Synthesis of indolizine derivatives containing eight-membered rings: Via a gold-catalyzed two-fold hydroarylation of diynes

Article abstract of DOI:10.1039/c7cc09250d

A gold-catalyzed method for the construction of indolizines with eight-membered rings has been developed. The reaction proceeded through a two-fold hydroarylation with indole or pyrrole derivatives containing a 1,6-diyne using a tri(1-adamantyl)phosphine

Full text of DOI:10.1039/c7cc09250d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Synthesis of Indolizine Derivatives Containing Eight-membered
†
Ring by a Gold-catalyzed Two-fold Hydroarylation of Diynes
Received 00th January 20xx,
Accepted 00th January 20xx
Ruixing Liu, Qiang Wang, Yin Wei and Min Shi*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A gold-catalyzed method for the construction of indolizines with and co-workers reported a gold(I)-catalyzed synthesis of
eight-membered ring has been developed. The reaction functionalized indolizines via intramolecular
7
proceeded through a two-fold hydroarylation with indole or hydroarylation/aromatization of pyrrole derivatives.
pyrrole derivatives containing an 1,6-diyne using tri(1-
Figure 1 Representative natural products with indolizine skeleton
adamantyl)phosphine gold complex as the catalyst, affording 1,8-
COOH
NH2
disubstituted indolizines in moderate to good yields in DCE at 80
O
N
o
CHO
O
1
N
C. The potential usefulness of these indolizines as blue or green
1
H
O
HN
O
8
N
OLEDs has been also disclosed.
N
N
N
N
Et
EtO
2
C
R
Ph
Ph
PLA2 inhibitory
fluorophores
homofascaplysin C
antimycobacterial
Seoul-Flour
Indolizines are fundamental structural motifs found in many
natural products and pharmaceuticals because the indolizine
skeleton as a key constituent possesses a diverse range of
biological activities, and it can be considered as an important
1
a-1c
scaffold for the preparation of novel pharmaceuticals.
Indolizine derivatives have many pharmaceutical activities,
including anti-tuberculosis, bacterial, cancer, inflammatory
and histaminic activities. For example, anti-inflammatory
activities of PLA2 inhibitory have been extensively investigated
1
d
thus far (Figure 1). On the other hand, fluorescent materials
with indolizine skeleton are also very useful. The structure-
photophysical property of Seoul-Flour has been studied
1
e
systematically. Similar as Seoul-Flour, fluorophores with
indolizine skeleton has been also designed by Wang and co-
1
f
workers in 2015 (Figure 1). Therefore, the exploration of
efficient and useful synthetic protocols to rapidly construct
multisubstituted indolizine derivatives is highly desirable in the
area of synthetic organic chemistry and material science.
Thus far, several synthetic methods for the construction of
indolizines have been reported in the previous literature
Scheme 1 Different strategies for the synthesis of indolizine.
Among these synthetic methods, C-H functionalization and
transition-metal-catalyzed cycloisomerization of alkynylpyridine
derivatives both need to cut C3 and C4 bond to keep pyridine
scaffold on the basis of retro-synthetic analysis (Scheme 1). With
regard to this synthetic strategy, one of practical synthetic
pathways needs to build up an ortho-substituted pyridine
containing an alkyne chain or its equivalents. Recently, Liu and co-
workers have also reported a highly efficient synthesis of indolizines
from propargylic pyridines and its equivalents in the presence of
metal catalyst such as Cu or Pd, in which propargylic pyridines could
including:
1)
cyclization
of
pyridines
with
2
alkenyldiazoacetates, 2) metal-catalyzed C-H functionalization
3
reactions, 3) transition-metal-catalyzed cycloisomerization of
4
alkynylpyridine
derivatives,
4)
transannulation
of
5
pyridotriazoles with alkynes and others. Furthermore, Liu
6
a.
State Key Laboratory of Organometallic Chemistry, University of Chinese Academy
of Sciences, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032 China. mshi@sioc.ac.cn.
Electronic Supplementary Information (ESI) available: Experimental procedures,
†
characterization data of new compounds, and CCDC 1518151, 1544297, 1557465.
See DOI: 10.1039/b000000x
This journal is © The Royal Society of Chemistry 20xx
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o
be transformed to indolizines in good yields through intramolecular transformation is to carry out the reaction in DCE at 80 C using
8
a,8b
attack of nitrogen atom to alkynyl moiety (Scheme 1a).
3 2
Ad PAuCl (5 mol %) and AgNTf (5 mol %) as the catalysts.
Moreover, Alper’s group also developed palladium-catalyzed
oxidative carbonylation methods to synthesize indolizine
Table 1 Optimization of the reaction conditions for the synthesis of 2a
8
c
derivatives with similar substrates (Scheme 1a). The other
methods based on this strategy are also starting from ortho-
9
substituted methylene pyridines. The representative example is
the copper-mediated oxidative cross-coupling/cyclization of
methylene pyridine derivatives with simple olefins developed by Jia
9
a
[a]
entry
ligand
silver
salt
time
(h)
10
10
10
10
10
10
10
10
10
10
48
48
48
10
10
48
48
48
48
48
solvent
yield
(%)
46
0
0
21
7
8
and co-workers (Scheme 1a).
[b]
We envisaged another synthetic approach from pyrrole rather
than pyridine to construct indolizine through cutting C8 and C9
bond from the point view of retro-synthetic analysis, and
subsequently designed indole or pyrrole derivatives containing an
alkene-tethered diyne (this work, Scheme 1b). This working
hypothesis relies on various successful examples in the
hydroarylation of allenylheteroarenes or alkynylheteroarenes
1
2
3
4
5
6
7
8
9
JohnPhos
P(4ꢀCF Ph)
PPh
AgSbF
AgSbF
AgSbF
AgSbF
AgSbF
6
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
toluene
THF
3
3
6
3
6
t
P Bu
XPhos
JackiePhos
3
6
6
AgSbF
AgSbF6
6
t
BuBrettPhos
SPhos
9
AgSbF
AgSbF
AgSbF
AgSbF
AgOTf
AgNTf
ꢀ
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
6
37
41
59
75
61
87
0
n
1
0
P BuAd
2
6
reported by many research groups. For example, our group has
recently reported formal cycloadditions of indolyl-allenes for the
1
1
0
1
PAd
PAd
PAd
PAd
PtCl
ꢀ
PAd
PAd
PAd
PAd
PAd
3
3
3
6
6
catalyst-dependent stereodivergent and regioselective synthesis of 12
1
1
1
1
1
1
3
4
5
6
3
2
indole-fused heterocycles (Scheme 1b, previous work). Moreover,
2
diyne systems have been also frequently used as substrates for
2
2
2
2
2
2
0
1
2
gold-catalyzed transformation. Therefore, if hydroarylation of
3
63
61
48
22
46
indole or pyrrole containing an alkene-tethered diyne could take 17
3
3
3
3
1
1
2
8
9
0
dioxane
MeCN
DCM
place both at C2 and C3 position upon gold catalysis, we can easily
access a sort of new indolizine derivatives bearing eight-membered
ring (Scheme 1b) (see Supporting Information for the synthesis of [a] The reaction was conducted with 1a (0.1 mmol), LAuCl (0.005 mmol) and
o
starting materials).
silver salt (0.005 mmol) in 1,2-dichloroethane (0.5 mL) at 80 C.
[
b] Yield of isolated product purified by column chromatography on silica gel.
To achieve this hypothesis, our initial examination began with a
typical reaction using 1a as a substrate and JohnPhosAu(I) chloride
With the optimized reaction condition in hand, we then turned
1
3
and AgSbF
as the catalysts. To our delight, the desired two-fold our attention towards the examination of substrate scope, and the
6
hydroarylation product 2a was obtained in 46% yield in DCE (Table results are summarized in Scheme 2. All of the reactions proceeded
1
4
1
, entry 1). Encouraged by this result, we further screened the efficiently under the optimal condition, furnishing the desired
reaction conditions by changing the phosphine ligand of the gold products in moderate to high yields. For different diyne moiety in
catalyst to find out the optimal one. Simple phosphine ligands such substrates 1b-1d, the desired products were given in 75-80% yields
t
as P(4-CF Ph) and PPh did not produce 2a, and P Bu showed poor (1b containing two regioisomers, see Scheme 3). Introducing
3
3
3
3
catalytic efficiency under otherwise identical conditions (Table 1, halogen substituent on indole moiety afforded the desired products
entries 2-4). We also attempted to use Buchwald’s phosphine 2e-2i in 74-95% yields. Substrates 2j and 2k bearing methyl and
ligands such as XPhos, JackiePhos, tBuBrettPhos and SPhos in this methoxy groups on the indole moiety also gave the corresponding
reaction, but affording 2a in poor yields as well (Table 1, entries 5-8). products 2j and 2k in 78% and 83% yields, respectively. However,
The use of nBuPAd
2
gave 2a in better yield, suggesting that when nitro substituent was introduced on indole moiety, only trace
sterically bulky and electron-rich phosphine ligand is beneficial to of product 2l or 2m was formed presumably because the electron-
the formation of 2a (Table 1, entry 9). Therefore, we prepared tri(1- withdrawing substituent on indole ring would reduce its
adamantyl)phosphine ligand (PAd
3
)
according to Carrow’s nucleophilicity to attack the alkynyl moiety. Replacing indole with
1
5
procedure and found that the use of PAd gave 2a in 59% yield pyrrole produced the desired product 2n in 67% yield. The structure
3
(
Table 1, entry 10). Extending the reaction time to 48 hours of 2a has been confirmed by X-ray diffraction. Its ORTEP drawing
produced 2a in 75% yield (Table 1, entry 11). Next, we also and the CIF data are presented in the Supporting Information.
examined the silver salts and found that AgNTf is the best additive,
During the isolation and purification of 2b, we identified that
leading to the production of 2a in 87% yield (Table 1, entries 12-13). another cyclization product was also formed along with 2ba upon
The control experiments indicated that PtCl or AgNTf
alone could gold catalysis (Scheme 3). The structures of the two products have
2
2
2
1
6
not catalyze the reaction (Table 1, entries 14-15). The examination been unambiguously determined by X-ray diffraction (see
of solvent effects revealed that 1,2-dichloroethane (DCE) was the Supporting Information). For terminal alkyne, indole’s C3 of 1b has
best one (Table 1, entries 16-20). On the basis of above examination, two pathways (path a and path b) for cyclization through 8-endo or
we identified that the best reaction condition for this 7-exo dig ring closure upon gold catalysis, giving products 2ba in 9%
2
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yield bearing an eight-membered ring structure and 2bb in 71%
yield bearing a seven-membered ring structure (see Scheme S1 in
the Supporting Information). On the basis of X-ray crystal structure
of 2bb, we found a π-π stacking interaction between indolizine and
Ts group, which is not observed in the eight-membered ring
structure. For inner alkyne 1a, only 8-exo dig ring closure took place
to exclusively give indolizine derivative 2a containing an eight-
membered ring structure. We credited that the π-π stacking
interaction of 2bb played a dominant role for the formation of
seven-membered ring in the second cyclization.
Scheme 4 Substrate scope of different connecting group
Et
NTs
NTs
Ph
NTs
NTs
NTs
N
N
N
N
N
F
2
ba (9%)a
2bb (71%)a
2c (75%)b
2d (76%)b
e (83%)b
2
NTs
NTs
NTs Cl
NTs
NTs
Br
N
N
Cl
Br
N
N
N
Scheme 5 A Plausible Reaction Mechanism
2
f (95%)b
2g (95%)b
2i (74%)b
2
h (88%)b
2j (78%)b
Besides the special biological activities and fluorescent
properties of indolizine mentioned above, we accidentally found
that these compounds have a unique fluorescent emission and
measured their photonic property by fluorescence spectroscopy.
We found that most of products barely emit from 254 nm to 400
nm, but has green fluorescence from 400 nm to 580 nm at the
visible range, suggesting the usefulness as highly promising
MeO
NTs
NO2
NTs
NTs
NTs
N
N
N
N
O2N
2n (67%)b
2
k (83%)b
2l (trace)
2m (trace)
[
[
a] The yield of 2bb and 2ba is calculated by the ratio of 1:8 determined by NMR spectroscopic data.
b] Yields for isolated products.
Scheme 2 Substrate scope of the synthesis of indolizine derivatives.
1
8
candidates for OLEDs.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
1.0
0.8
0.6
0.4
0.2
0.0
2
c
2e
2j
2
a
2
d
2k
2bb
2n
Scheme 3 Two different structures of 2ba and 2bb.
300
350
400
450
500
550
600
650
700
300
350
400
450
500
550
600
650
700
Wavelength /nm
Wavelength /nm
Figure 2 Fluorescence emission spectra of products.
This gold-catalyzed transformation also tolerated different
nitrogen protecting groups such as benzenesulfonyl, benzylsulfonyl
and methanesulfonyl groups, affording the desired products 3a-3c
Most of products 2 emit green to yellow fluorescence, but
in excellent yields. The connector of two alkynes could be O or C, pyrrole product 2n has a unique fluorescence at 443 nm, which is
giving the corresponding products 3e and 3f in 82% and 43% yields, cerulean (Figure 2). The substituent on aromatic ring can slightly
respectively without formation of any seven-membered ring adjust emission wavelength, suggesting that they would just
structure even if they contain terminal alkynes (Scheme 4). This enhance chemical diversity and handling property. Products 2a and
may be due to that products 3e and 3f did not have an aromatic 2bb have fluorescence at 500 nm and the other products such as 2c
ring to make a π-π stacking interaction with indolizine as that of and 2d have fluorescence at 490 nm approximately.
product 2bb, thereby exclusively giving the desired products having
Table 2 Fluorescence Quantum Yield
eight-membered ring structures. However, when the connecting
group was C(CO Me) , the reaction gave a complex mixture (for
Compound
ΦF
Compound
ΦF
2
2
more information, see Scheme S2 in the Supporting Information).
A plausible mechanism for this transformation is outlined in
2a
2c
0.78
0.62
0.75
0.68
0.73
2a
3c
3e
3f
0.78
0.51
0.41
0.59
1
7a
2
e
Scheme 5. The cationic gold catalyst activates an alkynyl moiety
17b-c
2
j
in 1a to afford intermediate A,
which undergoes intramolecular
2
k
nucleophilic attack of C2 site in indole moiety to give the
corresponding indolizine intermediate B. Protodeauration of
intermediate B and the activation of another alkynyl moiety in 1a by
gold catalyst produce intermediate C, which undergoes nucleophilic
attack of C3 site in indole moiety and protodeauration again to
afford the desired product 2a via intermediate D.
2 2
sample were measured in CH Cl , c = 1.0 μM.
We also measured fluorescence quantum yields (Φ
and 3 and found that different connecting group at the eight-
membered ring can affect Φ (Table 2). The fluorescence property
relies on the core skeleton of indolizine with eight-membered ring.
The substituents at eight-membered ring and phenyl ring in
indolizine can tune the maximum emission wavelength. All of these
factors are shown in Scheme 4.
F
) of products
2
F
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6
.
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3
6
319; (b) X. Wu, X. Li and S. Chen, Asian J. Org. Chem., 2017, 6,
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1
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Scheme 4 Structure-photonics relationship.
In summary, we have developed a novel protocol for the
synthesis of indolizine scaffold from indole or pyrrole derivatives
7
8
.
.
(a) X. Li, J. Zhao, X. Xie and Y. Liu, Org. Biomol. Chem., 2017, 15,
containing
hydroarylations proceeded smoothly in the presence of tri(1-
adamantyl)phosphine coordinated gold complex (PAd AuCl) and
AgNTf mainly through an 8-exo-dig cyclization, affording indolizine
derivatives containing eight-membered ring in moderate to good
yields. We also preliminarily investigated their structure-photonics
relationship. Further investigations on expanding this gold-
catalyzed cyclization and application of this method are undergoing
in our laboratory.
an alkene-tethered
diyne.
These
two-fold
8
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We are grateful for the financial support from the National
Basic Research Program of China [(973)-2015CB856603], the
Strategic Priority Research Program of the Chinese Academy of
Sciences (Grant No. XDB20000000), the National Natural Science
Foundation of China (20472096, 21372241, 21572052, 20672127,
21421091, 21372250, 21121062, 21302203, 20732008, 21772037
1
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Supporting Information.
and 21772226).
Conflicts of interest
There are no conflicts of interest to declare.
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DOI: 10.1039/C7CC09250D
Journal Name
COMMUNICATION
A novel strategy for a gold(I)-catalyzed synthesis of
indolizine derivatives containing eight-membered ring
has been developed, which may have potential usefulness
as blue or green OLEDs.
Synthesis of Indolizine Derivatives Containing Eight-
membered Ring by
a Gold-catalyzed Two-fold
Hydroarylation of Diynes
*
Ruixing Liu, Qiang Wang, Yin Wei and Min Shi
This journal is © The Royal Society of Chemistry 20xx
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