Chemistry Reference
In-Depth Information
Pd(OAc)
2
PCy
3
HBF
4
Cs
2
CO
3
DMAc, 130 °C
+
Br
48%
124
-xyl
(
endo
:
exo
=5:1)
2
R
R
R
H
hyperconjugation
H
hyperconjugation
R=Me,OH
endo
-
124
-R
exo
-
124
-R
Scheme 41 Synthesis of xylylsumanene 124-xyl [
141
,
142
]
successfully obtained in 48% yield (Scheme
41
). Unlike 117 (E
SiMe
3
), 124-xyl
was obtained as a mixture of
endo
- and
exo
-conformers in a 5:1 ratio.
A recent study indicated that the stereoelectronic effect of a curved aromatic
structure dominates the stability of
endo
/
exo
-124-R (Scheme
41
)[
142
]. Steric
repulsion is the main factor to facilitate the conformer
exo
-124-SiMe
3
, whereas
some mono-substituted sumanenes, such as methylsumanene 124-Me, hydroxy-
sumanene 124-OH, and 2,6-xylylsumane 124-xyl, favor formation of the stable
endo
-conformation. In methylsumanene and hydroxysumanene, the hyper-
conjugation between the benzylic proton and the sumanenyl convex face causes
the
endo
conformer to be more stable than the
exo
conformer. In 124-xyl, the CH-
¼
π
interaction, which is caused by the methyl group in xylyl substituent and the concave
face of sumanene, favors the
endo
-conformer.
Mono-substituted sumanenes 125 can easily be obtained by electrophilic aro-
matic substitution reaction [
143
]. However, this protocol is unsuitable for prepa-
ration of di- and trisubstituted sumanenes due to the low regioselectivity.
Trisubstituted sumanenes, such as
C
3
symmetric triformylsumanene and its deri-
vatives, were synthesized regioselectively by using suitable reaction intermediates
syn
-tri(norborneno)benzenes (cf. Scheme
36
)[
144
].
E
E=I,75%
NO
2
,65%
CHO, 60%
COCH
3
,64%
COPh, 68%
125
The enantioselective synthesis of a chiral buckybowl was reported by Sakurai
and Higashibayashi in 2008 [
145
]. As in the synthesis of sumanene,
C
3
symmetric
syn
-tris(norborneno)-benzene 126 is the key intermediate in this synthetic approach
(Scheme
42
). The synthesis started with enantiopure iodonorbornanone. The
