The concise synthesis of spiro-cyclopropane compounds via the dearomatization of indole derivatives

Article abstract of DOI:10.1021/ol500948r

A concise synthesis of spiro-cyclopropane compounds from indole derivatives and sulfur ylides has been developed via a dearomatization strategy. Moreover, the spiro-cyclopropane compounds could be conveniently transformed to rearomatized indole derivatives in the presence of acids.

Full text of DOI:10.1021/ol500948r

Letter
pubs.acs.org/OrgLett
The Concise Synthesis of Spiro-Cyclopropane Compounds via the
Dearomatization of Indole Derivatives
Jing Luo,† Bo Wu,‡ Mu-Wang Chen,‡ Guo-Fang Jiang,*,† and Yong-Gui Zhou*,‡
†State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, China
‡State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian
116023, China
S
* Supporting Information
ABSTRACT: A concise synthesis of spiro-cyclopropane compounds
from indole derivatives and sulfur ylides has been developed via a
dearomatization strategy. Moreover, the spiro-cyclopropane com-
pounds could be conveniently transformed to rearomatized indole
derivatives in the presence of acids.
The use of an arylsulfonyl group as an auxiliary group is still a
prominent synthetic strategy in organic synthesis.18 Recently,
the utilization of the arylsulfonyl group in connection with
indole has attracted intriguing interests. The sulfonyl moiety at
the benzylic position of 3-substituted indoles acts as a good
leaving group, which enables the generation of vinylogous
imines intermediates under basic conditions. The generating
vinylogous imines intermediates are equal to α,β-unsaturated
imines, which are able to react with numerous nucleophiles.19
Considering that sulfur ylides act as a nucleophile under the
basic condition,20 the vinylogous imine intermediates can be
generated from arenesulfonylindole under the mild basic
conditions. We envisioned that ylides could react with the
vinylogous imine intermediates to furnish the spiro-cyclo-
propane compounds (Scheme 1). Herein, we report an efficient
earomatization reactions are important transformations of
Daromatic compounds because they directly lead to a
variety of ring systems.1 Additionally, they are highly efficient
for constructing quaternary carbon centers2 and used as the key
steps in the total synthesis of many natural products.3 In this
regard, dearomatization reactions of indoles,4 pyridines,5
phenols,6 pyrroles,7 etc., have been developed. Among them,
compounds with an indole skeleton have attracted enormous
attention because of the important biological activities of their
derivatives.8 Dearomatization reaction of indoles has been
extensively studied over the past decades, with particular
focuses on alkylative dearomatization9 and oxidative dearoma-
tization.10 In addition, the Diels−Alder pathway with various
dienophiles11 and organocatalyzed Michael/Mannich cycliza-
tion cascade reactions were also demonstrated in dearomatiza-
tion reactions of indoles.12 Besides, several transition-metal-
catalyzed dearomatization reactions of indoles were also
documented.13 Although great progress has been made in the
dearomatization reaction of indoles, most of the methods
required harsh conditions and designed catalysts that are
currently commercially unavailable. Thus, to develop a facile
and efficient method for a dearomatization reaction of indole
derivatives from readily available starting materials would be of
considerable significance in organic chemistry.
Scheme 1. Synthesis of Spiro-Cyclopropane Compounds via
the Dearomatization of Indoles
Spiro-cyclopropane structure motifs are ubiquitous and
prevalent in various anticancer agents and pharmaceuticals.14
In addition, they also serve as valuable synthetic intermediates
for a wide range of organic compounds.15 Therefore, the
construction of spiro-cyclopropane skeletons has achieved
extensive attention, and a plethora of efficient methods have
been developed for the synthesis of these important structures,
including from alkenes by Simmons−Smith and related
reactions or with ylides, transition-metal-catalyzed carbene
transfer, Michael-initiated ring closure (MIRC), and organo-
catalytic cyclopropanation, which have been described in the
literature.16,17
dearomatization reaction of indole derivatives in which
arylsulfonyl group acts as a leaving group with sulfonium salts
under mild conditions to synthesize spiro-cyclopropane
compounds.
Initially, our investigation was launched with 2-methyl-
substituted arenesulfonylindole 1a (1.0 equiv), sulfonium salt
2a (1.5 equiv), K2CO3 (3.0 equiv) as the base in CH2Cl2
Received: January 22, 2014
Published: May 6, 2014
© 2014 American Chemical Society
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Letter
(Table 1). Delightfully, the spiro-cyclopropane products 3a and
3a′ were obtained with 71% yield and a ratio of 3:1. Next, a
Table 2. Scope for the Reaction of Arenesulfonylindoles 1
with Sulfonium Salts 2
a
Table 1. Optimization for the Reaction of 2-
Methylarenesulfonylindole 1a with Sulfonium Salt 2a
a
b
c
entry
R1/R2/R3
Me/Ph/H
2
3, yield (%)
dr
b
c
entry
solvent
base
yield (%)
dr
1
2a
2b
2c
2d
2e
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
3a, 83
3b, 78
3c, 60
3d, 55
3e, 71
3f, 56
3g, 88
3h, 86
3i, 84
3j, 82
3k, 75
3l, 81
3m, 69
3n, 88
3o, 83
3p, 70
3q, 82
3r, 81
3s, 83
3t, 85
13:1
2
Me/Ph/H
13:1
1
CH2Cl2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
DIPEA
Na2CO3
KOtBu
74
<5
26
87
55
83
84
83
66
75
61
<5
3:1
−
3
Me/Ph/H
>20:1
>20:1
>20:1
>20:1
14:1
2
toluene
4
Me/Ph/H
3
CH3OH
4:1
6:1
19:1
7:1
8:1
13:1
6:1
7:1
15:1
−
5
Me/Ph/H
4
EtOH (E)
iPrOH (P)
E/P (1:1)
E/P (1:5)
E/P (1:10)
E/P (1:10)
E/P (1:10)
E/P (1:10)
E/P (1:10)
6
Me/n-Pentyl/H
Me/4-MeC6H4/H
Me/3-MeC6H4/H
Me/2-MeC6H4/H
Me/4-ClC6H4/H
Me/3-ClC6H4/H
Me/4-BrC6H4/H
Me/3-MeOC6H4/H
Me/Ph/5-Me
Me/Ph/5-F
5
7
6
8
13:1
7
9
13:1
8
10
11
12
13
14
15
16
14:1
9
12:1
10
11
12
15:1
11:1
a
12:1
Condition: 0.25 mmol 1a, 0.375 mmol 2a, 0.75 mmol base, 3.3 mL of
b
10:1
solvent, room temperature, 12 h. Isolated yields of the mixture of 3a
c
and 3a′. All of the dr (3a/3a′) were determined by 1H NMR
Ph/4-BrC6H4/H
H/Ph/H
1:5
d
17
>20:1
>20:1
>20:1
>20:1
spectroscopy. E/P: EtOH/iPrOH.
d
18
H/4-ClC6H4/H
H/4-BrC6H4/H
H/n-Pentyl/H
d
19
d
20
series of solvents were examined, and to our delight, high yield
was obtained from EtOH (87%, Table 1, entry 4), and iPrOH
gave the excellent 19:1 of diastereoselectivity (Table 1, entry 5).
Thus, the next screening was focused on the mixed solvents.
When the ratio of iPrOH/EtOH was 10:1, the yield could be
retained, and the diastereoselectivity could be raised up to 13:1
(Table 1, entry 8); several other bases were also examined, and
moderate yields and diastereoselectivities were obtained (Table
1, entries 9−11). So, the optimal conditions were established,
K2CO3 as the base and iPrOH/EtOH (10:1) as the solvent.
With the optimized conditions in hand, we explored the
reaction scope using a variety of 2-substituted arenesulfony-
lindoles 1 and sulfonium salts 2 as listed in Table 2. A range of
sulfonium salts were examined, and medium to good yields and
excellent diastereoselectivitives were achieved (55−78% yields,
dr = 13:1 to >20:1, Table 2, entries 2−5). Especially, when R
was aryl carbonyl, up to >20:1 diastereoselectivity were
obtained. The steric and electronic property of the aryl
substituents had little effect on the yield and diastereoselectivity
for substituents R2 (Table 2, entries 7−13); all of them could
get satisfactory yields and diastereoselectivity. The electronic
nature of the indole core had little influence on the outcome of
the reactions; when the sulfonylindoles with fluorine or methyl
as substituent at the 5-position of the indole ring, both gave
excellent yields and satisfactory diastereoselectivities (Table 2,
entries 14−15). It is worthy of note that when the 2-position of
the indole ring was phenyl, the diastereoselectivity was reversed
(Table 2, entry 16), which may ascribe to the change of
geometry of CC bond and relative stability of vinylogous
imine intermediate due to steric hindrance of phenyl group. To
our delight, the 2-position unsubstituted sulfonyl indoles were
also suitable reaction partners using ethanol as solvent and
provided the desired products in good yields and excellent
diastereoselectivities (Table 2, entries 17−20).21
a
Condition: 0.25 mmol 1, 0.375 mmol 2, 0.75 mmol K2CO3, 3.0 mL
of iPrOH, 0.3 mL of EtOH, room temperature, 12 h. Isolated yields
of the mixture of 3 and 3′. All of the dr (3/3′) were determined by
b
c
d
1H NMR spectroscopy. 3.3 mL of EtOH was used.
The structure and stereochemistry of spiro-cyclopropanes 3a
(Figure 1), 3p, and 3p′ were verified by the combination of
Figure 1. X-ray crystal structure of compound 3a.
NMR, HRMS spectroscopy, and single-crystal X-ray diffraction
anaysis (for X-ray crystal structure of 3p and 3p′, see the
Supporting Information).22
Furthermore, a preliminary study on the enantioselective
version of this dearomatization reaction was also tried. Using
the known chiral sulfonium salts23 originally developed by
Aggarwal, only moderate enantioselectivity 64% ee and low
yield (29%) were obtained (Scheme 2). These promising
results demonstrated the potential for synthesis of the chiral
spiro-cycopropane derivatives, although more efficient catalytic
systems need to be developed.
It is noted that the model substrates 1a and 2a could also be
carried out at a gram scale. As illustrated in Scheme 3, the target
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Organic Letters
Letter
Scheme 2. Synthesis of Chiral Spiro-Cyclopropane with the
Chiral Sulfonium Salt
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, characterization data, X-ray struc-
tures, data for the determination of enantiomeric excess, and
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
products 3a and 3a′ could be obtained in 82% yield and the
Corresponding Authors
diastereoselectivity could be kept with 13:1.
*E-mail: gfjiang@hnu.edu.cn.
*E-mail: ygzhou@dicp.ac.cn.
Scheme 3. Scale-up of Model Substrates
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support from the National
Natural Science Foundation of China (J1210040) and the
National Basic Research Program of China (2010CB833300).
In order to explore the application of our methodology, we
tried to transform them to rearomatized 2,3-disubstituted
indole derivatives (Scheme 4). Pleasingly, when trifluoroacetic
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