A facile synthesis of homotriptycenes from anthranol derivatives

Article abstract of DOI:10.1055/s-2005-921929

Substituted trans-10-benzyl-9-anthranols 5a,b and substituted 10, 10-dibenzyl-9-anthranol 8e undergo intramolecular cyclization in the presence of formic or oxalic acid to give homotriptycenes 9a,b,e. Depending on the amount of acid used, a competitive 1,

Full text of DOI:10.1055/s-2005-921929

LETTER
3166
A Facile Synthesis of Homotriptycenes from Anthranol Derivatives
Synthesis of
H
omo
e
triptycenes
r
f
rom
A
o
nthranol
D
er
n
ivatives g Cao,*a,b Chunmei Gao,a,c Herbert Meierd
a
LCLC, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, P. R. of China
b
College of Chemistry, South China University of Technology, Guangzhou 510641, P. R. of China
Fax +86(20)85232903; E-mail: caodr@gic.ac.cn
c
Graduate School of CAS, Beijing 100039, P. R. of China
d
Institute of Organic Chemistry, University of Mainz, 55099 Mainz, Germany
Received 26 August 2005
lished by a 2D NOESY NMR study. The proton 9-H
shows a cross-peak to the enantiotopic protons of the CH2
group, but not to the proton 10-H.
Abstract: Substituted trans-10-benzyl-9-anthranols 5a,b and sub-
stituted 10,10-dibenzyl-9-anthranol 8e undergo intramolecular cy-
clization in the presence of formic or oxalic acid to give
homotriptycenes 9a,b,e. Depending on the amount of acid used, a
competitive 1,4-dehydration to anthracene derivatives 10a,b was
observed for 5a,b. The latter process was the only reaction pathway
for anthranols that do not possess electron-donating substituents on
benzyl moiety (5c,d → 10c,d).
Alternatively, 10,10-disubstituted-9-dihydro-9-anthranol
8e was obtained by bisalkylation of anthrone 1¢ with 6e to
generate 7e (50%). Subsequent reduction of the carbonyl
group by NaBH4 then gave 8e in 94% yield.
Treatment of the mono- or dialkylated anthranol deriva-
tives 5a–d and 8e, respectively, with formic or oxalic acid
led to homotriptycenes 9a,b,e9 and/or to the anthracenes
10a–d. Protonation of the hydroxy group leads to a carbe-
nium ion which then acts as an electrophile to promote the
electrophilic aromatic substitution of the benzene ring of
the benzyl moiety. This route 5 → 9 represents formally a
1,7-elimination of H2O, which competes with a 1,4-elim-
ination of H2O. The latter process furnishes the an-
thracene derivatives 10. Whereas the 1,7-elimination
pathway is an almost quantitative process for 5a and 5b,
it does not work for compounds 5c and 5d, which do not
contain activated electron-donating substituents at the
correct positions in the benzene ring. Therefore 5c and 5d
underwent the 1,4-elimination to give 10c and 10d, re-
spectively.
Key words: aromatization, condensation, electrophilic substitu-
tion, transannular ring closure
Triptycene and its derivatives have attracted great atten-
tion because of (1) their rigid aromatic three-dimensional
structure and conformational properties,1 (2) unique elec-
trochemical and photochemical properties,2 (3) potential
pharmaceutical properties,3 and (4) applications in mate-
rials science and supramolecular chemistry.4 However,
only a few examples for the synthesis of homotriptycenes
have been reported. Cristol5 synthesized homotriptycene
and its derivatives by ring enlargement of 1-aminoethyl-
triptycene, but the route was lengthy and required a te-
dious separation from the concomitant isomers. Szeimies6
reported an interesting route for the preparation of homo-
triptycene using a thermal dehydrogenation of annulated
dibenzohomobarrelene, but this method has limited appli-
cations, because a multi-step synthesis was required for
the starting propellane and there was also a limited varia-
tion of substituents that could be introduced into the ho-
motriptycene skeleton. Saito7 also reported an alternative
method using the cycloaddition of the strained benzocy-
clopropene to anthracenes. We now describe a simple
route for the preparation of homotriptycenes from 9-an-
thranols (Scheme 1, Table 1).
The 10,10-dibenzyl substituted compound 8e, on the other
hand, does not have the possibility of undergoing 1,4-
elimination and thus homotriptycene 9e was the exclusive
reaction product upon treatment with acids.
We also observed that the reaction rates were also depen-
dent upon the amount of acid for the 1,4-elimination
(Table 2). Table 2 demonstrates that the ratio of 9:10 be-
comes higher with increasing amount of formic acid. Ob-
viously, formic acid is involved in the rate determining
step of the 1,7-elimination, whereas this is not the case, or
at least to a minor extent (medium effect), for the 1,4-
elimination.
9-Anthranol 1, which is in equilibrium with its tautomer
anthrone 1¢, reacted with substituted benzaldehydes 2a–d
to give 10-benzylidene-9-anthrones 3a–d in pyridine–pi-
peridine.8 Catalytic hydrogenation (5% Pd/C) of 3a–d af-
forded 10-benzyl-9-anthrones 4a–d in 58–90% yield.
Subsequent reduction of 4 with NaBH4 then produced 10-
benzyl-9,10-dihydro-9-anthranols 5a–d in 87–92% yield.
The trans-configuration of compounds 5a–d was estab-
The structural identities of compounds 5, 8, 9 and 10 were
1
established by one- and two-dimensional H NMR and
13C NMR spectroscopy. The characteristic data were sum-
marized in Table 3 and Table 4.
Since the starting materials 5 or 8 are easily accessible,
this reported facile preparation of homotriptycenes 9 is a
more convenient route than other reported procedures.
The final ring closure described here is related to a reac-
tion that was found in an addition product between anthra-
nol with lignin model quinone methides.10
SYNLETT 2005, No. 20, pp 3166–3168
1
9
.1
2
.2
0
0
5
Advanced online publication: 28.11.2005
DOI: 10.1055/s-2005-921929; Art ID: W02605ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Homotriptycenes from Anthranol Derivatives
3167
OH
R1
R1
R1
R1
R2
R1
R2
R1
R2
R1
R2
R1
1
CHO
2a–d
H2
Pd/C
NaBH4
O
O
O
OH
5a–d
R1
3a–d
R1
4a–d
R2
R1
CH2Cl
R1
R1
1`
R2
R1
R2
R1
R2
R1
R2
R1
R1
6e
NaBH4
OH
O
8e
7e
R1
R1
R2
R1
R2
R3
2R1
9
H+
+
5a–d, 8e
15
1
16
9a,b,e
10a–d
Scheme 1
Table 1 Preparation of Homotriptycenes from 9-Anthranols
R1
R2
R3
Yield of 9 (%)a
Mp (°C)
165
Yield of 10 (%)a Mp (°C)
a
b
c
OMe
H
H
96
96
–
4
4
133
107
140
133
OMe
OMe
OMe
H
H
174
H
H
100
100
–
d
e
H
H
–
Benzyloxy
H
3,5-Dibenzyloxybenzyl
100
198
Table 2 Product Distribution of the Acid-Catalyzed Dehydration of
5a in CH2Cl2 at Room Temperature
Acknowledgment
We are grateful to the National Natural Science Foundation of
China (20272059; 20572111), the Guangdong Natural Science
Foundation of China (04002263), and the Deutsche Forschungs-
gemeinschaft for financial support.
Entry Molar ratio
Time
(h)
Conversion
(%)
Ratio of
of 5a:HCOOH
9a:10aa
1
2
3
4
1:0.67
1:2
2
100
100
100
100
64:36
67:33
92:8
2
References
1:12
1:56
0.5
0.5
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Synlett 2005, No. 20, 3166–3168 © Thieme Stuttgart · New York

3168
D. Cao et al.
LETTER
a
Table 3 Selected 1H NMR and 13C NMR Data of Compounds 5a–d and 8e in CDCl3
Compd
9-H
10-H
a-CH2
OH
C-9
C-10
a-CH2
5a–d
4.90 0.13
4.25 0.03
2.91 0.03
1.99 0.04
66.8 0.1
48.2 0.3
45.5 0.7
3J = 10.8 Hz
3J = 6.3 Hz
3J = 6.3 Hz
3J = 10.8 Hz
8e
5.01
3.61, 3.40
0.32
67.5
49.0
52.4, 48.5
3J = 11.6 Hz
3J = 11.6 Hz
a TMS as internal standard.
a
Table 4 Selected 1H NMR and 13C NMR Data of Homotriptycenes 9a,b,e in CDCl3
Compd 1-H
9-H
4.19b
4.21c
–
a-CH2
3.21b
OCH3 or OCH2
3.63, 3.89
C-1
C-9
a-CH2
37.4
OCH3 or OCH2
55.1, 56.1
9a
9b
9e
5.69
5.55
5.84
41.9
43.4
44.7
45.6
45.6
46.9
3.21c
3.67, 3.81, 4.02
4.65, 4.83, 5.11
36.9
55.8, 60.8, 61.8
69.8, 70.0, 70.7
3.03, 3.90
40.1, 42.6
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Synlett 2005, No. 20, 3166–3168 © Thieme Stuttgart · New York