Chemistry Reference
In-Depth Information
Fig. 9
ORTEP drawings of the crystal structure of
37a
viewed from two different perspectives
with hydrogen atoms omitted for clarity [
37
]. Reprinted with permission from [
37
]. Copyright
(2001) American Chemical Society
The helical twist in
37a
is also manifested by the appearance of a set of AB
1
H
NMR signals with a large geminal coupling constant of 21.0 Hz from the
diastereotopic methylene hydrogen atoms on the five-membered rings. The AB
signals remain well separated and exhibit no line broadening at 125
C, indicating
the rate of helix inversion is relatively slow on the NMR time scale. The activation
barrier is estimated to be at least 19.4 kcal/mol, which is significantly higher than
that of
24
. The buttressing effect from the two fused indeno rings at the 2,3- and 6,7-
positions and the two sterically demanding
tert
-butyl groups at the 1- and
8-positions are responsible for the higher configurational stability. The higher
configurational stability allowed the resolution of a derivative with a hydroxy
group at the
para
position of each of the phenyl substituents. The enantiomers
were found to be configurationally stable at 25
C for at least several days [
41
]. The
13
C NMR spectra of
37a-c
also indicate restricted rotations of the aryl substituents
at ambient temperature.
Treatment of
37a
with LDA followed by paraformaldehyde produced
38
as a
mixture of three diastereomers (Scheme
7
)
42
]. The two consecutive
Wagner-Meerwein rearrangements of
38
were promoted by P
2
O
5
at 110
C, leading
to 17,18-diphenyldibenzo[
a
,
o
]pentaphene (
39
) having an extended conjugation and
bearing the two phenyl groups at the sterically most hindered positions. The two
tert
-butyl groups were removed during the course of the reaction.
The synthetic sequence outlined in Scheme
6
also found success in producing
40
bearing four phenyl substituents (Fig.
10
)[
43
]. The X-ray structure of
40
if viewed
from the direction perpendicular to the helical axis resembles that of a segment of a
spiral staircase with the four phenyl groups as the four parallel steps or that of a
single stranded DNA containing four bases. The two outer phenyl substituents on
rings D and E are twisted toward opposite directions by 184.3
. The diindenophe-
nanthrenyl system is severely distorted with a 29.3
twist angle between the mean
planes of rings A and D and a more pronounced 59.2
twist angle between the mean
planes of rings A and C.
Replacing the aryl substituents at the alkynyl termini in
35
with pyridyl, 2,2
0
-
bipyridyl, and 2,2
0
;6
0
,2
00
-terpyridyl groups led to 4,5-diheteroarylphenanthrene
