Lewis or Br?nsted acid-catalysed reaction of propargylic alcohol-tethered alkylidenecyclopropanes with indoles and pyrroles for the preparation of polycyclic compounds tethered with indole or pyrrole motif

Article abstract of DOI:10.1039/c9ob02211b

We developed a facile synthetic method to access cyclopenta[b]naphthalene derivatives via the Lewis or Br?nsted acid catalysed cascade nucleophilic addition, electronic cyclization, ring-opening rearrangement of propargylic alcohol-tethered alkylidenecyclopropanes with indole and pyrrole derivatives. The reaction exhibited a broad substrate scope and good functional group tolerance under metal-free conditions, affording the desired products in moderate to good yields.

Full text of DOI:10.1039/c9ob02211b

Organic &
Biomolecular Chemistry
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Lewis or Brønsted acid-catalysed reaction
of propargylic alcohol-tethered
alkylidenecyclopropanes with indoles and pyrroles
for the preparation of polycyclic compounds
tethered with indole or pyrrole motif†
a,b
Cite this: Org. Biomol. Chem., 2020,
18, 135
Hao-Zhao Wei,a Liu-Zhu Yua and Min Shi
*
We developed a facile synthetic method to access cyclopenta[b]naphthalene derivatives via the Lewis or
Brønsted acid catalysed cascade nucleophilic addition, electronic cyclization, ring-opening rearrange-
ment of propargylic alcohol-tethered alkylidenecyclopropanes with indole and pyrrole derivatives.
The reaction exhibited a broad substrate scope and good functional group tolerance under metal-free
conditions, affording the desired products in moderate to good yields.
Received 12th October 2019,
Accepted 21st November 2019
DOI: 10.1039/c9ob02211b
rsc.li/obc
Lewis/Brønsted acid-catalysed nucleophilic substitution of pro-
pargylic alcohols is essential in organic synthetic chemistry,
Introduction
Cyclopenta[b]naphthalene structural motifs exist in numerous which can be converted into diverse acyclic, carbocyclic, and
natural products1 and functional materials.2 Besides, naphtha- heterocyclic synthetic building blocks.10 To design a novel
lene-based dyes consisting of cyclopenta[b]naphthalene and cascade reaction under mild conditions, we considered apply-
other groups possess desirable photophysical properties such ing indole or pyrrole as a nucleophile to trigger an inter-
as wavelength-dependent emission with changes in microenvi- molecular cascade reaction of propargylic alcohol-tethered
ronment polarity, high quantum yield, and decent photostabi- methylene cyclopropanes.
lity (Scheme 1).3
In 2016, our group reported a thermally induced ring-
Indole and its derivatives are extensively distributed in opening and cyclization reaction from ortho-aminoaryl-teth-
natural products and are general structural motifs in pharma- ered alkylidenecyclopropanes with in situ generated isocya-
ceuticals.4 Their different and productive biological activities
have resulted in their classification as “privileged” heterocycles
in medicinal chemistry.4a,g,5 Methylenecyclopropanes (MCPs)
are a type of highly strained molecule and have been widely
applied in organic synthesis,6 pharmaceutical chemistry,7 agri-
cultural chemistry,8 and even materials science.9 MCPs can
undergo an assortment of reactions, which are facilitated
owing to the release of the intramolecular strain of its small
ring and exocyclic C–C bond. Propargylic alcohols can undergo
various cascade processes to furnish structurally attractive car-
bocycles or heterocycles by forming in situ generated allenes.
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals,
School of Chemistry & Molecular Engineering, East China University of Science
and Technology, 130 Meilong Road, Shanghai 200237, China
bState Key Laboratory of Organometallic Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Ling-Ling Lu, Shanghai, 200032, China. E-mail: mshi@mail.sioc.ac.cn
†Electronic supplementary information (ESI) available: Detailed experimental
procedures and analytical data. CCDC 1584674. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9ob02211b
Scheme 1 Representative examples containing cyclopenta[b]naphtha-
lene skeleton.
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Table 1 Screening of reaction conditions
T
[°C]
Yieldb/2a
[%]
Entrya
Catalyst
Solvent
2a : 3a e
1
2
3
4
5
6
7
8
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
Sc(OTf)3
MeCN
MeCN
MeCN
EA
rt
40
80
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
50
41
32
43
—
22
37
—
40
—
55
—
—
51
63
50
60
57
1.6 : 1
1.6 : 1
1.8 : 1
1.0 : 1
—
1.1 : 1
1.5 : 1
—
1.3 : 1
—
2.0 : 1
—
PhCl
PhMe
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
DMSO
PhMe
PhCl
AgSbF6
9
CF3SO3H
CH3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
CF3COOH
10
11
12
13
14
15
16
17c
18d
—
1.2 : 1
1.9 : 1
1.2 : 1
1.9 : 1
1.4 : 1
PhF
PhCl
PhCl
a Reaction conditions: 1a (0.10 mmol, 1.0 equiv.), indole (0.10 mmol,
1.0 equiv.), catalyst (20 mol%), solvent (1.0 mL), T, for 6 h. b 1H NMR
yield using 1,3,5-trimethoxybenzene as an internal standard.
c Reaction conditions: 1a (0.40 mmol, 0.2 equiv.), indole (0.20 mmol,
1.0 equiv.), catalyst (20 mol%), solvent (2.0 mL). d Reaction conditions:
1a (0.20 mmol, 1.0 equiv.), indole (0.20 mmol, 1.0 equiv.), catalyst
(20 mol%), solvent (2.0 mL), isolated yield. e The ratio of 2a and 3a was
determined by 1H NMR spectroscopic data.
Scheme 2 Previous works and this work.
nates or isothiocyanates to afford furoquinoline and thienoqui-
noline derivatives (Scheme 2, eqn (1) and (2)).6g In 2010, Hu
and co-workers reported a method to construct 4,9-diphenyl-
2,3-dihydro-1H-cyclopenta[b]naphthalene derivatives via the
palladium(0)-catalysed reaction of diynes with aryl halides
through C–C coupling and C–H bond activation of the incor-
porated aryl group (Scheme 2, eqn (3)).11 Based on these pre-
vious findings, herein, we report the Lewis or Brønsted acid
catalysed and pronucleophile-involved [3 + 2] cyclization reac-
tion of propargyl alcohol-tethered methylene cyclopropanes
with the in situ generated allene moiety for the facile synthesis
of cyclopenta[b]naphthalene derivatives under metal-free
conditions.
this reaction in the presence of BF3·OEt2 (Table 1, entries 2
and 3). The examination of solvent effects revealed that aceto-
nitrile is the most suitable solvent for this reaction with
BF3·OEt2 as the catalyst at room temperature (Table 1, entries
4–6). Several other Lewis acids such as Sc(OTf)3 and AgSbF6
were tested in acetonitrile, but no better result was obtained
(Table 1, entries 7 and 8). Next, Brønsted acids such as
trifluoromethylsulfonic acid (TfOH), acetic acid (CH3CO2H)
and trifluoroacetic acid (CF3COOH) were selected as the cata-
lyst for this reaction and we identified that the use of
CF3COOH as the catalyst gave 2a in 55% yield along with 3a as
the by-product (Table 1, entries 9–11). It should be noted that
the acids used as catalysts should have enough acidity to
remove the hydroxyl group to generate the carbocationic inter-
mediate, but too strong Brønsted acid such as TfOH can
Results and discussion
We initially investigated the reaction outcome using 1a as a decompose the MCP group, which will cause the production of
model substrate in the presence of indole (1.0 equiv.) and 2a in lower yield. After screening solvents in the presence of
found that the desired cyclized product 2a was obtained in CF3COOH at room temperature, we found that chlorobenzene
50% yield accompanied with by-product 3a at room tempera- was the solvent of choice, giving 2a in 63% yield (Table 1,
ture in acetonitrile using BF3·OEt2 (20 mol%) as the catalyst entries 12–16). Using 2.0 equiv. of 1a to react with 1.0 equiv. of
(Table 1, entry 1). Then, we evaluated different temperatures indole or carrying out the reaction in a diluted solution in
ranging from 40 °C to 80 °C and identified that room tempera- 2.0 mL of MeCN did not further improve the yield of 2a
ture is the most appropriate temperature in acetonitrile for (Table 1, entries 17 and 18) (for more details on the screening
136 | Org. Biomol. Chem., 2020, 18, 135–139
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ranging from 40% to 46%. When R2 was a methoxyphenyl
group or a heteroaromatic ring, the reaction proceeded
smoothly, producing the desired products 2f and 2g in 30%
and 49% yields, respectively. The electron-donating and elec-
tron-withdrawing substituent could be also introduced at the
benzene ring in the propargylic alcohol moiety, giving the
desired products 2h–2k in moderate to good yields. It should
be noted that 2k was presented as two atropisomers owing to
the rotation obstruction of the C–C bond connecting the
indole moiety to the cyclopenta[b]naphthalene motif. The reac-
tion was sensitive to the R2 group in the presence of indole
(Scheme 3). When substrates 1l (R2 = Me) and 1n (R2
=
4-ClC6H4) were applied for the reaction, traces of the corres-
ponding cyclized products were obtained because these groups
with a lower electron density compared to the phenyl group
would reduce the reactivity of the MCP group. In addition,
Fig. 1 X-ray structures of 2a.
of reaction conditions including other Brønsted acids, see when substrate 1m (R2 = 4-BnOC6H4) was used in the reaction,
Tables S1 and S2 in the ESI†). It should be mentioned here
that the production of 3a could not be avoided due to the two
the BnO group could act as a leaving group in the presence of
CF3COOH, rendering that the target product was not obtained.
competitive addition reactions, which produced 2a and 3a (see Furthermore, when a substrate bearing two methyl groups at
Scheme 6 for the reaction mechanism).12 The structure of 2a
the propargylic moiety was applied for this reaction with
was unambiguously established by X-ray diffraction. The
ORTEP drawing of 2a is shown Fig. 1, and the corresponding strate having one phenyl group and one methyl group at the
indole as the nucleophile, no reaction occurred. For the sub-
CIF data is presented in the ESI.†
propargylic moiety, the corresponding by-product was mainly
obtained. We attribute the results to the property of the carbo-
With the optimized reaction condition in hand, we next sur-
veyed a range of substrates for this reaction. We first turned cationic intermediates. The presence of aryl groups can stabil-
our attention toward the scope of the catalytic cascade cycliza-
tion of propargylic alcohol-tethered alkylidenecyclopropanes
ize the carbocationic intermediate formed in the propargylic
moiety, and thus the process from 1 to intermediates A/B (see
(1b–1n) with indole as the nucleophile, and the results are mechanistic explanation shown in Scheme 6) can proceed,
summarized in Scheme 3. The R1 substituent could be elec-
subsequently improving the regioselectivity.
tron-donating and electron-withdrawing at different positions,
Subsequently, we also examined the substituted indoles in
affording the desired products 2b–2e in moderate yields this reaction using 1a as a substrate and found that similar
results were obtained, affording the desired products 2o–2w in
moderate to good yields ranging from 40%–69%, suggesting
that the electronic property of the substituent at the indole
nucleophile did not have a significant impact on the reaction
outcome (Scheme 4).
Scheme 4 Scope of the catalytic cyclization of propargylic alcohol-
tethered alkylidenecyclopropanes with substituted indoles as nucleo-
philes. Reaction conditions: 1a (0.20 mmol, 1.0 equiv.), substituted
Scheme 3 Scope of the catalytic cyclization of propargylic alcohol-
tethered alkylidenecyclopropanes with indole as a nucleophile. Reaction
conditions: 1 (0.20 mmol, 1.0 equiv.), indole (0.20 mmol, 1.0 equiv.),
CF3COOH (0.2 equiv.), PhCl (2.0 mL) at r.t. for 6 h.
indole (0.20 mmol, 1.0 equiv.), CF3COOH (20 mol%), PhCl (2.0 mL), at
r.t. for 6 h. a Reaction conditions: 1 (0.20 mmol, 1.0 equiv.), substituted
indole (0.20 mmol, 1.0 equiv.), BF3·Et2O (20 mol%), PhCl (2.0 mL), at r.t.
for 8 h.
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two types of resonances of the carbocationic intermediate. The
nucleophilic attack of indole or pyrrole to the 2-position of the
carbocationic intermediate afforded intermediate C, which
underwent 6π-electrocyclization to give intermediate D. Then,
a cyclopropane ring-opening rearrangement took place from
intermediate
D to afford the corresponding product 2.
Meanwhile, the direct nucleophilic substitution of HNu at the
1-position of the carbocationic intermediate gave by-product 3.
Scheme 5 Scope of the catalytic cyclization of propargylic alcohol-
tethered alkylidenecyclopropanes with pyrrole and its derivatives as
nucleophiles. Reaction conditions: 1 (0.20 mmol, 1.0 equiv.), substituted
pyrrole (0.20 mmol, 1.0 equiv.), CF3COOH (20 mol%), PhCl (2.0 mL), at
r.t. for 6 h.
Conclusions
In conclusion, we developed a novel Lewis or Brønsted acid-
catalysed reaction of propargylic alcohol-tethered alkylidene-
cyclopropanes with indole and pyrrole derivatives for the prepa-
ration of cyclopenta[b]naphthalene skeletons tethered with
indole and pyrrole motifs in moderate to excellent yields
under mild conditions. This simple synthetic methodology
avoided the use of transition metal catalysts, providing a
useful tool for the synthesis of polycyclic compounds. Further
exploration on the synthesis of useful polycyclic compounds
with this new synthetic protocol is underway.
Using pyrrole and its derivatives as nucleophiles for this
cascade nucleophilic cyclization reaction, to our delight, the
corresponding products 4a–4f were exclusively obtained in over
90% yield probably because the nucleophilicity of these pyr-
roles is weaker than that of indoles, leading to the nucleophilic
substitution of the hydroxyl group in 1 being impossible
(Scheme 5). Some other nucleophiles including furan, thio-
phene, imidazole, benzimidazole, and benzothiazole were
used for this reaction under the standard conditions, but no
corresponding product was obtained due to their weak
nucleophilicity.
Experimental
General procedures for the preparation of substrates
To
a
stirred solution of iodine-substituted methyl-
Based on these results and the previous reports, a plausible
mechanism for this cascade nucleophilic cyclization of pro-
pargylic alcohol-tethered alkylidenecyclopropanes with pro-
nucleophiles is proposed in Scheme 6.6g,13 Firstly, a carbocationic
intermediate was formed upon treating 1 with a Lewis/
Brønsted acid. The corresponding intermediates A and B are
enecyclopropanes (1.1 equiv.) and propargylic alcohol (1.0
equiv.) in Pr2NH (30 mL), PdCl2(PPh3)2 (2 mol%) and CuI
i
(2 mol%) were added under an argon atmosphere. The result-
ing mixture was stirred at 80 °C for 8 h. After the separation of
ammonium salt by filtration and the removal of solvent under
reduced pressure, the residue was purified by column chrom-
atography on silica gel to afford the corresponding substrates
in good yields ranging from 85% to 98% (for more details, see
chapter 3 in the ESI†).
General procedures for the synthesis of the cyclized products
To a stirred solution of propargylic alcohol-tethered alkylidene-
cyclopropanes (0.2 mmol) and substituted indole (0.2 mmol)
in chlorobenzene (2 mL), catalyst (20 mol%) was added, and
the resulting mixture was stirred at room temperature for 6 h.
After removing the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel
to afford the corresponding cyclized products.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful for the financial support from the National
Basic Research Program of China [(973)-2015CB856603], the
Scheme 6 Plausible reaction mechanism.
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Strategic Priority Research Program of the Chinese Academy of
Sciences (Grant No. XDB20000000) and sioczz201808, the
National Natural Science Foundation of China (220472096,
21372241, 21572052, 20672127, 21421091, 21372250,
21121062, 21302203, 20732008, 21772037, 21772226,
21861132014 and 91956115), and the Fundamental Research
Funds for the Central Universities 222201717003.
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