Efficient two-step synthesis of structurally diverse indolo[2,3-: B] quinoline derivatives

Article abstract of DOI:10.1039/c8ob03033b

A general and efficient synthesis of diverse tetracyclic indolo[2,3-b]quinoline derivatives was achieved through palladium-catalyzed domino carboannulation/cross-coupling and DDQ-mediated double cross-dehydrogenative C-N bond formation. This approach provides a straightforward, atom-economical and concise route to easily access a diverse range of tetracyclic indolo[2,3-b]quinolines and their analogues in excellent yields with good tolerance of functional groups.

Full text of DOI:10.1039/c8ob03033b

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DOI: 10.1039/C8OB03033B
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Efficient Two Steps Synthesis of Structurally Diverse Indolo[2,3-
b]quinolines Derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Sandip Kundal, Baitan Chakraborty, Kartick Paul, Umasish Jana*
www.rsc.org/
A general and efficient synthesis of diverse tetracyclic indolo[2,3- In addition, recently, these scaffolds have been the part of
b]quinolines derivatives was achieved through a palladium- designing and synthesis of modern drugs.[ Therefore, due to
catalyzed domino carboannulation/cross coupling and DDQ- their significant applications in drug discovery research, several
mediated double cross-dehydrogenative C-N bond formations. This synthetic methods have been developed for the construction of
6]
[
7]
approach provides a straight forward, atom-economic and coincise indolo[2,3-b]quinolone ring structures since past decades.
route to easy access diverse range of tetracyclic indolo[2,3- Among the recent methods, some appear to be quite unique in
b]quinolines and its analogues in excellent yields with good the fabrication of their designs and, at the same time,
8]
tolerance of functional groups.
usefulness of their strategies.[ For example, Seidel’s group
achieved an easy access to the indolo[2,3-b]quinolines by the
condensation between indoles and 2-aminobenzaldehydes in
the presence of TFA or p-TsOH under refluxing condition in
Tetracyclic indoloquinolines are privileged structural motifs
embedded in a wide variety of natural alkaloids and bioactive
[
1]
compounds (Fig 1). In recent years, they drew much attention
due to their striking biological activities, particularly as DNA
intercalating and antimalarial properties and many other
important pharmacological activities.[ For example, 5-
methylindolo[2,3-b]quinoline (neocryptolepine), is isolated
from Cryptolepissanguinolenta and usually used in traditional
[8a]
toluene. Liang et al. reported an iodine-mediated synthesis of
indolo[2,3-b]quinolones via electrophilic
substitution/amination reaction of indole with 1-(2-
tosylaminophen-yl)ketones, in the presence of Cs CO (2 equiv.)
Yin et al. developed a synthesis of indolo[2,3-
b]quinolines from isoindigo derivatives in the presence of SnCl
2]
2
3
o
[8b]
at 90 C.
2
,
[
3]
West African medicine for the treatment of malaria. Similarly,
o
in combination with AcOH/HCl under heating at 120 C in
6
the
H-indolo[2,3-b]quinoline (Norcryptotackieine) isolated from
[8e]
moderate yields.
catalyzed synthesis of indoloquinoline by the reaction between
indoles and isoxazoles in the presence of AgSbF /NaOAc at 100
C. However, most of these methods usually have some
Wang’s group has published a Rh(III)-
Justiciabetonica,[
4]
and
such
exhibits
as
leaves
of
importantpharmacological
properties
5
potentantiplasmodial, antiproliferative, and antitumor
o
[8f]
activities.[
5]
limitations, such as, the use of prefunctionalized indole or
quinoline derivatives as starting materials, low overall yield of
the products and lack of flexibility. Considering the huge
pharmaceutical applications of indolo[2,3-b]quinolines
derivatives, the development of practical, straightforward,
flexible and efficient synthetic methodologies is still in fancy.
The synthesis of pharmaceutically important molecules
involving new strategies that are highly selective, atom- and
energy-efficient, and environmentally benign is a primary
challenge to synthetic chemist in this century. During our recent
investigations towards the synthesis of polycyclic heterocycles
and carbocycles through novel annulations reactions, very
recently, we have adopted a novel and efficient strategy for the
synthesis of polycyclic heterocycles involving palladium-
catalyzed intramolecular carbopalladation/Suzuki coupling and
subsequent cycloisomerisation under mild conditions. In
Me
N
N
N
N
H
N
N
Me
Norcryptotackieine
Cryptolepine
Neocryptolepine
Me
N
N
N
N
Me
Isoneocryptolepine
Isocryptolepine
Figure 1. Examples of natural indoloquinoline derivatives
[
9]
Department of Chemistry, Jadavpur University, Kolkata 700032, West
Bengal, India
*
E-mail: jumasish2004@yahoo.co.in, umasish@gmail.com.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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continuation to these investigations,[9e,f,g,h] we first synthesied After preparing the desired substrate 2a, we attempted to
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DOI: 10.1039/C8OB03033B
3
-indoline derivative 2 bearing ortho-amino group on the aryl develop a suitable reaction condition for the oxidative cross-
3
_
ring of methylene unit using palladium-catalyzed intramolecular dehydrogenative coupling (CDC) of allylic Csp H of indoline ring
domino carbopalladation-Suzuki coupling of 2-halo-N- with –NH group to achieve the synthesis of indolo[2,3-
2
propargylanilide derivatives 1 with arylboronic acids. We b]quinoline 3a with respect to various oxidizing agents,
anticipated that compound could undergo oxidative cross- solvents, and temperature. Considering the efficiency of DDQ
dehydrogenative coupling (CDC) with 2,3-dichloro-5,6-dicyano- for metal free oxidant in CDC reactions, we first carried out the
3
_
1
,4-benzoquinone (DDQ) between allylic Csp H of indoline ring intramolecular oxidative C–H amination of 2a in the presence of
with free –NH group. Very recently, DDQ has been proven as DDQ (1 equiv). It was observed that the substrates 2a rapidly
2
an efficient metal free oxidant for CDC reactions of carbon- converted into the mixture of compounds at room
carbon bond formation reactions,[ but the formation of C–N temperature. We inferred that due to the insufficient amount
bond via DDQ-mediated cross-dehydrogenative couplings (CDC) of DDQ, a mixture of compounds was formed including some
are relatively rare until recently.[ Therefore, the present unreacted starting materials. Next we attempted the said
strategy for the synthesis of indolo[2,3-b]quinolines should be reaction with 2 equiv. of DDQ, we were pleased to observe that
very practical, challenging and attractive in the modern context the starting compound 2a was completely converted to 3a at
of organic chemistry research. In this communication, we report room temperature in quantitative yield within 1 h (Table 1,
a new, efficient and general approach for the synthesis of entry 1). Other solvents such as 1,2-dichloroethane and
10]
11]
diversely substituted
indolo[2,3-b]quinolines derivatives nitromethane also worked well, but furnished slightly lower
through two stages domino reaction.
yields of 3a, in 90% and 93%, respectively (Table 1, entries 3 and
). Next, other oxidizing agents, such as TBHP, CAN, PIDA and
4
R
Oxidative
Annulation
DDQ (2 equiv.)
R
TEMPO, were evaluated for this process using 2a in DCM at
room temperature. It was found that TBHP, PIDA and TEMPO
(Table 1, entries 4, 6 and 7) did not work under these reaction
conditions, while CAN afforded 85% yield (Table 1, entry 5).
However, the cycloammination reaction with DDQ in
combination with TEMPO (Tetramethyl morphline N-Oxide) as
a co-oxidant (Table 1, entry 8) also worked efficeintly and gave
Br
Heck-Suzuki
NH2
N
H2N
N
Ts
RB(OH)2
N
CH2Cl2
rt
N
1
2
3
Ts
Ts
Scheme 1 Our Strategy for the Construction of Indolo[2,3-b]quinolines
Our approach commenced with the synthesis of 3-indoline derivative
2
a by palladium-catalyzed domino Heck-Suzuki coupling of 2-bromo- the desired product quntitatively. Thus, DDQ (2 equiv) in DCM
N-propargylanilide 1a with phenylboronic acid according to our at room temperature was adopted as the standard reaction
recently developed method.[9e,f] The reaction of compound 1a and condition for the additional study.
phenylboronic acid was carried out using 5 mol% of Pd(OAc)
mol% of tricyclohexylphosphine (PCy ) at 75 °C in the presence of 2.5
M K CO , to afford the desired 3-substituted indoline derivative 2a in
2% yield (Scheme 2). The carbopalladation reaction proceeds
through an intramolecular syn-carbopalladation via a 5-exo-dig
cyclisation process in preference to 6-endo-dig cyclisation with the
alkyne unit to give a σ-alkylpalladium(II) intermediate, and a
subsequent intermolecular Suzuki coupling with phenylboronic acid
derivatives gave the desired product 2a in a stereoselective fashion.
The preference of 5-exo-dig cyclisation over 6-endo-dig cyclisation
could be rationalized on the basis of lower energy transition state for
2
, 10
Table 1. Optimization of Reaction Conditions for the Synthesis of 3aa
3
Entry Reagent
Solvent Temp Time Yield(%)
2
3
8
1
2
DDQ
DDQ
DCM
DCE
rt
rt
1
1.5
quant
90
3
4
5
6
7
8
DDQ
CH
3
NO
2
rt
rt
rt
rt
rt
rt
1
93
CAN
DCM
DCM
DCM
DCM
1.5
3
85
TBHP
PIDA
TEMPO
NR
NR
NR
quant
3
3
5
-exo-dig cyclisation as bulky palladium complex ends up at the less
DDQ+TEMPO DCM
1
hindered side of the product, and the length of tether is also play an
important role for this mode of cyclisation.
[a]Reaction conditions: 2a (0.3 mmol), reagents (0.6 mmol), solvent (2 mL).
To demonstrate the generality of this strategy, we intended to
synthesize a series of indolo[2,3-b]quinolines following the
aforementioned two steps reactions. The preparations of
intermediate substrates 2b–2f were carried out by domino
Heck-Suzuki coupling in high yields (79% to 90%) (Table 2).
Substrates containing various groups, such as electron-donating
Ph
Pd(OAc)2
PCy3
Br
NH2
NH2
PhB(OH)2
N
N
Ts
Ts
2a, 82%
1
a
[a]Reaction conditions: 1a (0.3 mmol), phenylboronic acid (0.45 mmol), Pd(OAc)
2
–Me and weakly electron-withdrawing –Cl on the 2-bromo-N-
(
7
0.015 mmol), PCy
3
(0.03 mmol), 2.5 M K
2
CO
3
(1mL), 2 mL ethanol-toluene (1:1) at
o
propargylanilide ring, underwent smooth reaction in very good
yields, 83% and 79%, respectively (Table 2, entries 2b–2c).
Similarly, phenylboronic acids bearing –OMe and –Cl were well
tolerated under the reaction conditions, (Scheme 2, entries 2e–
5 C.
Scheme 2 Preparation of Substrate 2a by Heck-Suzuki coupling.[a]
2
| J. Name., 2012, 00, 1-3
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2
f) and gave the desired products in good yields, 85% and 81%, the synthesis of 3a. Gratifyingly, it was found that thVieewsAurbticsletrOantlienes
respectively. Substrate 2d, in which –Me group is present on the 2b–2e rapidly converted into the desiredDpOrIo: d10u.1c0ts393/bC8–O3eB0a3t0t3h3Be
aryl ring of 2-alkynyl aniline moiety, afforded 2d in 90% yield room temperature in quantitative yield, and the substrate 2f
(Table 2). Next, the 3-indoline derivatives 2b–2f were subjected afforded 92% yield of the desired product 3f (Table 2).
to DDQ-mediated oxidative cycloamination process as Therefore, it appears that there was no significant electronic
described for the synthesis of 3a. Gratifyingly, it was found that effect of the substituent on the aryl ring onto the olefinic motif
the substrates 2b–2e rapidly converted into the desired during the C–N bond formation.
products 3b–3e at the room temperature in quantitative yield, Next, to expand the synthetic utility of this strategy, we
and the substrate 2f afforded 92% yield of the desired product prepared the substrates 2g and 2h by Pd-catalyzed reductive
3
f (Table 2). Therefore, it appears that there was no significant carobopalladation of 2-bromo-N-propergylanilide derivatives
electronic effect of the substituent on the aryl ring onto the using Pd(OAC) /PCy , in the presence of 2.5 M K CO in toluene-
2
3
2
3
[
9c]
olefinic motif during the C–N bond formation.
ethanol mixture according to our previous method. The
desired 3-indoline derivatives 2g and 2h were obtained in 89%
and 88%, respectively. Next, when these compounds were
treated with 2 equiv of DDQ in DCM at room temperature and
afforded desired products 3g and 3h in 82% and 75% yields,
respectively (Scheme 3).
Table 2. Synthesis of Indolo[2,3-b]quinolines via Heck-Suzuki and DDQ-Mediated C-H
Amination.[a]
R2
NH2
Ar
R2
Ar
R2
Br
R1
R1
R1
i
ii
R2
R2
N
3b-3f
H2N
N
N
N
1
b-1f Ts
Ts
2b-2f
Ts
NH2
H
R2
R1
H
R1
Br
i
R1
ii
Entry
Substrates
3-indolines
Ph
Indolo[2,3-b]quinolines
NH2
N
Me
Ph
N
N
N
Me
Ts
Ts
2g, 3 h, 89%
R1 = Me, R2 = H, Ar = Ph
Ts
3
1
a, R1 = H, R2 = H
1
g, 2 h, 82%
H2N
N
N
N
Ts
Ts
1e, R1 = H, R2 = Cl
2h, 3 h, 88 %
3h, 2.5 h, 75%
3b, 1 h, quant
2b, 3 h, 83 %
Ph
Cl
Ph
Cl
[a]Reaction conditions: (i) Compound 1 (0.3 mmol), Pd(OAc)
(0.03 mmol), 2.5 M K CO
Compound 2 (0.2 mmol), DDQ (0.4 mmol), CH
(0.015 mmol), PCy
3
(1 mL), 2 mL ethanol-toluene (1:1 mixture), 75 °C. (ii)
Cl (2 mL), rt.
R1 = Cl, R2 = H, Ar = Ph
2
2
H2N
N
N
N
2
3
Ts
Ts
2
c, 3 h, 79 %
3c, 1 h, quant
2
2
Ph
Me
Ph
Me
R1 =H, R2 = Me, Ar = Ph
Scheme 3 Substrates Scope for Carbopalladation and Cycloamination.
3
N
N
N
H2N
Ts
Ts
2
d, 3 h, 90 %
3d, 1 h, quant
R2
C6H4 p-OMe
C6H4 p-OMe
X
R2
R2
Ph
4
R1 = H, R2 = H,
NH2
R1
Ph
R1
Ar = C6H4 p-OMe
N
N
N
H2N
R1
(i)
PhB(OH)2
(ii)
Ts
Ts
Br
X
X
2e, 3 h, 85 %
3e, 1 h, quant
C6H4 p-Cl
N
X
H2N
DDQ
X
N
C6H4 p-Cl
N
X
N
Ts
R1 = H, R2 = H,
Ts
Ts
i-2k
5
2
3i-3k
Ar = C6H4 p-Cl
1f-1h
N
H2N
N
N
Ts
f, 3 h, 81 %
Ts
3f, 1 h, 92 %
entry
Substrate 1
Product 2
Ph
Product 3
Ph
2
[a]Reaction conditions: (i) Compound 1 (0.3 mmol), ArB(OH)
3
Pd(OAc) (0.015 mmol), PCy (0.03 mmol), 2.5 M K CO (1 mL), 2 mL ethanol-
(0.45 mmol),
2
2
3
2
2 2
toluene (1:1), 75 °C. (ii) Compound 2 (0.2 mmol), DDQ (0.4 mmol), CH Cl (2
mL), rt.
Br
N
N
N
N
NH2
Me
2
H N
1
N
Ts
Ts
N
N
Ts
2
i, 3.5 h, 77%
3i, 2 h, 85%
1
f
To demonstrate the generality of this strategy, we intended to
synthesize a series of indolo[2,3-b]quinolines following the
aforementioned two steps reaction conditions. The
preparations of substrates 2b–2f were carried out by domino
Heck-Suzuki coupling in high yields (79% to 90%) (Table 2).
Substrates containing various groups, such as electron-donating
Me
Ph
Ph
Me
N
Br
NH2
N
N
2
N
H2N
N
N
N
Ts
Ts
Ts
1g
2j, 3.5 h, 80%
3j, 3.5 h, 83%
Me
Me
N
Ph
Me
Ph
Me
Me
–Me and weakly electron-withdrawing –Cl on the 2-bromo-N-
N
Me
Br
N
N
N
N
propargylanilide ring, underwent smooth reaction in very good
yields, 83% and 79%, respectively (Table 2, entries 2b–2c).
Similarly, phenylboronic acids bearing –OMe and –Cl were well
NH2
N
2
H N
3
N
N
N
Ts
Ts
Ts
1
h
2k, 4 h, 75%
3k, 3.5 h, 75%
[a]Reaction conditions: (i) Compound 1 (0.3 mmol), PhB(OH)
2
(0.45 mmol),
tolerated under the reaction conditions, (Scheme 2, entries 2e– Pd(OAc)2 (0.015 mmol), PCy3 (0.03 mmol), 2.5 MK CO (1 mL), 2 mL 75 °C. (ii)
2
3
2
2 2
f) and gave the desired products in good yields, 85% and 81%, Compound 2 (0.2 mmol), DDQ (0.4 mmol), CH Cl (2 mL), rt.
respectively. Substrate 2d, in which –Me group is present on the
aryl ring of 2-alkynyl aniline moiety, afforded 2d in 90% yield
Scheme 4 Synthesis of aza-indolo[2,3-b]quinolines[a]
(Table 2). Next, the 3-indoline derivatives 2b–2f were subjected
to DDQ-mediated oxidative amination process as described for
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Finally, to make this strategy even more general and flexible, we common and direct mechanism is initial one step hydride
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incorporated additional nitrogen atoms into both of the rings at two transfer from the substrate to DDQDO
I:
1
0.10
39/
C
8
O
B
0
3
0
33B
a
n
d
a
c
a
r
b
o
c
a
t
io
n
ends of the tetracyclic system; the results are described in Scheme 4. intermediated is generated. The other mechanistic pathways
Similarly, 3-indoline derivative containing 2-amino pyridine ring for are initial electron transfer from the substrates to DDQ to form
the radical cation of the substartes. DDQ mediated radical
pathway is generally eastablished by the inhibition of reaction
in the presence of radical scavenger.[ We noticed that the
radical scavanger, TEMPO (2 equiv) did not affect the DDQ
mediated dehydrogenative cycloamination process in the
present reaction system (Table 1, entry 8). Therefore, we
assumed that the activation of allylic Sp C–H bond might be
initiated through an hydride ion transfer mechanism from the
substrate 2 to DDQ resulting generation of an allylic carbocation
the discovery of new pharmaceutically a(Scheme 4, 2j) furnished
indolo[2,3-b]naphthyridine 3j in 83% yield. Finally, we were pleased
to find that aza-indoline tethered 2-amino pyridine derivative 2k
furnished the hitherto unknown tetracyclic aza-indolo[2,3-
b]naphthyridine 3k in 75% yield. Along this line, it is important to
note that, as indolo[2,3-b]quinolines acted as DNA-intercalator, in
this context the synthesis of these aza-heterocyclic ring systems
would be highly attractive in order to assemble a library of new
tetracyclic framework ctive compounds.
13]
3
2
aa/DDQH¯ ion pair. The allylic carbocation intermediate 2aa
is stabilized by resonance and furnished a benzylic cation 2bb.
Then cycloamination took place by intramolecular nucleohilic
attack of –NH group to the allylic cationic centre resulting
2
dihydropyridinium ion intermediate 2dd. Finally, deprotonation
of the substrate 2dd by DDQH¯ produced dihyropyridine
Although, the Heck Suzuki reaction in most of the cases gave a
clean reaction product, but we noticed that the Heck-Suzuki
coupling reaction of aryllboronic acid bearing strong electron-
withdrawing groups such as –CHO and –CN, furnished a
nonseparable mixture. To avoid the cumbersome purification of
these compounds, the crude products were directly reacted
with DDQ, as depicted in Scheme 5. To our delight, the desired
products were also obtained in high yields in the two steps, such
as 3l and 3m, in 74% and 71%, respectively.
2
intermediate 2ee with concomitant formation of DDQH . In the
next step, hydride transfer from 2ee to DDQ and subsequent
deprotonation leading to the desired indolo[2,3-b]quinoline
derivative 3.
R
R
R
DDQ
Ar
Ar
H
NH2
NH2
H
DDQH
ArB(OH)2
DDQ
N
N
H2N
N
H
Br
DDQH
H
N
Ts
Ts
Ion pair
Ts
2bb
H2N
N
NH2
2aa
Ion pair
N
2
Ts
Ts
N
Ar = C6H4-pCN 3l, 2 h, 74%[b]
Ar = C6H4-pCHO 3m, 2.5 h, 71 %[b]
Ts
1a
R
R
R
-
DDQH2
[
a]
N
H
Reaction Conditions: i) 1a (0.3 mmol), PhB(OH)
2
(0.45 mmol), Pd(OAc)
2
(0.015
Cl
N
N
N
H
H
H
N
N H
o
Ts
ee
H
H
mmol), PCy
(
3
2
(0.03 mmol), K CO
3
(2.5 M, 1mL), 75 C, 3h. Ii) DDQ (0.6 mmol), CH
2
2
Ts
H
2
DDQ
DDQH
Ion pair
Ts
2cc
2 mL), rt. [b]Two step yields.
2dd
DDQH
Ion pair
Scheme 5 Synthesis of indoloquinolines without isolation of intermidate.[a]
DDQH2
R
Furthermore, as the natural products do not contain any
protecting groups, and hence, we also attempted detosylation
of 3a in the presence of dil NaOH solution and methanol under
reflux. Pleasantly, the detosylated product 4 was obtained in
N
N
Ts
3
Scheme 7 Proposed Mechanism for DDQ-mediated Cycloamination
8
2% yield (Scheme 6).
This work not only represented a new strategy to provide a
conceptually alternative route, but also due to easy availability
of starting materials and efficiency of the double annulations
processes, this method could offer a practical and arguably an
ideal strategy for the rapid access to diversified indolo[2,3-
b]quinoline derivatives with high atom economy and step
economy. Moreover, the products obtained in our strategy
could easily be converted to natural occurring indolo[2,3-
b]quinolines such as Norcryptotackieine, by simple detosylation
of 3g. To the best of our knowledge this is the first report of
preparation of indolo[2,3-b]quinoline derivatives through
carbopalladation/cross-coupling and subsequent DDQ
Ph
Ph
NaOH
MeOH, reflux
5h
N
N
N
Ts
N
H
3a
4, 82%
Scheme 6. Detosylation of indolo[2,3-b]quinoline3a
The
mechanism
of
domino
Heck-Suzuki/reductive
carbopalladtion has already been described in our previous
reports.
[
9b,c]
On the basis of our experimental results and
3
mediated allylic Csp –H amination reaction from 2-bromo-N-[3-
following the previous literature,[ a tentative mechanism for
the DDQ mediated oxidative C–H amination is delineated in
Scheme 6. Different mechanistic pathways have been
postulated in the literatures for the DDQ mediated oxidation
depending on subtrates and reaction conditions. The most
12]
(
2-aminophenyl)prop-2-ynyl]-N-tosylanilide.
1
13
All the structures were characterized by H, C NMR and HRMS
spectra and one of the structure 3e confirmed by X-ray
diffraction (See supporting information).
4
| J. Name., 2012, 00, 1-3
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Page 5 of 5
O rP gl ea na si ce &d Bo i on mo to al e dc juu l sa tr mC ha er mg i ins ts ry
Journal Name
COMMUNICATION
L. Wang, W. –J .Lu, C. -Q. Pang, T. Maeda, W. PeVinewg,ArMtic.leaOisnelinre,
Conclusions
I. E. El-Sayed, T. Inokuchi, J. Med. Chem.2013, 56, 1431.
DOI: 10.1039/C8OB03033B
In summary, we have developed a novel and efficient two steps
strategies involving Pd-catalyzed carbocyclisation/cross-
coupling and subsequent DDQ-mediated intramolecular double
oxidative amination reaction to construct medicinally useful
indolo[2,3-b]quinoline derivatives in an atom efficetnt maner.
This method was found to be general and displays a wide
substrate scopes, good functional groups tolerance, and
provides excellent yields of the desired products. Importantly,
this strategy has also been utilized for the synthesis of
6
7
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Parameswaranb, Curr. Org. Synth. 2016, 13, 58; (c) P. T.
Parvatkar, P. S. Parameswaran, S. G. Tilve, Curr. Org. Chem.,
2
011, 15, 1036.
8
Recent report of synthesis of indolo[2,3-b]quinolines, see: (a)
M. K. Vecchione, A. X. Sun, D. D. Seidel, Chem. Sci., 2011, 2,
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aza-indolo[2,3-b]quinolines,
indolo[2,3-
2
178; (b) S. Ali, Y. Li, S. Anwar, F. Yang, Z. Chen, Y. Liang, J. Org.
b]napthyridine and aza-inolo[2,3-b]napthyridine derivatives for
the first time. Further studies toward applications of these
concise protocol to access other bioactive scaffolds are
currently underway in our laboratory.
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Acknowledgements
S.K. and B.C. are thankful to the UGC, New Delhi, India, and K.P. is
thankful to the CSIR, New Delhi, India, for their fellowships. Authors
gratefully acknowledge the financial assistance grant under the UGC
CAS programme, New Delhi, India and DST PURSE-II programme,
New Delhi, India, of Department of Chemistry, Jadavpur University.
9
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Conflicts of interest
There are no conflicts to declare.
1
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