Heat Versus Basic Conditions: Intramolecular Dehydro-Diels-Alder Reaction of 1-Indolyl-1,6-heptadiynes for the Selective Synthesis of Substituted Carbazoles

Article abstract of DOI:10.1055/s-0036-1588461

The intramolecular dehydro-Diels-Alder reaction of 1-indolyl-1,6-heptadiynes proceeds smoothly under rather mild heating conditions to give substituted carbazoles in moderate to good yields. The reaction of 7-aryl-1-indolyl-1,6-heptadiynes under heating gives the corresponding carbazoles chemoselectively in high yields, whereas the reaction under basic conditions gives naphthalenes as major products.

Full text of DOI:10.1055/s-0036-1588461

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
T. Kudoh et al.
Letter
Synlett
Heat Versus Basic Conditions: Intramolecular Dehydro-Diels–Alder
Reaction of 1-Indolyl-1,6-heptadiynes for the Selective Synthesis
of Substituted Carbazoles
Takayuki Kudoh
O
O
O
Syo Fujisawa
Megumi Kitamura
Akira Sakakura*
Base
Heat
N
Bn
N
N
Graduate School of Natural Science and Technology, Okayama
University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530,
Japan
18 examples
up to 87% yield
Bn
Naphthalenes
Bn
R
R
Carbazoles
R
sakakura@okayama-u.ac.jp
Heat versus Base: Chemoselective Dehydro-Diels–Alder Reaction
Received: 01.05.2017
Accepted after revision: 19.05.2017
Published online: 06.07.2017
same naphthalenes.7 This reaction involves cyclic allenes8
as reactive intermediates that rapidly undergo hydrogen
migration to reduce their ring strain. Since 1-aryl-1,6-hep-
tadiynes are easily synthesized by means of Sonogashira
coupling,9 these dehydro-Diels–Alder reactions are useful
for the synthesis of substituted naphthalenes.
In addition to being useful for the synthesis of naphtha-
lenes, the intramolecular dehydro-Diels–Alder reaction of
1-aryl-1,6-heptadiynes is expected to be useful for the syn-
thesis of other condensed aromatic compounds. In this
study, we investigated the synthesis of substituted carba-
zoles by the dehydro-Diels–Alder reaction of 1-indolyl-1,6-
heptadiynes. Scheme 1 shows our plan for the synthesis of
substituted carbazoles 2 and 5. 1-Substituted carbazoles 2
might be synthesized by the intramolecular dehydro-Diels–
Alder reaction of 1-(indol-3-yl)-1,6-diynes 1, whereas the
reaction of 1-(indol-2-yl)-1,6-diynes 4 should give carba-
zoles 5 bearing a substituent at the 4-position.
We first examined the reaction conditions for the dehy-
dro-Diels–Alder reaction of 1-(indol-3-yl)-1,6-diyne 1a10
for the synthesis of 1-substituted carbazole 2a (Scheme 2).
When the reaction of 1a was conducted under basic condi-
tions (Triton B, DMSO, r.t.), substituted naphthalene 3a was
obtained in 84% yield, and the desired carbazole 2a was not
formed. This result showed that Triton B selectively ab-
stracts proton Hb rather than proton Ha to promote the an-
ionic Diels–Alder reaction4 of 1a, in which the alkynylben-
zene moiety acts as a diene and the alkynylindole acts as a
dienophile. In sharp contrast, the reaction of 1a under heat-
ing (toluene, 100 °C) gave the desired carbazole 2a in 36%
yield, while 3a was obtained in 7% yield. It is conceivable
that, under heating, the reaction of 1a proceeds by a con-
certed pathway, and the high HOMO level of the alkynylin-
dole π-system permits the alkynylindole moiety to act as a
diene to give 2a as the major product. The low yield of 2a is
likely to have resulted from the oxidative decomposition of
DOI: 10.1055/s-0036-1588461; Art ID: st-2017-u0315-l
Abstract The intramolecular dehydro-Diels–Alder reaction of 1-indo-
lyl-1,6-heptadiynes proceeds smoothly under rather mild heating con-
ditions to give substituted carbazoles in moderate to good yields. The
reaction of 7-aryl-1-indolyl-1,6-heptadiynes under heating gives the
corresponding carbazoles chemoselectively in high yields, whereas the
reaction under basic conditions gives naphthalenes as major products.
Key words carbazoles, indoles, naphthalenes, dehydro-Diels–Alder re-
action, intramolecular Diels–Alder reaction, chemoselectivity
The carbazole skeleton is an important nitrogen-con-
taining heteroaromatic moiety that is widely found in natu-
ral products, pharmaceuticals, agrochemicals, and other
functional molecules.1 Although several methods for the
synthesis of carbazoles have been developed,2 new methods
for this synthesis are needed.
The Diels–Alder reaction is a powerful tool for the stereo-
selective synthesis of carbocyclic compounds. This reaction
has some synthetically useful variants. For example, when
two double bonds in the substrates are replaced by triple
bonds, the reaction gives aromatic compounds. The dehy-
dro-Diels–Alder reaction,3 a dehydrogenative variant of the
Diels–Alder reaction of an arylalkyne and an alkyne, is es-
pecially useful because it gives condensed aromatic com-
pounds (Scheme 1). For example, when 1-aryl-1,6-hepta-
diynes are treated with a base such as Triton B or t-BuOK,
the corresponding 1-substituted naphthalenes are ob-
tained.4–6 These reactions involve aryl-substituted bisal-
lenes or allenyl anions as reactive intermediates generated
by the abstraction of propargylic protons, and intramolecu-
lar cyclization proceeds under rather mild conditions. Alter-
natively, heating can promote the intramolecular dehydro-
Diels–Alder reaction of 1-aryl-1,6-heptadiynes to give the
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
T. Kudoh et al.
Letter
Synlett
Diels–Alder Reaction of 1-Aryl-1,6-heptadiynes for the Synthesis of Condensed Aromatics
X
C
X
C
Basic conditions
concerted cyclization
or
stepwise cyclization via
biradical intermediates
C
base-promoted
isomerization
or
X
6
1
X
R'
R'
OMe
MeO
R
R
(concerted) cyclization
hydrogen migration
C
R
1-aryl-1,6-heptadiynes
substituted condensed aromatics
X
R'
Heat conditions
H
Plan for the Synthesis of Substituted Carbazoles 2 and 5
R
2
R
4
X
X
3
X
X
N
N
N
1
N
R'
R
R'
R
R'
R'
1-(indol-3-yl)-1,6-heptadiynes 1
2
5
1-(indol-2-yl)-1,6-heptadiynes 4
Scheme 1 Dehydro-Diels–Alder reaction of 1-(indol-3-yl)-1,6-heptadiynes for the synthesis of 1- and 4-substituted carbazoles 2 and 5
1a. When the reaction was conducted in the presence of hy-
droquinone (0.4 equiv) in degassed toluene, the yield of 2a
was dramatically improved to 77%.11,12
Having determined the optimized conditions for the se-
lective synthesis of carbazoles 2, we next examined the
substrate scope and limitations of the intramolecular dehy-
ties of 1a and 1b, which have electron-donating groups on
the terminal aromatic ring, were not very high, and signifi-
cant amounts of the corresponding naphthalenes 3 were
obtained as byproducts (Scheme 1 and Table 1, entry 1).
These results might be attributable to the rather high LUMO
level of the alkynylbenzene moiety of 1a and 1b. On the
other hand, substrates 1d and 1e, bearing aromatic rings
with electron-withdrawing substituents, showed high reac-
tivities and gave the corresponding carbazoles 2d and 2e
exclusively in yields of 87 and 86% (entries 3 and 4). The re-
action of the nitro-substituted substrate 1e proceeded es-
pecially rapidly (entry 4). The rather low LUMO levels of the
alkynylbenzene moieties of 1d and 1e should accelerate the
desired reaction and suppress the generation of naphtha-
lenes 3d and 3e. The dehydro-Diels–Alder reaction of the 2-
furyl substrate 1f gave a significant amount of the substi-
tuted benzofuran 3f as a byproduct, although the reactivity
of 1f was high (entry 5). The N-Methyl derivative 1g
showed better reactivity than 1a (entry 6), probably be-
cause there was less steric repulsion between the methyl
group and the terminal 4-methoxyphenyl group. On the
other hand, the N-benzoyl derivative 1h showed poor reac-
tivity, giving the corresponding carbazole 2h in only 5%
yield (entry 7). The electron-withdrawing nature of the
benzoyl group renders the 3-alkynylindole moiety electron
deficient and, therefore, less reactive. The dehydro-Diels–
Alder reaction of the methoxycarbonyl substrate 1i went to
completion in only four hours, and gave the desired carba-
zole 2i in 83% yield (entry 8). The high electron-withdraw-
ing nature of the methoxycarbonyl group probably reduces
the LUMO level of the alkyne to accelerate the reaction. In
contrast, the reactivities of 1j (R1 = H; R2 = Bn) and 1k [R1 =
dro-Diels–Alder reaction of 1-(indol-3-yl)-1,6-diynes
1
with various substituents on the alkyne terminals (Table 1).
The reaction of 1b, bearing a dimethylamino group, gave
the corresponding carbazole 2b in 77% yield, along with
naphthalene 3b in 15% yield (Table 1, entry 1). The reactivi-
Ha
O
Hb
conditions
3
N
Bn
1a
OMe
O
O
1
+
N
Bn
N
Bn
2a
3a
OMe
OMe
Triton B, DMSO, r.t.
toluene, 100 °C, 72 h
0%
84%
7%
36%
hydroquinone (0.4 equiv),
toluene (degassed),
100 °C, 80 h
77%
16%
Scheme 2 Basic conditions versus heating conditions for the dehydro-
Diels–Alder reaction of the 1-(indol-3-yl)-1,6-diyne 1a
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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T. Kudoh et al.
Letter
Synlett
Table 1 Dehydro Diels–Alder Reactions of 1-(indol-3-yl)-1,6-diynes 1a
O
O
O
hydroquinone
+
3
1
toluene, 100 °C
R
R
R
N
N
N
R'
R'
R'
1
2
3
Entry
Substrate
R1
R2
Bn
Time (h)
2 [Yield (%)]
3 [Yield (%)]
1
2
1b
1c
1d
1e
1f
4-Me2NC6H4
Ph
96
36
72
6
2b (77)
2c (83)
2d (86)
2e (87)
2f (50)
2g (73)
2h (5)
3b (15)
3c (8)
3d (0)
3e (0)
3f (34)
3g (7)
3h (11)
–
Bn
Bn
Bn
Bn
Me
Bz
3
4-NCC6H4
4-O2NC6H4
2-furyl
4
5
8
6
1g
1h
1i
4-MeOC6H4
4-MeOC6H4
CO2Me
72
24
4
7
8
Bn
Bn
Bn
2i (83)
2j (59)
2k (0)
9b
10b
1j
H
30
30
–
1k
(CH2)4Me
–
a The reaction was conducted in the presence of hydroquinone (0.4 equiv) in degassed toluene at 100 °C.
b The reaction was conducted at 150 °C.
Table 2 Dehydro-Diels–Alder Reactions of 1-(indol-2-yl)-1,6-diynes 4a
(CH2)4Me; R2 = Bn] were too low for the reaction to proceed
at 100 °C. When the reaction of 1j was conducted at 150 °C,
the corresponding carbazole 2j was obtained in 59% yield
(entry 9). However, the reaction of 1k at 150 °C resulted in
the decomposition of 1k, and did not give the desired 2k
(entry 10).
R
O
hydroquinone
2
toluene, 100 °C
N
Bn
4
R
The dehydro-Diels–Alder reaction of 1-(indol-2-yl)-1,6-
heptadiynes 4 was also investigated for the synthesis of 4-
substituted carbazoles 5 (Table 2), and the reactivities of
these substrates were compared with those of diynes 1. The
reactions of 4a, 4c, and 4d gave the corresponding 4-substi-
tuted carbazoles 5a, 5c, and 5d as major products in good
yields (Table 2, entries 1–3). The reactivities of these diynes
were higher than those of the corresponding 1-(indol-3-yl)-
1,6-heptadiynes 1. In particular, substrate 4d, with an elec-
tron-withdrawing cyano group, showed a much higher re-
activity than did 1d (Table 2, entry 3 versus Table 1, entry
3). The reactivity of 4f bearing a 2-furyl group was similar
to that of 1f (Table 2, entry 4 versus Table 1, entry 5). The
reaction of 4f gave carbazole 5f (41%) and naphthalene 6f
(37%) in similar yields. 7-Methoxycarbonyl-substituted 1,6-
heptadiyne 4i showed an excellent reactivity, similar to that
of 1i, and gave 5i in 82% yield within three hours (entry 5).
As in the case of 1j and 1k, the reactivities of 7-unsubstitut-
ed diyne 4j and 7-pentyl-substituted diyne 4k were poor,
so their dehydro-Diels–Alder reactions were conducted at
150 °C (entries 6 and 7). Notably, the reaction of 4k gave
the desired 4-alkyl-substituted carbazole 5k, although the
R
O
4
O
+
N
N
Bn
Bn
5
6
Entry Substrate
R
Time (h) 5 [Yield (%)]
6 [Yield (%)]
1
4a
4c
4d
4f
4-MeOC6H4
Ph
48
30
12
7
5a (60)
5c (82)
5d (82)
5f (41)
5i (82)
2j (68)
5k (11)
6a (16)
6c (7)
6d (0)
6f (37)
–
2
3
4-NCC6H4
2-furyl
CO2Me
H
4
5
4i
3
6b
7b
4j
30
30
–
4k
pentyl
–
a The reaction was conducted in the presence of hydroquinone (1 equiv) in
degassed toluene at 100 °C.
b The reaction was conducted at 150 °C.
yield was poor (11%; entry 7), whereas the reaction of 1k
did not give the desired product under the same reaction
conditions (Table 1, entry 10).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
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Letter
Synlett
Next, we examined the effect of the linker moiety on
the reactivity and chemoselectivity (Scheme 3). The reac-
tion of 1-(indol-2-yl)-1,6-heptadiyne 7 with a (2-nitroben-
zene)sulfonamide linker gave carbazole 8 (61%) along with
naphthalene 9 (34%). The low chemoselectivity (8/9) might
be attributed to the electron-withdrawing nature of the (2-
nitrobenzene)sulfonamide linker, which might render the
2-alkynylindole electron deficient. Despite the electron-
withdrawing nature of its linker, diyne 7 showed a similar
reactivity to that of 4a, probably because of the Thorpe–In-
gold effect.13 The reactivity of the 1-(indol-3-yl)-1,6-hepta-
diyne 10 with a sulfide linker was poor, and the corre-
sponding carbazole 11 (21%) and naphthalene 12 (24%)
were obtained in similar yields. This poor result might re-
sult from the greater length of the C–S bond in the linker
compared with that of the C–O bond in 1a, or from the sen-
sitivity of the sulfur linker to oxidation.
give a mixture of the desired lactones 13 and 14 in 36%
yield. The oxidation occurred preferentially at the less-hin-
dered site to give 13 as a major product.
O
O
O
CrO3 (12 mol%)
H5IO6 (2 equiv)
O
+
2a
MeCN, r.t.
N
N
Bn
Bn
OMe
OMe
13
14
36% (13/14 10:1)
Scheme 4 Benzylic oxidation of carbazole 2a
In conclusion, we have developed a simple method for
the synthesis of substituted carbazoles by the intramolecu-
lar dehydro-Diels–Alder reaction of 1-indolyl-1,6-hepta-
diynes, which proceeded under rather mild heating (100 to
150 °C). The high reactivity is due to the high HOMO level of
the alkynylindole moiety, which acted as a diene. This
method was applied to the synthesis of 1-substituted car-
bazoles 2 from 1-(indol-3-yl)-1,6-heptadiynes 1 and 4-sub-
stituted carbazoles 5 from 1-(indol-2-yl)-1,6-heptadiynes 4,
with various substituents on the alkyne terminal.
MeO
NNs
hydroquinone
toluene, 100 °C, 48 h
2
N
7
Bn
MeO
MeO
Funding Information
NNs
4
NNs
+
This project was supported in part by JSPS KAKENHI (26410120)
a
Jp
a
n
S
o
ecity
o
f
r
hte
P
or
m
o
itn
of
S
ecin
c
e
2(
6
4
1
0
1
2
0)'
N
Bn
N
8, 61%
9, 34%
Bn
Acknowledgment
S
The authors gratefully thank the Division of Instrumental Analysis,
Department of Instrumental Analysis and Cryogenics, Advanced Sci-
ence Research Center, Okayama University, for NMR and HRMS mea-
surements.
hydroquinone
3
toluene
150 °C, 72 h
N
Bn
Supporting Information
OMe
10
S
S
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1588461.
S
u
p
p
o
nrtogI
f
rmoaitn
S
u
p
p
ortiInfogrmoaitn
1
+
N
Bn
References and Notes
N
Bn
OMe
OMe
11, 21%
12, 24%
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Scheme 3 Dehydro Diels–Alder Reaction of 1-Indolyl-1,6-diynes 7 and
10 with Nitrogen and Sulfur Linkers
Benzylic oxidation of the dihydrofuran moiety of carba-
zoles 2 and 5 should give the corresponding lactones, which
were expected to be useful precursors for transformation
into various functional groups. We therefore examined the
oxidation of carbazole 2a (Scheme 4). By following Yamaza-
ki’s procedure,14 the oxidation of 2a was conducted with a
catalytic amount of CrO3 and two equivalents of H5IO6 to
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

E
T. Kudoh et al.
Letter
Synlett
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dure
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Hydroquinone (4.4 mg, 0.044 mmol) was added to a solution of
diyne 1a (44 mg, 0.11 mmol) in degassed toluene (1 mL) in a 12
mL glass pressure vessel. The mixture was stirred at 100 °C for
80 h then cooled to r.t. The solvent was removed under reduced
pressure, and the residue was purified by column chromatogra-
phy [silica gel, toluene–EtOAc (10:1)] to give carbazole 2a and
naphthalene 3a.
2a: Colorless solid; yield: 35.2 mg (0.086 mmol, 77%); mp
35.5 °C. 1H NMR (300 MHz, CDCl3): δ = 8.12 (d, J = 7.8 Hz, 1 H),
7.96 (s, 1 H), 7.4−7.1 (m, 6 H), 7.02 (d, J = 8.8 Hz, 2 H), 6.74 (d, J =
8.8 Hz, 2 H), 6.6–6.5 (m, 2 H), 5.33 (s, 2 H), 5.10 (s, 2 H), 4.88 (s,
2 H), 3.82 (s, 3 H).
3a: Colorless solid; yield: 7.3 mg (0.018 mmol, 16%); mp 66.9 °C.
1H NMR (300 MHz, CDCl3): δ = 7.78 (d, J = 8.9 Hz, 1 H), 7.63 (s, 1
H), 7.44 (d, J = 8.4 Hz, 1 H), 7.4−7.0 (m, 11 H), 5.47 (d, J = 16.0 Hz,
1 H), 5.42 (d, J = 16.0 Hz, 1 H), 5.29 (s, 2 H), 5.05 (s, 2 H), 3.60 (s,
3 H).
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the absence of hydroquinone, 2a was obtained in 51% yield,
along with 3a in 9% yield.
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