Lewis acid catalyzed [3+2] coupling of indoles with quinone monoacetals or quinone imine ketal

Article abstract of DOI:10.1002/ejoc.201402490

The one-pot synthesis of benzofuroindolines and tetrahydroindolo[2,3-b] indoles was accomplished through a mild and concise [3+2] coupling of indoles and quinone monoacetals or quinone imine ketal promoted by a Lewis acid. A wide variety of benzofuroindolines and tetrahydroindolo[2,3-b]indoles were prepared in moderate to good yields. Copyright

Full text of DOI:10.1002/ejoc.201402490

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201402490
Lewis Acid Catalyzed [3+2] Coupling of Indoles with Quinone Monoacetals or
Quinone Imine Ketal
Chang Shu,[a,b] Li-Hua Liao,[a,b] Yi-Jun Liao,[a,b] Xiao-Yan Hu,[a,b] Yong-Hong Zhang,[a,b]
Wei-Cheng Yuan,[a] and Xiao-Mei Zhang*[a]
Keywords: Nitrogen heterocycles / Fused-ring systems / C–C coupling / Lewis acids / Acetals
The one-pot synthesis of benzofuroindolines and tetra- als or quinone imine ketal promoted by a Lewis acid. A wide
hydroindolo[2,3-b]indoles was accomplished through a mild
variety of benzofuroindolines and tetrahydroindolo[2,3-
and concise [3+2] coupling of indoles and quinone monoacet- b]indoles were prepared in moderate to good yields.
Introduction
Fused indolines are important building blocks for a large
number of natural products and pharmaceuticals.[1] As a
consequence, the synthesis of fused indolines such as
pyrroloindolines,[2] furoindolines,[2c,2d,2j,2q,3] benzofuro-
indolines,[4] tetrahydroindolo [2,3-b]indoles,[5] and other
types of fused indolines[6] has attracted considerable atten-
tion, and diverse strategies for their synthesis have been de-
veloped. The benzofuroindoline core is a unique motif
Figure 1. Representative benzofuroindoline natural products.
found in many important natural alkaloids,[1e,1g,4] such as
diazonamides[4a–4d] and azonazine[4e,4f] (Figure 1). Given
that diazonamide A has been found to be a very potent an-
ted by several groups.[4o–4w] For example, in 2011, Chen and
ticancer agent owing to its high antitumor activity (IC50
Ͻ 5 nm),[4a,4b] it has received considerable attention from
chemists to synthesize these benzofuroindoline skel-
etons.[4g–4l] The prevalence of these indole-derived heterocy-
clic frameworks continues to inspire the development of
new reactions for their construction.
co-workers employed a Brønsted acid catalyzed direct
[3+2] coupling of β-carbolines with quinones as a key strat-
egy in the formal synthesis of (+)-haplophytine (Scheme 1,
c).[4w]
Quinone monoacetals and quinone imine ketals have at-
tracted much attention owing to their particular reactivities
and broad utilities in organic synthesis.[7,8] In 1994, Swen-
ton et al. employed acetic acid to catalyze the [3+2] cou-
plings of quinone monoacetals with vinyl thiol.[8a] Recently,
Kita et al. presented the [3+2] couplings of quinone
monoacetals with various alkenes promoted by Brønsted
acids.[8b,8c] Inspired by these reports and the work of
Chen[4w] on the [3+2] coupling of β-carbolines with quin-
ones, we envisioned that, through dearomatization,[9] 3-sub-
stituted indoles might undergo [3+2] coupling with quinone
monoacetals or quinone imine ketals to generate benzo-
furoindolines and tetrahydroindolo[2,3-b]indoles in one pot.
Herein, we present the Lewis acid promoted [3+2] coupling
of 3-substituted indoles with quinone monoacetals or quin-
one imine ketals. Through this mild and concise transfor-
mation, a wide variety of benzofuroindolines and tetra-
hydroindolo[2,3-b]indoles were prepared in moderate to
good yields (Scheme 1, d), which thus provides a good ex-
tension of the work of Chen and co-workers.
In 2012, Bisai et al.[4m] reported the Lewis acid catalyzed
Friedel–Crafts alkylation of 3-hydroxy-2-oxindole that pro-
vides access to benzofuroindolines in three steps (Scheme 1,
a). Soon after, Vincent[4n] and co-workers reported FeCl3-
mediated Friedel–Crafts hydroarylation with electrophilic
N-acetylindoles and phenols to afford 3,3-disubstituted
indolines, followed by oxidation to yield the desired benzo-
furoindolines (Scheme 1, b). Furthermore, the direct con-
struction of benzofuroindolines through the [3+2] coupling
of 3-substituted indoles with quinones was also documen-
[a] Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences,
Chengdu 610041, China
E-mail: xmzhang@cioc.ac.cn
http://www.cioc.ac.cn
[b] University of Chinese Academy of Sciences,
Beijing 100049, China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402490.
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X.-M. Zhang et al.
SHORT COMMUNICATION
following investigations. Then, several other solvents were
screened. However, they all delivered lower yields than tolu-
ene (Table 1, entries 11–14). Hence, toluene was identified
as the most favorable solvent for the reaction. Upon con-
ducting the reaction at 50 °C, a lower yield of the product
was obtained (Table 1, entry 15), because quinone
monoacetal decomposed quickly at that higher tempera-
ture. If the reaction was conducted at 0 °C, only a trace
amount of the product was observed.
Table 1. The [3+2] coupling of 3-methylindole (1a) with 4,4-dimeth-
oxycyclohexa-2,5-dienone (2a) promoted by Lewis acids.[a]
Entry Catalyst
Solvent
T [°C]
Yield[b] [%]
1
2
3
4
5
6
7
8
none
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
ether
25
25
25
25
25
25
25
25
25
25
25
25
25
25
50
0
n.r.
n.d.
n.d.
n.d.
30
trace
33
86
67
47
62
CF3CO2H
CF3SO3H
C6F5CO2H
AlCl3
FeCl3
SnCl4
Zn(OTf)2
Cu(OTf)2
Sc(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
Zn(OTf)2
9
10
11
12
13
14
15
16
0
19
66
55
THF
CH3CN
toluene
toluene
trace
[a] Unless otherwise specified, the reaction was performed with 1a
(0.20 mmol), 2a (0.30 mmol), and acid (0.02 mmol) in solvent
(2 mL) for 24 h. [b] Yield of isolated product based on 1a; n.r.: no
reaction, n.d.: not determined.
Scheme 1. Strategies for the formation of benzofuroindolines.
With the optimized reaction conditions in hand, the
scope of the reaction was investigated. In the presence of
Zn(OTf)2 (10 mol-%), various 3-substituted indoles and
Results and Discussion
First, we initiated the reaction of 3-methylindole (1a) quinone monoacetals were subjected to the [3+2] coupling.
with 4,4-dimethoxycyclohexa-2,5-dienone (2a) in toluene at The results are summarized in Table 2. Substituents on the
25 °C for 24 h. If the reaction was conducted in the absence benzene ring of the indole clearly affected the results. 3-
of catalyst, no product was generated (Table 1, entry 1). In Methyl-5-fluoroindole (1b) gave only a trace amount of the
the presence of Brønsted acids, such as acetic acid, tri- product, perhaps as a result of the strong electron-with-
fluoroacetic acid, and pentafluorobenzoic acid, quinone drawing nature of the fluoro group, which reduced the nu-
monoacetal 2a decomposed completely and no desired cleophilicity of the indole (Table 2, entry 2). Nonetheless,
product was detected (Table 1, entry 2–4). Afterwards, vari- the 5-chloro-, 5-bromo-, and 5-methylindole derivatives
ous Lewis acids were evaluated in the reaction. Several underwent the reaction smoothly to generate the corre-
metal chlorides were found to exhibit poor reactivity in the sponding products in good yields (Table 2, entries 3–5). A
reaction (Table 1, entries 5–7). To our delight, upon using stronger electronic effect was observed with 3-methyl-6-
some metal triflates better yields were observed (Table 1, fluoroindole (1f) and 3-methyl-6-bromoindole (1g), which
entries 8–10); zinc trifluoromethanesulfonate [Zn(OTf)2] did not afford any products (Table 2, entries 6 and 7). Reac-
gave the desired product in the highest yield of 86% tions of indoles with bulkier substituents in the 3-position
(Table 1, entry 8). Thus, Zn(OTf)2 was determined to be the gave inferior yields of the products (Table 2, entries 8–11).
optimal Lewis acid in the reaction and it was used in the No reaction was observed with 3-phenylindole (1l; Table 2,
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Lewis Acid Catalyzed [3+2] Coupling of Indoles
entry 12). Afterwards, several other quinone monoacetals Table 3. The [3+2] coupling of 3-substituted indoles 1 and quinone
imine ketal 4 promoted by Zn(OTf)2.[a]
bearing different substituents were also subjected to the re-
action. They all reacted with 3-methylindole smoothly to
generate the products in moderate to good yields (Table 2,
entries 13–16). Notably, bulkier substrate 2d gave product
3o in good yield after a prolonged reaction time (Table 2,
entry 15).
Table 2. The [3+2] coupling of 3-substituted indoles 1a–l and quin-
one monoacetals 2a–e promoted by Zn(OTf)2.[a]
Entry
5
Yield[b] [%]
1
2
3
4
5
6
7
8
5a: R1 = H, R2 = Me
86
76
72
57
68
75
67
46
76
60
0
5b: R1 = 5-F, R2 = Me
5c: R1 = 5-Cl, R2 = Me
5d: R1 = 5-Br, R2 = Me
5e: R1 = 6-F, R2 = Me
5f: R1 = 6-Br, R2 = Me
5g: R1 = H, R2 = Et
5h: R1 = H, R2 = cyclohexyl
5i: R1 = H, R2 = CH2CH2OTBS
5j: R1 = H, R2 = CH2CH2NPhth
5k: R1 = H, R2 = Ph
9
10
11
12
5l: R1 = H, R2 = cyclopentyl
42
[a] Unless otherwise specified, the reaction was performed with 1
(0.20 mmol), 4 (0.30 mmol), and Zn(OTf)2 (0.02 mmol) in toluene
(2 mL) for 24 h. [b] Yield of isolated product based on 1.
The relative configuration of fused indoline 5d was deter-
mined to be cis by X-ray crystal structure analysis[10] (Fig-
ure 2). All other fused indolines were assigned relative con-
figurations by analogy.
Figure 2. X-ray crystal structure of cis-fused indoline 5d.
[a] Unless otherwise specified, the reaction was performed with 1
(0.20 mmol), 2 (0.30 mmol), and Zn(OTf)2 (0.02 mmol) in toluene
Furthermore, we also attempted to employ chiral cata-
(2 mL) for 24 h; TBS = tert-butyldimethylsilyl, Phth = phthalyl. lysts in the reaction (Scheme 2). However, a poor yield of
[b] Yield of isolated product based on 1. [c] The reaction was per-
formed for 48 h.
the racemic product was obtained with chiral Zn(OTf)2-Box
(6). Several chiral phosphoric acids were also tested in
which R-TRIP (7) provided the product in moderate yield
Besides, this reaction protocol was also effective for quin- with the highest enantiomeric excess value of 33%.
one imine ketal 4. We found that 4 was more reactive than
To study the mechanism of the reaction, in situ MS
quinone monoacetal 2. Under the optimal reaction condi- analysis was performed. As can be seen in Figure 3, a trace
tions, indoles bearing many sorts of substituents were toler- amount of quinone oxonium was detected in the solution
able in the reaction with 4 to afford the corresponding tetra- of quinone monoacetal 2a in toluene (Figure 3a). After the
hydroindolo[2,3-b]indoles 5 in moderate to good yields. addition of Zn(OTf)2, the peak for quinone oxonium in-
Even indoles 1b, 1f, and 1g, which failed to provide the creased remarkably (Figure 3, b). After 3-methylindole was
products in the reaction with quinone monoacetal 2a, par- added, the peak for quinone oxonium decreased and the
took in the reaction to afford the products in fairly good peak for product 3a emerged (Figure 3, c).
yields (Table 3, entries 2, 5, and 6). Only 3-phenylindole (1l)
On the basis of the above MS (ESI) analysis, we propose
was still inactive in this case (Table 3, entry 11).
the reaction pathway shown in Scheme 3. In the presence
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X.-M. Zhang et al.
SHORT COMMUNICATION
active quinone oxonium to give intermediate B, which un-
dergoes aromatization immediately to give phenol interme-
diate C. Finally, spontaneous cyclization affords benzofuro-
indoline 3a.
Scheme 2. Catalytic asymmetric variant of this process.
Scheme 3. Plausible reaction pathway.
Conclusions
In conclusion, we developed a mild and concise [3+2]
coupling of indoles and quinone monoacetals or quinone
imine ketal promoted by a Lewis acid. Through this trans-
formation, a series of structurally unique benzofuroindol-
ines and tetrahydroindolo[2,3-b]indoles were synthesized in
moderate to good yields. The relative configuration of one
product was determined to be cis on the basis of X-ray crys-
tal structure analysis. Afterwards, in situ MS (ESI) analysis
was performed and a key quinone oxonium intermediate
was detected. Accordingly, a plausible reaction pathway
was proposed.
Experimental Section
General Procedure for the [3+2] Coupling of 3-Substituted Indoles 1
with Quinone Monoacetal 2 or Quinone Imine Ketal 4: 3-Substituted
indole 1 (0.20 mmol) and quinone monoacetal 2 (0.30 mmol) or
quinone imine ketal 4 (0.30 mmol) were added to a flame-dried vial
equipped with a magnetic stirring bar. Then, toluene (2 mL) was
added to dissolve the mixture. Afterwards, Zn(OTf)2 (10.8 mg,
0.02 mmol) was added. The mixture was stirred at 25 °C for 24 h.
The solvent was evaporated, and the residue was subjected to
chromatography (silica gel, petroleum ether/EtOAc 20:1 to 10:1) to
afford desired product 3 or 5.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and spectral and analytical data for
the benzofuroindolines and tetrahydroindolo[2,3-b]indoles; HPLC
chromatograms for chiral benzofuroindoline 3a.
Figure 3. MS (ESI) was performed with the standard procedure for
MS detection. (a) 2a in toluene, (b) 2a and Zn(OTf)2 (10 mol-%) in
toluene stirred for 30 min, (c) 3-methylindole (1a) was added.
Acknowledgments
The authors are grateful for financial support from the National
Basic Research Program of China (973 Program, 2010CB833300)
and the National Sciences Foundation of China (NSFC) (grant
of the Zn(OTf)2 Lewis acid, one of the methoxy groups of
quinone monoacetal 2a is removed and quinone oxonium
A is generated. Then, 3-methylindole attacks the highly re- numbers 21372218 and 21172217).
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[10] CCDC-983833 (for 5d) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: April 25, 2014
Published Online: June 16, 2014
Eur. J. Org. Chem. 2014, 4467–4471
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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