Combining Visible-Light-Photoredox and Lewis Acid Catalysis for the Synthesis of Indolizino[1,2-b]quinolin-9(11H)-ones and Irinotecan Precursor

Article abstract of DOI:10.1021/acs.orglett.7b03395

One-step construction of substituted indolizino[1,2-b]quinolin-9(11H)-ones was achieved by combining visible-light-photoredox and Lewis acid catalysis for an intramolecular Povarov cycloaddition reaction under mild conditions. In this catalytic process, t

Full text of DOI:10.1021/acs.orglett.7b03395

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Combining Visible-Light-Photoredox and Lewis Acid Catalysis for the
Synthesis of Indolizino[1,2‑b]quinolin-9(11H)‑ones and Irinotecan
Precursor
Wuheng Dong, Yao Yuan, Bei Hu, Xiaoshuang Gao, Huang Gao, Xiaomin Xie, and Zhaoguo Zhang*
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China
*
S Supporting Information
ABSTRACT: One-step construction of substituted
indolizino[1,2-b]quinolin-9(11H)-ones was achieved by com-
bining visible-light-photoredox and Lewis acid catalysis for an
intramolecular Povarov cycloaddition reaction under mild
conditions. In this catalytic process, the visible-light-promoted
dehydrogenation protocol of tetrahydroquinolines constitutes
the key procedure. Moreover, this method can be applied to
the formal synthesis of the precursor of irinotecan, which
exhibited good anticancer activities.
7
f,g
ndolizino[1,2-b]quinolin-9(11H)-ones represent an impor-
tant class of N-containing heterocycle skeletons that are
present in a variety of natural products and biologically active
Yao developed cascade reactions triggered by Me OBF or
3 4
I
bis(triphenyl)oxodiphosphonium trifluoromethanesulfonate, af-
fording a variety of indolizino[1,2-b]quinolin-9(11H)-ones
compounds (Scheme 1a and 1b). Moreover, a Dy(OTf)3-
molecules. For instance, camptothecin was isolated from
1
camptotheca acuminate by Wani and Wall, which has been
2
proven to be a potent tumor inhibitor. Subsequent belotecan
Scheme 1. Povarov Cycloaddition Reaction for Synthesis of
Quinoline Derivatives
3
and topotecan are two commercially available anticancer drugs
that are the most frequently used anticancer drugs in clinical
4
settings. In addition, irinotecan and camptothecin analogues are
commonly used anticancer drugs (Figure 1). Therefore, a
catalyzed Povarov cycloaddition/aromatization cascade reaction
between an N-arylimine and an unactivated alkyne was also
found in the synthesis of this structure by Batey et al. in 2004
Figure 1. Representative drugs with indolizino[1,2-b]quinolin-9(11H)-
one frameworks.
7d
(
Scheme 1c). In the above cases, an alkyne, which serves as a
number of approaches for their synthesis have been developed,
5
dienophile, is often required.
which involve transition-metal-catalyzed cross-coupling, ring-
6
7
Despite these advances, more efficient and straightforward
protocols for synthesizing diverse indolizino[1,2-b]quinolin-
closing metathesis, Povarov cycloaddition, and radical
8
reactions. Among them, the Povarov cycloaddition reaction of
9
(11H)-ones using olefins as a dienophile under mild reaction
N-aryl imines derived from aldehydes and anilines with electron-
rich olefins was thought to constitute the most efficient method
conditions remains a major challenge.
9
for tetrahydroquinolines synthesis. To furnish the quinolone
structure, however, the dehydrogenation protocol requires a
stoichiometric oxidant and harsh conditions. Fortunak and
Received: November 1, 2017
10
7a,b
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03395
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
In the past few years, visible-light-induced chemical trans-
formations have attracted increased interest from organic
chemists due to their green and sustainable features. Recently,
this strategy has been successfully applied to catalytic
dehydrogenation of tetrahydroquinolines. For example, in
Table 1. Optimization of Reaction Conditions
11
2
016, Badu-Tawiah and co-workers presented a concise
synthesis of quinolines through visible-light-promoted dehydro-
genation of tetrahydroquinolines under ambient conditions
b
entry
photocatalyst
Eosin Y
Ru(bpy) Cl ·6H O
catalyst
solvent
yield (%)
1
2
1
2
3
4
5
6
7
8
BF ·Et O
CH CN
20
(Scheme 2a). Soon afterward, the Li group reported a dual
3
2
3
BF ·Et O
CH CN
>95
>95
>95
95
3
2
2
3
2
3
Ru(bpy) (PF )
BF ·Et O
CH CN
3
6
2
3
2
3
Scheme 2. Visible-Light-Promoted Dehydrogenation of
Tetrahydroquinolines
Ru(bpy) Cl ·6H O
TsOH·H O
CH CN
3
2
2
2
3
Ru(bpy) Cl ·6H O
Zn(OTf)2
CH CN
3
2
2
3
Ru(bpy) Cl ·6H O
TsOH·H O
MeOH
DMF
>95
83
3
2
2
2
Ru(bpy) Cl ·6H O
TsOH·H O
3
2
2
2
Ru(bpy) Cl ·6H O
TsOH·H O
DCM
64
3
2
2
2
9
Ru(bpy) Cl ·6H O
TsOH·H O
toluene
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
28
3
2
2
2
c
10
11
12
13
14
15
16
Ru(bpy) Cl ·6H O
TsOH·H O
>95
3
2
2
2
d
f
e
Ru(bpy) Cl ·6H O
TsOH·H O
>95 (96)
3
2
2
2
Ru(bpy) Cl ·6H O
30
25
0
3
2
2
g
h
i
TsOH·H O
2
Ru(bpy) Cl ·6H O
TsOH·H O
0
3
2
2
2
j
Ru(bpy) Cl ·6H O
TsOH·H O
0
3
2
2
2
a
Conditions: 1a (61.0 mg, 0.3 mmol), 2a (32.7 mg, 0.3 mmol),
photocatalyst (2 mol %), catalyst (10 mol %), solvent (3 mL),
b
1
irradiation with 23 W household light bulb at rt for 24 h. HNMR
yield was reported using 4,4′-ditertbutylbiphenyl as an internal
c
d
e
standard. Photocatalyst (0.5 mol %). Catalyst (5 mol %). Isolated
f
g
h
yield. Without TsOH·H O. Without photocatalyst. Without
photocatalyst and TsOH·H O. Reaction was run in the dark. Under
2
i
j
photo- and cobalt-catalyzed dehydrogenation of N-heterocycles
2
1
3
(Scheme 2b). Inspired by these two reports, we envisaged an
N atmosphere.
2
intramolecular Povarov cycloaddition reaction between in situ
generated benzylidene anilines derived from arylamines and
salicylaldehyde bearing a tethered alkene partner, followed by a
visible-light-induced photocatalytic dehydrogenation to yield the
target products (Scheme 2c). As part of our ongoing research
into the synthesis of heterocycles under visible light, we herein
report an efficient and alternative route to indolizino[1,2-
b]quinolin-9(11H)-ones using a photoredox strategy.
showed that the loading of acid and photocatalyst could be
decreased to 5 mol % and 0.5 mol %, respectively, without
diminishing product yield (Table 1, entries 10 and 11). Finally,
some control experiments revealed that both photocatalyst and
visible light were necessary. Specifically, only 25% of the desired
product was obtained when the photocatalyst was absent, and no
desired product was detected when the reaction was conducted
Considering that the alkene could serve as synthons for
either in the dark or under a N atmosphere (Table 1, entries 12−
2
14
introducing other functional groups, we chose 1-((2E,4E)-
16).
hexa-2,4-dien-1-yl)-6-oxo-1,6-dihydropyridine-2-carbaldehyde
With the optimized reaction conditions elucidated, we next
investigated the scope of this cascade process by utilizing various
substituted anilines in combination with 1-((2E,4E)-hexa-2,4-
dien-1-yl)-6-oxo-1,6-dihydropyridine-2-carbaldehyde (1a) as the
reaction partner (Scheme 3; for the crystal structures of 3a, 3d,
3j, 3k, and 3m; see Supporting Information). The substituent on
the aniline plays a critical role in the success of the
dehydrogenation of tetrahydroquinolines. Both weakly elec-
tron-donating groups (Scheme 3, 3b) and strongly electron-
donating groups (Scheme 3, 3c, 3d) located para to the aniline
resulted in excellent yields. However, aniline with electron-
withdrawing groups at this position exhibited an obvious
negative effect on the yield (Scheme 3, 3e, 3f). Increasing steric
demand ortho to the amino group did not compromise reaction
efficiency, and a high yield of target product was obtained
(Scheme 3, 3h, 3i). The scope of the reaction was next examined
(1a) as the model substrates to test the feasibility of visible-light
photoredox catalysis (Table 1). Initially, we investigated the
reaction of 1-((2E,4E)-hexa-2,4-dien-1-yl)-6-oxo-1,6-dihydro-
pyridine-2-carbaldehyde (1a) and 4-aminophenol (2a) using
Eosin Y (2 mol %) as a photocatalyst with BF ·Et O (10 mol %)
3
2
as a catalyst in acetonitrile under the irradiation of a 23 W
household compact fluorescent lamp at room temperature for 24
h. Fortunately, the starting material was completely converted to
the corresponding tetrahydroquinolines (II), along with the
target product 3a in 20% yield (entry 1). Since we thought that
the photocatalyst might play an important role in the
dehydrogenation of tetrahydroquinolines, other commonly
used transitional metal photocatalysts were examined to improve
reaction efficiency. To our satisfaction, when both Ru(bpy) Cl ·
3
2
6
H O and Ru(bpy) (PF ) were used as the photocatalyst, the
2 3 6 2
1
desired product could be obtained at more than a 95% yield.
Other acids, such as p-methylbenzene sulfonic acid and zinc
trifluoromethanesulfonate, exhibited a similarly high catalytic
efficiency (Table 1, entries 4 and 5). In addition, solvent
screening proved that acetonitrile and methanol were superior to
other solvents (Table 1, entries 6−9). Further optimization
with respect to the substituent effect (R ) on the alkene terminus.
With substrates bearing no substituent or more than one
substituent at the C−C double bond, corresponding products
were obtained in excellent yield (Scheme 3, 3j, 3k). Moreover,
1
the R -alkyl substituent can be replaced by an aryl substituent, as
documented for the phenyl (Scheme 3, 3l). A good yield was also
B
DOI: 10.1021/acs.orglett.7b03395
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Substrate Scope
chloride and DMSO. Subsequently, under the optimized
conditions, the Povarov cycloaddition/dehydrogenation aroma-
tization cascade reaction of aldehyde 7 and 4-aminophenol gave a
good yield (92%) of the product 8, which constitutes the key
6l,15
synthetic intermediate to 10-hydroxycamptothecin
irinotecan 9.
Based on the above results and related reports,
and
16
6m,13
we
propose a plausible mechanism to account for the reaction in
Scheme 5. The intermediate II is formed via an intramolecular
Scheme 5. Proposed Mechanism for the Catalysis
a
Conditions: 1 (0.3 mmol), 2 (0.3 mmol), photocatalyst (0.5 mol %),
catalyst (5 mol %), solvent (3 mL), irradiation with 23 W household
b
light bulb at rt for 24 h. 96 h irradiation.
attained when a conjugated diene was replaced by styrene
functionality (Scheme 3, 3m). However, no desired product was
obtained, except for a mixture of imine and corresponding
tetrahydroquinolines in the substrate of ethenyl or crotyl
functionality (Scheme 3, 3n and 3o).
Povarov cycloaddition reaction between the in situ generated
Furthermore, we utilized this mild and efficient cascade
reaction to synthesize the irinotecan precursor from the known
methyl 4-ethyl-8-oxo-7,8-dihydro-1H-pyrano[3,4-c]pyridine-6-
imine I derived from 1a and 2a. Then, the intermediate II was
II
oxidized by the excited Ru * to give the radical amine cation III
I
7
f
and the reduced species Ru . The intermediate III was converted
carboxylate 4 (Scheme 4).
to intermediate IV or V. The desired product 3a was then
obtained after a further oxidative dehydrogenation reaction.
In summary, we have developed an intramolecular Povarov
cycloaddition reaction to construct substituted indolizino[1,2-
b]quinolin-9(11H)-one, and visible-light-promoted dehydro-
genation of tetrahydroquinolines as the key procedure.
Furthermore, this reaction has been successfully applied to the
formal synthesis of the precursor of irinotecan 9, which possesses
anticancer activities. The applicability of readily available starting
materials and the use of air as the sole oxidant under mild
reaction conditions make this process very attractive. Further
applications of this transformation toward other heterocyclic
products are currently underway.
Scheme 4. Synthetic Route to the Irinotecan Precursor
ASSOCIATED CONTENT
■
*
S
Supporting Information
The corresponding pyridone 5 was obtained by the N-
allylation of 4 with (E)-5-bromopenta-1,3-diene in 50% yield,
and then the reduction of pyridone 5 selectively achieved the
corresponding alcohol 6 (92%). The important intermediate,
aldehyde 7, has been previously synthesized in 85% yield by the
Swern oxidation of the corresponding alcohol 6 with oxalyl
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03395.
Preparation of substrates; general procedure; X-ray crystal
structure of 3a, 3d, 3j, 3k, and 3m; characterization data;
1
13
H and C NMR spectra (PDF)
C
DOI: 10.1021/acs.orglett.7b03395
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
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CCDC 1573982, 1573984−1573985, and 1573987−1573988
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.a-
c.uk/data_request/cif, or by emailing data_request@ccdc.cam.
ac.uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
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AUTHOR INFORMATION
Corresponding Author
E-mail: zhaoguo@sjtu.edu.cn.
■
*
(
(
b) Muhuhi, J.; Spaller, M. R. J. Org. Chem. 2006, 71, 5515−5526.
c) Jacob, R. G.; Perin, G.; Botteselle, G. V.; Lenardao, E. J. Tetrahedron
Lett. 2003, 44, 6809−6812. (d) Maiti, S.; Panja, S. K.; Sadhukhan, K.;
Ghosh, J.; Bandyopadhyay, C. Tetrahedron Lett. 2012, 53, 694−696.
10) (a) Rohlmann, R.; Stopka, T.; Richter, H.; Garcia Mancheno, O. J.
ORCID
̃
Xiaomin Xie: 0000-0002-5798-291X
Zhaoguo Zhang: 0000-0003-3270-6617
Notes
(
̃
Org. Chem. 2013, 78, 6050−6064. (b) Richter, H.; García Mancheno, O.
The authors declare no competing financial interest.
Org. Lett. 2011, 13, 6066−6069. (c) Desrat, S.; van de Weghe, P. J. Org.
Chem. 2009, 74, 6728−6734.
(
11) (a) Prier, C. K.; Rankic, D. A.; MacMillan, D. W. Chem. Rev. 2013,
ACKNOWLEDGMENTS
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The authors acknowledge the financial support provided by the
National Natural Science Foundation of China (No. 21232003).
We also express gratitude for the support and valuable
suggestions regarding SC-XRD measurements from Ms. Xiao-li
Bao and Ms. Ling-ling Li of the Instrumental Analysis Center of
Shanghai Jiao Tong University.
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