Asymmetric Synthesis of Furo[3,4-b]indoles by Catalytic [3+2] Cycloaddition of Indoles with Epoxides

Article abstract of DOI:10.1002/chem.201502448

A highly efficient N,N′-dioxide-Ni^II catalyst system for the catalytic [3+2] cycloaddition of indoles with epoxides through C-C cleavage of oxiranes was accomplished under mild conditions. It provided a promising approach for chiral furo[3,4-b]indoles in up to 98 % yield with up to 91 % enantiomeric excess (ee) and >95:5 diastereomeric ratio (d.r.). A promising approach: Catalytic de-aromatic [3+2] cycloaddition of indoles with epoxides by C-C cleavage of oxiranes is accomplished by using a highly efficient N,N′-dioxide-Ni^II catalyst system. A range of chiral furo[3,4-b]indoles is obtained with high enantiomeric excesses and diastereomeric ratios.

Full text of DOI:10.1002/chem.201502448

DOI: 10.1002/chem.201502448
Communication
&
Enantioselective Catalysis |Hot Paper|
Asymmetric Synthesis of Furo[3,4-b]indoles by Catalytic [3+2]
Cycloaddition of Indoles with Epoxides
[a]
[a]
[a]
[a]
[a]
[a]
Weiliang Chen, Yong Xia, Lili Lin, Xiao Yuan, Songsong Guo, Xiaohua Liu, and
[a, b]
Xiaoming Feng*
dition of indole and epoxide by CÀC bond cleavage of oxiranes
II
Abstract:
A highly efficient N,N’-dioxide–Ni catalyst
have advantages in atom-economy and safety. In 2012, the
system for the catalytic [3+2] cycloaddition of indoles
with epoxides through CÀC cleavage of oxiranes was ac-
complished under mild conditions. It provided a promising
approach for chiral furo[3,4-b]indoles in up to 98% yield
with up to 91% enantiomeric excess (ee) and >95:5 dia-
stereomeric ratio (d.r.).
II
Zhang group reported a Ni -catalyzed regio- and diastereose-
[10,11]
lective [3+2] cycloaddition of indoles and oxiranes.
How-
ever, when the enantioselective variant of this reaction was
tested, only 19% ee was obtained by the application of BOX as
the chiral ligand (Scheme 1a). On the other hand, we have ach-
ieved the catalytic asymmetric [3+2] cycloaddition of oxiranes
[
12]
with alkynes and aldehydes recently.
In this context, we
present herein our ongoing efforts in developing an efficient
II
The catalytic asymmetric dearomatization reaction of indoles
represents an attractive and useful tactic for the conversion of
indoles into optically active polycyclic C2-, C3-fused indoline
N,N’-dioxide–Ni complex for the catalytic asymmetric [3+2] cy-
cloaddition of indoles with oxiranes by selective CÀC bond
[
1]
motifs, which are privileged molecular architectures in bio-
logically active heterocyclic compounds and organic electron-
[2]
Table 1. Optimization of different chiral N,N’-dioxide-metal complexes.
ics. As a result, various methodologies have been developed,
[3]
including alkylative dearomatization, oxidative dearomatiza-
[
4]
[5,6]
tion, dearomatic annulation, and so on.
In terms of chemi-
cal efficiency, the enantioselective C2-, C3-annulation of indoles
by means of cycloaddition reactions has received a growing
[
5,7]
acclaim.
Carbonyl ylides are promising 1,3-dipolar building block for
annulation, and have been utilized to construct many complex
[8]
oxa-heterocyclic skeletons. Among these oxa-heterocyclic
skeletons, 1H-furo[3,4-b]indole exhibits considerable biological
activity, and has been employed as anti-infective, immune, and
[9]
antitumor agents as shown in Figure 1. Moreover, 1H-
furo[3,4-b]indole, which was achieved by cycloaddition of a car-
bonyl ylide and indole, is a key intermediate to access a variety
[a]
[b]
[c,d]
Entry
Metal salts
Sc(OTf)3
Ligand
Solvent
CH Cl
Yield [%]
ee [%]
[8c,d]
of natural products, for instance, vindoline,
(+)-fendleridine,
[8e]
1
2
3
4
5
6
7
8
9
L-PrPr2
40
63
72
59
66
85
76
84
83
84
76
race
82
83
40
43
65
60
15
85
87
88
53
90
2
2
2
2
2
2
2
2
2
2
2
2
and (+)-1-acetylaspidoalbidine,
aspido-sperma, and so
Co(ClO
Ni(ClO
4
)
2
·6H
·6H
2
O
O
L-PrPr
L-PrPr
2
CH
CH
Cl
Cl
[
8g]
forth. In these impressive studies, diazo compounds or oxa-
diazole derivatives were employed to generate carbonyl ylides
to access natural products; in addition the asymmetric cycload-
4
)
2
2
2
Ni(ClO ) ·6H O
4
4
4
4
4
4
4
4
4
4
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
L-PiPr2
L-RaPr
2
CH Cl
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
Ni(ClO
)
)
)
)
)
)
)
)
)
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
·6H
O
O
O
O
O
O
O
O
O
2
CH
CH
CH
CH
CH
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
L-PrEt
L-PrMe
L-PrPh
2
2
+
+
[
a] W. Chen, Y. Xia, Dr. L. Lin, X. Yuan, S. Guo, Prof. Dr. X. Liu, Prof. Dr. X. Feng
Key Laboratory of Green Chemistry & Technology
Ministry of Education, College of Chemistry
Sichuan University, Chengdu 610064 (P. R. China)
Fax: (+86)28-8541-8249
L-PrPr
L-PrPr
L-PrPr
L-PrPr
L-PrPr
3
3
3
3
3
10
DCE
TCE
toluene 48
TCE 87
1
1
1
1
2
3
[e]
E-mail: xmfeng@scu.edu.cn
[
b] Prof. Dr. X. Feng
[a] Unless otherwise noted, all reactions were performed with L–metal
(10 mol%, 1.05:1), 1a (0.10 mmol) in solvent (0.5 mL) at 308C for 48 h.
[b] Isolated yield. [c] Determined by chiral HPLC analysis. [d]>19:1 d.r.
Collaborative Innovation Centre of Chemical Science and Engineering
Tianjin (P. R. China)
1
+
was obtained in all cases determined by H NMR analysis. [e] LiNTf2
[
] These individuals contributed equally to this work
(15 mol%) was added. DCE=dichloroethane, TCE=1,1,2,2-tetrachloro-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502448.
ethane.
Chem. Eur. J. 2015, 21, 15104 – 15107
15104
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
and ee was realized, when L-PrPr , with an isopropyl group at
3
para-position of L-PrPr , was employed, and 83% yield with
2
8
5% ee was obtained (Table 1, entry 9). Further attempts to im-
prove the results were focused on the solvents. Except for re-
activity, CHCl CHCl2 was more efficient in enantioselectivity
2
[
12]
than other solvents (Table 1, entries 9–12). As predicted, the
addition of LiNTf (15 mol%) enhanced the yield and enantio-
2
Figure 1. Selected 1H-furo[3,4-b]indoles with biological and pharmacological
activity.
selectivity to 87% yield and 90% ee, respectively (Table 1,
entry 13).
With the optimal conditions identified, various substituted
indoles with N–Me protecting groups were investigated, and
[
14]
the results are listed in Scheme 2. C4-Subsituted indoles un-
derwent the reaction efficiently, giving high yields (80–91%)
and enantioselectivities (88–90% ee), while the diastereoselec-
tivities were quite different, which might arise from the steric
hinderance of bromine (Scheme 2, 3a–3c). It seemed that in-
doles with electron-withdrawing groups exhibited slight higher
enantioselectivities than those with electron-donating groups
(
Scheme 2, 3d, 3e, 3h versus 3 f and 3i). Except for indoles
with a 5-Br group, C5-, C6- and C7-subsituted indoles per-
formed well, affording the corresponding products in excellent
yields, enantioselectivities and diastereoselectivities (86–96%
yield, 86–91% ee and >95:5 d.r.; Scheme 2, 3d–3l). Compared
to other positions, lower diastereoselectivities were observed,
when the indoles with substituents at C6 position were tested
(Scheme 1, 3b, 3e, 3 f, 3g versus 3h, 3i). To evaluate the
Scheme 1. The asymmetric synthesis of chiral furo[3,4-b]indoles.
[13]
cleavage of epoxides (Scheme 1b).
A variety of chiral 1H-
furo[3,4-b]indoles were obtained in high yields with excellent
enantioselectivities and diaste-
reoselectivities (up to 98% yield,
9
1% ee, >95:5 d.r.).
We initially carried out the
screening of an efficient Lewis
acid for catalytic [3+2] cycload-
dition of epoxides 1a and in-
doles 2a at 308C in CH Cl with
2
2
L-PrPr as a ligand. As shown in
2
Table 1, Sc(OTf) showed moder-
3
ate reactivity (40% yield), but
only a racemic product was ob-
tained (Table 1, entry 1). Co-
(
ClO ) ·6H O gave a lower yield
4 2 2
and ee, compared with Ni-
ClO ) ·6H O (Table 1, entries 2
(
4
2
2
versus 3). Investigation of li-
gands showed that l-proline de-
rived L-PrPr was superior to (S)-
2
pipecolic acid derived L-PiPr2
and l-ramipril derived L-RaPr in
2
both the reactivity and selectivi-
ty (Table 1, entry 3 versus en-
tries 4 and 5). With the decrease
of steric hinderance at the ortho-
position of a phenyl ring from
iPr to Et, Me and H, the enantio-
selectivity decreased from 83 to
6
5, 60 and 15% ee, respectively
(
Table 1, entries 3 versus 6–8). A
further improvement of yield Scheme 2. Substrate scope of indoles for the catalytic asymmetric [3+2] cycloaddition.
Chem. Eur. J. 2015, 21, 15104 – 15107 www.chemeurj.org ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15105

Communication
effect of C3-substituents of indole, 3-ethyl-1-methyl-1H-indole
was tested, which gave the corresponding product in 96%
[15]
yield, 90% ee and >95:5 d.r. (Scheme 1, 3m). Additionally,
when 1,2,3-trimethylindole was used, the corresponding prod-
uct 3n was obtained in 78% yield with 56% ee and >95:5 d.r..
The absolute configuration of the product 3a (2R,3S,4R) was
[16]
determined by X-ray crystallographic analysis.
The other
products exhibited a similar Cotton effect in their circular di-
chroism (CD) spectra (see the Supporting Information for de-
tails).
Figure 2. Proposed catalytic model and X-ray structure of the product 3a.
Subsequently, we investigated the substrate scope of the
epoxides. When dimethyl 3-(3,4-dimethylphenyl)oxirane-2,2-di-
carboxylate was employed, the corresponding product 3o was
obtained in 73% yield with 90% ee and >95:5 d.r.
upper right corner to the Re face, affording product 3a
(2R,3S,4R).
(
Scheme 3a). However, epoxides without electron-donating
In summary, we have developed an efficient catalytic system
for furo[3,4-b]indoles through asymmetric [3+2] cycloaddition
[17]
groups gave inferior results.
of indoles and epoxides by CÀC cleavage of oxiranes for the
II
first time. In the presence of the L-PrPr –Ni complex, a variety
3
of corresponding chiral furo[3,4-b]indoles were obtained in ex-
cellent yields (up to 98%) with high enantioselectivities (up to
9
1% ee) and diastereoselectivities (up to >95:5 d.r.). Addition-
ally, a transition-state model was proposed to explain the origi-
nal asymmetric introduction. This, along with the asymmetric
cycloaddition of epoxides with other classes of dipolarophiles,
constitutes the subject of our sustained efforts.
Experimental Section
Typical experimental procedure for asymmetric synthesis of
furo[3,4-b]indoles by catalytic [3+2] cycloaddition of indoles
with epoxides
N,N’-dioxide L-PrPr3 (7.4 mg, 0.105 mmol), Ni(ClO ) ·6H O (3.7 mg,
4 2
2
0
.01 mmol), LiNTf (15 mol%) and 4 Š MS (20.0 mg) were stirred in
2
CHCl CHCl (0.5 mL) at 358C under N atmosphere for 0.5 h, then
2
2
2
1
,3-dimethyl-1H-indole (21.8 mL, 0.15 mmol) was added. After the
decrease of temperature to 308C, dimethyl 3-(p-tolyl)oxirane-2,2-di-
carboxylate 1a (25.0 mg, 0.1 mmol) was added. The mixture was
stirred at 308C for 48 h. The reaction mixture was purified by flash
chromatography (petroleum ether:ethyl acetate=10:1) on silica
gel to afford the desired product.
Scheme 3. a) Catalytic asymmetric [3+2] cycloaddition of indole with oxir-
ane; b) gram-scaled version of the reaction; c) transformation of ester to al-
cohol.
To evaluate the synthetic value of this catalytic system,
a gram-scale reaction was performed (Scheme 3 b). In the pres- Acknowledgements
ence of the L-PrPr –Ni(ClO ) ·6H O complex (10 mol%), the
3
4 2
2
starting material 1a (3 mmol) reacted with 1.5 equivalents of
a, and the corresponding product 3a was obtained in 89%
yield (1.05 g) with 89% ee and>95:5 d.r.. By a simple recrystal-
lization using petroleum ether and ethyl acetate, enantiopure
product 3a could be obtained in 73% yield. Furthermore, this
enantiopure product 3a could be efficiently transformed into
We acknowledge the National Natural Science Foundation of
China (Nos. 21290182, 21321061, 21172151) and the National
Basic Research Program of China (973 Program: No.
2011CB808600) for financial support.
2
Keywords: asymmetric synthesis · epoxides · indoles · N,N’-
dioxide-metal complex
diols 4 upon treatment with NaBH in methanol with the re-
4
tention of stereochemistry (76% yield, >99% ee, Scheme 3c).
Based on the absolute configuration of 3a, and our previous
[
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II
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[
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be separated from the crude mixture due to the epoxide having the
3
2, 709.
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f
same R value as the product.
[
[
[
16] CCDC 1057117 (3a) contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre
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2
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Received: June 24, 2015
Published online on September 4, 2015
Chem. Eur. J. 2015, 21, 15104 – 15107
www.chemeurj.org
15107
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