One-Pot Catalytic Asymmetric Synthesis of Tetrahydrocarbazoles

Article abstract of DOI:10.1021/acs.orglett.5b01909

A one-pot asymmetric synthesis of 1,2,3,4-tetrahydrocarbazoles has been developed via an enantioselective [3 + 3] annulation of 2-alkynylindoles and donor-acceptor cyclopropanes. In the presence of chiral Lewis acids as catalysts, a series of optically active tetrahydrocarbazoles were furnished in high yields (63-87%) with good to excellent levels of enantioselectivity (up to 94% ee).

Full text of DOI:10.1021/acs.orglett.5b01909

Letter
pubs.acs.org/OrgLett
One-Pot Catalytic Asymmetric Synthesis of Tetrahydrocarbazoles
†
†
†
,§
,†
Qiong-Jie Liu, Wen-Guang Yan, Lijia Wang, X. Peter Zhang,* and Yong Tang*
†The State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
45 Lingling Road, Shanghai 200032, China
3
§
Department of Chemistry, University of South Florida, Tampa, Florida 33620-5250, United States
S Supporting Information
*
ABSTRACT: A one-pot asymmetric synthesis of 1,2,3,4-
tetrahydrocarbazoles has been developed via an enantioselective
[
3 + 3] annulation of 2-alkynylindoles and donor−acceptor
cyclopropanes. In the presence of chiral Lewis acids as catalysts,
a series of optically active tetrahydrocarbazoles were furnished
in high yields (63−87%) with good to excellent levels of
enantioselectivity (up to 94% ee).
olycyclic indoles, such as carbazoles and their derivatives,
propanes, which provides a one-pot access to a variety of
optically active tetrahydrocarbazoles.
P
are important heterocyclic compounds owing to their
frequent occurrence as key motifs in various bioactive nature
Initially, the reaction was carried out with 1.0 equiv of 2-
ethynylindole 1a and 2.0 equiv of cyclopropane 2a in 1,2-
dichloroethane (Table 1). After evaluation of several Lewis
1
,2
products and drug molecules (Figure 1). It was found that
14
acids, copper complexes were proven to be competent
catalysts for the asymmetric ring-opening reaction. By using in
situ generated Cu(SbF ) /L1 as catalyst, the reaction
6
2
proceeded very fast at 35 °C, affording the desired product
a in 98% yield with 55% ee after 50 min (entry 1). When
3
Cu(OTf) was employed, it gave rise to a 91% yield with a
2
moderate level of enantioselectivity (59% ee, entry 2).
Although the reaction provided a similar yield and
enantioselectivity in dichloromethane (entry 3), a dramatic
increase of enantioselectivity (77% ee) was observed by
employing toluene as solvent (entry 4). Aiming at improving
both the reactivity and the enantioselectivity of the ring-
opening reaction, a variety of chiral ligands were investigated.
Importantly, it was found that bisoxazoline ligands bearing
cyclohexyl backbones provided better enantioinduction. With
ligand L2, the ring-opening product 3a was afforded in 72%
yield with 83% ee (entry 5). Although trisoxazoline L3 led to a
lower reactivity and enantioselectivity, SaBOX ligand L4
bearing a benzyl side arm could promote the reaction and
gave a higher ee value (entries 6 and 7). Notably, SaBOX ligand
L5 containing a sterically demanding side arm could promote
the reaction efficiently, affording the ring-opening product 3a in
95% yield with 87% ee (entry 8). Since a further increase in
steric hindrance of the side arm group resulted in no
improvement of enantioselectivity of the reaction (entry 9),
the SaBOX ligand L7 with two side arm groups was employed.
To our delight, with L7, the product 3a was obtained in 97%
yield with 90% ee (entry 10). In addition, when the ratio of 1a/
Figure 1. Natural products containing tetrahydrocarbazole.
the tetrahydrocarbazoles D, especially in their optically active
forms, usually show high bioactivities, such as analgesic potency
2a
and anti-inflammatory potency. Great efforts have been made
to explore the effective methods for the enantioselective
construction of this subunit, including Friedel−Crafts alkyla-
3
−5
tion, the Diels−Alder reaction, and others.
In 2009, Kerr
and co-workers reported an extraordinarily efficient approach
that led to tetrahydrocarbazoles, which involved ring-opening
6
7
,8
of the donor−acceptor (D−A) cyclopropanes with indole
9
,10
and Conia−ene cyclization.
Recently, we have developed a
series of catalytic processes for enantioselective ring opening
and annulation reactions of D−A cyclopropanes with versatile
reagents such as nitrones, amines, enol silyl ethers, and
1
1
indoles, by Lewis acid based on side arm modified chiral
12,13
bisoxazoline ligands.
Here, we report our recent results on
the development of catalytic system for enantioselective [3 + 3]
Received: July 3, 2015
annulation of 2-alkynylindoles with donor−acceptor cyclo-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01909
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Reaction Optimization
alkynylindoles with cyclopropanes was examined. As shown in
Table 3, electron-rich indole derivatives, such as those bearing a
a
Table 3. Substrate Scope of Annulation Reaction
b
c
yield
ee
entry
R3
R4
4
(%)
(%)
1
2
3
4
5
6
7
8
9
H (1a)
PMP (2a)
PMP (2a)
PMP (2a)
PMP (2a)
PMP (2a)
4a
4b
4c
4d
4e
4f
82
86
87
76
71
85
77
77
69
79
68
71
86
77
78
63
89
93
90
89
88
94
91
77
77
84
57
77
82
87
90
86
4-Me (1b)
5-Me (1c)
6-Me (1d)
7-Me (1e)
b
c
entry
metal precursor
L
solvent
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
Cu(SbF6)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
L1
L1
L1
L1
L2
L3
L4
L5
L6
L7
L7
DCE
98
91
93
72
72
44
75
95
96
97
90
55
59
59
77
83
77
88
87
85
90
90
DCE
4-OMe (1f) PMP (2a)
5-OMe (1g) PMP (2a)
DME
4g
4h
4i
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
5-Cl (1h)
6-Cl (1i)
5-F (1j)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
PMP (2a)
PMP (2a)
PMP (2a)
1
1
1
1
0
1
2
3
4j
2-furyl (2b)
4k
4l
2-thienyl (2c)
4-TBSOC H (2d)
4m
4n
6
4
1
1
0
1
d
14
3,4-(MeO) C H (2e)
2 6 3
1
5
3,4,5-(MeO) C H (2f) 4o
3 6 2
a
Performed with 0.2 mmol of 1a, 0.4 mmol of 2a, and 3 mL of solvent.
Isolated yields. Determined by chiral HPLC. 1a/2a = 1.5/1, at 40
C.
b
c
d
16
CHCHPh (2g)
4p
a
Performed with 0.6 mmol of 1a and 0.4 mmol of 2a in 6 mL of
toluene for the first step; an additional 2 mL of toluene was added for
the second step. Isolated yields. Determined by chiral HPLC.
°
b
c
2
a was 1.5/1, both high yield and an excellent level of
enantioselectivity could be afforded as well (entry 11).
Next, we turned our attention to investigate the one-pot [3 +
methyl group at the 4-, 5-, 6-, and 7-positions (1b−e), could
react smoothly with cyclopropane 2a, giving the corresponding
3
2
] annulation reaction of 2-alkynylindole 1a with cyclopropane
[
3 + 3] products 4b−e in 71−87% yields with 88−93% ee
a (Table 2). With InCl as catalyst, the cyclization reaction
3
(
entries 2−5). Similar product yields (77−85% yields) and
a
slightly higher levels of enantioselectivity (91−94% ee) were
obtained for the reactions of 2a with indoles 1f and 1g, which
contain methoxy groups at the 4- and 5-positions, respectively
Table 2. One-Pot [3 + 3] Annulation Reaction
(
entries 6 and 7). Halo-substituted indole substrates 1h−j were
also tolerated in the current catalyst system, furnishing the
desired products 4h−j in good yields with up to 84% ee
(
entries 8−10). In addition to 2a, a variety of D−A
b
c
entry
DBU (mol %)
time (h)
yield (%)
ee (%)
cyclopropanes were compatible with this asymmetric [3 + 3]
annulation reaction. For example, the reactions of cyclo-
propanes bearing heterocyclic substituents such as 2-furyl (2b)
and 2-thienyl (2c) with 1a resulted in the desired products in
good yields with moderate to good enantioselectivities (entries
11 and 12).
1
2
3
4
0
5
5
6
73
80
82
78
85
87
89
90
10
15
6
72
a
Performed with 0.3 mmol of 1a and 0.2 mmol of 2a in 3 mL of
toluene for the first step; an additional 1 mL of toluene was added for
the second step. Isolated yields. Determined by chiral HPLC.
D−A cyclopropanes with other aromatic substituents, such as
d−f containing electron-donating groups on the phenyl ring,
b
c
2
could also perform well in the asymmetric [3 + 3] annulation
reaction, affording the corresponding products 4m−o in high
yields with high levels of enantioselectivity (entries 13−15). In
addition, styrenyl-substituted D−A cyclopropane 2g was also a
suitable substrate, delivering the corresponding product 4p in
63% yield with 86% ee (entry 16). The absolute configuration
of 4h was established as S by X-ray crystallography (Figure 2).
Previous studies on the crystal structures of SaBOX/Cu(II)
complexes indicated that the aryl groups of the side arms always
bend toward the metal center. Accordingly, an asymmetric
induction model was proposed to explain the observed
enantioselectivity (Scheme 1). In this model, it is evident that
the upper and lower sides of the Cu(II)/oxazoline ring square
o
proceeded smoothly at 120 C after 5 h, furnishing the desired
product 4a in 73% yield but with a slight drop of the ee value
85% ee, entry 1). When 5 mol % of DBU (1,8-
(
diazabicyclo[5.4.0]undec-7-ene) was added as additive, both
the yield and enantioselectivity were improved (entry 2).
Increasing the amount of the base to 10 mol %, the annulation
product 4a was obtained in 82% yield with 89% ee (entry 3).
However, a further increase of DBU to 15 mol % led to a
dramatically sluggish reaction, delivering the [3 + 3] annulation
product in 78% yield with 90% ee even after 3 days (entry 4).
Under the optimized reaction conditions, the substrate scope
of the enantioselective one-pot [3 + 3] annulation reaction of 2-
B
DOI: 10.1021/acs.orglett.5b01909
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Experimental procedures, characterization data for all
new compounds, and crystallographic data (PDF)
X-ray data for enantiopure product 4h (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
*
*
E-mail: xpzhang@usf.edu.
E-mail: tangy@mail.sioc.ac.cn.
Figure 2. X-ray crystal structure of annulation product 4h.
Notes
The authors declare no competing financial interest.
Scheme 1. Proposed Stereochemical Model
ACKNOWLEDGMENTS
We are grateful for financial support from the National Natural
Science Foundation of China (Nos. 21121062, 21432011, and
■
2
1272250), the National Basic Research Program of China
(
973 Program) (2015CB856600), the Chinese Academy of
Sciences, the National Science Foundation (CHE-1152767 and
CHE-1465106; X.P.Z.), and the National Institutes of Health
(
R01-GM098777; X.P.Z.).
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