Ming-Phos/Gold(I)-Catalyzed Diastereo- and Enantioselective Synthesis of Indolyl-Substituted Cyclopenta[ c]furans

Article abstract of DOI:10.1021/acs.orglett.8b02701

A highly enantioselective gold(I)-catalyzed intermolecular tandem cyclization/[3 + 2] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with 3-stylindoles was achieved by using Ming-Phos as a chiral ligand. A variety of chiral highly substituted cyclopenta[c]

Full text of DOI:10.1021/acs.orglett.8b02701

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Ming-Phos/Gold(I)-Catalyzed Diastereo- and Enantioselective
Synthesis of Indolyl-Substituted Cyclopenta[c]furans
Yidong Wang, Zhan-Ming Zhang, Feng Liu, Yinyan He,* and Junliang Zhang*
†
†
†
,‡
,†
†Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East
China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China
‡
Department of Obstetrics and Gynecology, Shanghai General Hospital, Shanghai Jiaotong University, 100 Haining Road, Shanghai
2
00080, China
*
S Supporting Information
ABSTRACT: A highly enantioselective gold(I)-catalyzed
intermolecular tandem cyclization/[3 + 2] cycloaddition of
2
-(1-alkynyl)-2-alken-1-ones with 3-stylindoles was achieved
by using Ming-Phos as a chiral ligand. A variety of chiral
highly substituted cyclopenta[c]furans were obtained in good
yields (up to 99%) with excellent diastereoselectivities
(
>20:1) and enantioselectivities (up to 97% ee). The salient
features of the present protocol include mild conditions,
excellent yields, and high diastereo- and enantioselectivities,
using readily available starting materials and a chiral ligand.
he synthesis of highly substituted furans has been an
Scheme 1. Synthesis of Cyclopenta[c]furans via the
Annulations of 2-(1-Alkynyl)-2-alken-1-ones
T
intense topic over past years, due to their pervasiveness in
1
natural products and pharmaceuticals, as well as their unique
2
and useful building blocks in total synthesis. Although many
3
classical strategies for their assembly have been developed,
how to efficiently construct 3,4-fused bicyclic furans from
4
readily accessible acyclic precursors is still a challenging task.
5
In this context, 2-(1-alkynyl)-2-alken-1-ones have been
extensively used for the synthesis of racemic 3,4-fused bicyclic
furans derivatives via the metal-catalyzed intermolecular
6a
6b
cycloaddition with nitrones, imines, 1,3-diphenyl-isobenzo-
6
c
6e
6g
furan, unactivated alkenes, and triazines. Recently, the
Selander group explored an elegant indium(III)-mediated
three-component reaction between 2-(1-alkynyl)-2-alken-1-
ones with aldehydes and secondary amines, leading to
6f
cyclopenta[c]furans (Scheme 1a). Despite much progress
that has been made for the synthesis of racemic 3,4-fused
bicyclic furans via the intermolecular annulations of 2-(1-
alkynyl)-2-alken-1-ones, the development of the asymmetric
version still poses a considerable challenge, especially for the
asymmetric gold catalysis, due to the linear binding mode of
gold complexes causing the chiral ligand to be far away from
7,8
the generated stereocenter. To the best of our knowledge,
only one example for the synthesis of optically active
cyclopenta[c]furan has been developed by us to date (Scheme
cycloaddition failed after screening a series of different types of
8
c
commercially available ligands. For example, the privileged
1
b). Therefore, novel protocols for rapidly assembling chiral
8a,10
chiral bisphosphine ligand (R)-L1 derived gold complex
cyclopenta[c]furan are in high demand.
In 2011, we reported a gold-catalyzed diastereoselective [3 +
led to very low enantioselectivity (16% ee) (Table 1, entry 1).
Meanwhile, the enantioselectivity was also not satisfactory
2
] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with 3-
styrylindoles, providing an efficient and convenient synthetic
6
d
route to highly substituted cyclopenta[c]furans. Unfortu-
Received: August 24, 2018
nately, the attempt to develop the asymmetric version of this
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02701
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
examined, and AgSbF is the best choice (up to 94% ee)
6
(
Table 1, entries 12 and 19−20). Finally, running the reaction
at −20 °C led to a slight improvement in enantioselectivity
96% ee), but the ee was decreased if the temperature was
(
brought down to −40 °C (Table 1, entries 21−22).
With the optimal reaction conditions in hand, various 2-(1-
alkynyl)-2-alken-1-ones 1 were explored to investigate the
generality of this asymmetric cycloaddition, and the results are
summarized in Table 2. The substrate scope is quite general,
Table 2. Scope with Respect to Various 2-(1-Alkynyl)-2-
a
alken-1-ones 1
b
entry
ligand
AgX
solvent
t (°C)
ee (%)
1
2
3
4
5
6
7
8
9
(R)-L1
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgNTf2
AgSbF6
AgSbF6
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Toluene
THF
CH Cl
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
−20
−40
(+)-3aa, 16
(−)-3aa, 0
(S,R,R)-L2
(S,R,R)-L3
(S,Rs)-M1
(R,Rs)-M1
(S,Rs)-M2
(R,Rs)-M2
(S,Rs)-M3
(R,Rs)-M3
(S,Rs)-M4
(R,Rs)-M4
(S,Rs)-M5
(R,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(S,Rs)-M5
(−)-3aa, 55
(−)-3aa, 53
(+)-3aa, 53
(−)-3aa, 67
(+)-3aa, 43
(−)-3aa, 55
(+)-3aa, 62
(−)-3aa, 70
(+)-3aa, 55
(−)-3aa, 89
(+)-3aa, 49
(−)-3aa, 77
(−)-3aa, 88
(−)-3aa, 91
(−)-3aa, 93
(−)-3aa, 80
(−)-3aa, 93
(−)-3aa, 94
(−)-3aa, 96
(−)-3aa, 90
1
2
3
b
c
entry
R /R /R
Me/Ph/Ph (1a)
Me/Ph/4-MeC H (1b)
3
yield (%) ee (%)
1
2
3
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
97
92
95
95
83
96
90
88
73
95
96
96
93
91
93
92
96
93
92
93
6
4
d
Me/Ph/4-MeOC H (1c)
6 4
4
5
6
7
8
Me/Ph/1-Naphthyl (1d)
Me/Ph/4-CF C H (1e)
10
11
12
13
14
15
16
17
18
19
20
21
22
3
6
4
Me/Ph/4-NO C H (1f)
2
6
4
Me/4-ClC H /Ph (1g)
Me/4-BrC H /Ph (1h)
6
4
6
4
d
9
Me/4-MeOC H /4-MeOC H (1i)
6 4 6 4
e
1
0
Ph/4-MeOC H /Ph (1j)
3ja
6
4
2
2
c
a
DCE
All reactions were performed with 1 (0.3 mmol), 2a (0.315 mmol),
b
d
DCE
DCE
DCE
and catalyst (5 mol %) in CH
2
Cl
2
at −20 °C for 2 h. Yield of isolated
c
products. dr >20:1. Determined by HPLC on a chiral stationary
d
e
phase. At −5 °C, 18 h. Dr = 18:1.
c
c
CH Cl
2
2
2
AgSbF6
CH Cl
2
and all the reactions can deliver high yields with excellent
a
Reaction conditions: Chiral gold complexes (5 mol %), AgX (5 mol
), 1a (0.2 mmol), 2a (0.24 mmol). Unless specified otherwise,
enantioselectivities (89−96% ee). For example, the alkynyl
3
%
moiety (R ) with both an electron-rich and -deficient aryl
1
conversions were determined by H NMR analysis (99%, >20:1 dr).
group is compatible, furnishing the desired [3 + 2]-cyclo-
b
c
Determined by HPLC on a chiral stationary phase. 100 mg of 4 Å
adducts in 83−96% yields with excellent enantioselectivities
d
MS were added. Wet DCE, air atmosphere. DCE = 1,2-dichloro-
ethane.
(
91−96% ee) (Table 2, entries 1−6). Moreover, good results
were obtained for the reaction of 1g and 1h with 3-styrylindole
2
a (93−96% ee) (Table 2, entries 7−8). Notably, substituent
1
1
when using binol-derived phosphoramidites (Table 1, entries
1
R , which can be either an aliphatic or an aromatic group, has
little impact on the yield and enantioselectivity (Table 2, entry
2
−3). Recently, we designed and developed a new type of
9
chiral ligand, called Ming-Phos, which showed good perform-
ance in the gold-catalyzed asymmetric [3 + 3] cycloaddition of
1
0). The absolute configuration of the cycloadduct (−)-3ha
was unambiguously determined to be 4S,5S,6R by X-ray crystal
structure analysis (Figure 1; CCDC 1838128).
8b
2
-(1-alkynyl)-2-alken-1-ones with nitrones. With the Ming-
Phos ligand kit in hand, we next investigated a series of chiral
Ming-Phoses. It was found that the modification of the aryl
group and the relative configuration on carbon of the Ming-
Phos have a large impact on the enantioselectivity (from 49%
to 89% ee) (Table 1, entries 4−13). Among them, (S,Rs)-M5
gave the highest ee (Table 1, entry 13). The solvent screening
showed that toluene and THF led to a slight decrease in
enantioselectivity (Table 1, entries 14 and 15). On the other
hand, a slight increase in enantioselectivity was observed when
DCE was replaced with dichloromethane as solvent (Table 1,
entry 16). Notably, water exhibits a significantly negative
impact on this reaction. For example, the ee value was
increased to 93% by adding 4 Å MS and was decreased to 80%
when the reaction was conducted in undried dichloromethane
We next examined the scope of the 3-styrylindole
component (Table 3). A diverse array of 3-styrylindoles are
well compatible, delivering the corresponding cycloadducts in
92−99% yields with excellent enantioselectivity (94−97% ee),
(Table 1, entries 17 and 18). A series of silver salts were then
Figure 1. X-ray structure of (4S,5S,6R)-3ha.
B
DOI: 10.1021/acs.orglett.8b02701
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
6e
Table 3. Scope with Respect to Various 3-Styrylindole 2
benzyne. Moreover, the furan ring could undergo ring
opening under the oxidative conditions (m-CPBA), delivering
functionalized compound (−)-5aa. Notably, the chiral 3-
cyclopentyl indole skeleton is a crucial structure in the
12
bioactive compounds.
In summary, the gold(I) complex attached with the novel
Ming-Phos ligand is an effective catalyst system for the
catalytic asymmetric intermolecular [3 + 2] cycloaddition of 2-
(1-alkynyl)-2-alken-1-ones with 3-styrylindoles, furnishing the
corresponding indolyl-substituted cyclopenta[c]furans in ex-
cellent yields (up to 99%) and high enantioselectivities (up to
97%). The salient features of the present protocol include mild
conditions, excellent yields, and high diastereo- and
enantioselectivities, using readily available starting materials
and a chiral ligand.
b
c
entry
R4/R5
3
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
4-MeC H /H (2b)
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
93
99
97
93
99
86
92
99
98
98
73
94
94
95
96
96
96
97
97
90
92
87
6
4
4-MeOC H /H (2c)
6
4
4-FC H /H (2d)
6
4
4-ClC H /H (2e)
6
4
4-BrC H /H (2f)
6
4
4-CF C H /H (2g)
3
6
4
4-CNC H /H (2h)
6
4
4-NO C H /H (2i)
ASSOCIATED CONTENT
Supporting Information
2
6
4
■
Ph/Me (2j)
Ph/MeO (2k)
Ph/Br (2l)
3aj
3ak
3al
*
S
1
1
0
1
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02701.
a
All reactions were performed with 1a (0.3 mmol), 2 (0.315 mmol),
b
and catalyst (5 mol %) in CH Cl at −20 °C for 2 h. Yield of isolated
products. dr >20:1. Determined by HPLC on a chiral stationary
phase.
2
2
Experimental procedures, H and 13NMR spectra and
1
c
HPLC date for all products (PDF)
Accession Codes
irrespective of the electronic property of the functionality at
the styryl moieties (Table 3, entries 1−8). Moreover, different
substituents, such as MeO, Me, and Br, could be introduced to
the different positions of the indole moiety and the
corresponding [3 + 2]-cycloadducts could be obtained in
CCDC 1838128 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
7
3−98% yields with 87−92% ee (Table 3, entries 9−11).
It should be noted that this gold-catalyzed enantioselective
[
3 + 2] cycloaddition of 2-(1-alkynyl)-2-alken-1-ones with 3-
AUTHOR INFORMATION
Corresponding Authors
styrylindoles is easy to scale-up. A gram-scale (4.1 mmol)
reaction was investigated, delivering 1.93 g of (−)-3aa in 98%
yield, with a slight loss of enantioselectivity (90% ee) with the
use of a 1 mol % catalyst loading. After one recrystallization,
the ee value was up to 98%. Furthermore, in order to reveal the
synthetic utility of our method for generating useful and
interesting chiral building blocks, we performed two synthetic
transformations of the representative (−)-3aa (Scheme 2). For
instance, (−)-4aa was obtained with 99% ee, via a Diels−Alder
cycloaddition reaction of (−)-3aa with an in situ generated
■
*
*
E-mail: jlzhang@chem.ecnu.edu.cn (J.Z.).
E-mail: amelie0228@126.com (Y.H.).
ORCID
Junliang Zhang: 0000-0002-4636-2846
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Scheme 2. Gram-Scale Synthesis and Further
Transformation
We grateful for the financial support from the National Natural
Science Foundation of China (21672067) and the Shanghai
Eastern Scholar Program.
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D
DOI: 10.1021/acs.orglett.8b02701
Org. Lett. XXXX, XXX, XXX−XXX