 1
0
, and
8
0
in the overcrowded fjord regions.
