An expedient synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones via copper(II) triflate-catalyzed intramolecular cyclization of N- propargylaminonaphthoquinones

Article abstract of DOI:10.1016/j.tetlet.2013.04.125

An expedient synthesis of a series of 1,2-dihydrobenzo[g]quinoline-5,10- diones in good yields has been accomplished via three-component one pot sequential reactions of 2-hydroxynaphthalene-1,4-dione, substituted anilines and propargyl as well as 3-ethylp

Full text of DOI:10.1016/j.tetlet.2013.04.125

Tetrahedron Letters 54 (2013) 3735–3739
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Tetrahedron Letters
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An expedient synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones
via copper(II) triflate-catalyzed intramolecular cyclization of
N-propargylaminonaphthoquinones
⇑
Balasubramanian Devi Bala, Sivasubramanian Muthusaravanan, Subbu Perumal
Department of Organic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An expedient synthesis of a series of 1,2-dihydrobenzo[g]quinoline-5,10-diones in good yields has been
accomplished via three-component one pot sequential reactions of 2-hydroxynaphthalene-1,4-dione,
substituted anilines and propargyl as well as 3-ethylpropargyl bromides furnishing N-propargylamino-
naphthoquinones, and their concomitant copper(II) triflate-catalyzed intramolecular 6-endo-dig
cyclization.
Received 26 February 2013
Revised 27 April 2013
Accepted 30 April 2013
Available online 15 May 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Three-component reactions
1,2-Dihydrobenzo[g]quinoline-5,10-dione
N-Propargylaminonaphthoquinones
Copper(II) triflate
endo-dig Cyclization
Functionalized heterocyclic building blocks are of great impor-
tance in both medicinal and synthetic chemistry and development
of new efficient synthetic methodologies for these scaffolds remains
a great challenge in modern organic synthesis.1 1,2-Dihydro-
benzo[g]quinoline-5,10-dione framework is an important structural
motif prevalent in natural products with interesting biological prop-
erties.2 Interest in 1,2-dihydrobenzo[g]quinoline-5,10-dione skele-
ton is connected to its structural relationship with dielsiquinone
and marcanines A–E (Fig. 1). The dielsiquinone is a potent cytotoxic
natural product related to anthracyclines that provides the basis for
the development of safer anticancer drugs due to its less cardiotox-
icity than the clinically used anthracyclines.2a Marcanines A–E,
isolated from the stem bark of Goniothalamus marcanii, exhibit cyto-
toxic activity against several human tumor cell lines.2b Marcanine A
displays in vitro antimalarial activity against the drug-resistant K1
strain of Plasmodium falciparum.2c Previously reported synthesis of
1,2-dihydrobenzo[g]-quinoline-5,10-dione skeleton includes that
of marcanine,3 dielsiquinone,4,2a 1-aza-2,9,10-anthracenetriones,5
polyheterocyclic quinones,6 benzo[g]quinoline-2,5,10-trione,7 and
1,2,5,10-tetrahydrobenzo[g]quinoline-5,10-dione.8
have appeared on the utility of inter- or intramolecular cyclization
of alkyne functionality in the assembly of heterocycles such as oxaz-
olines and oxazoles,9 acridines and quinolines,10 isoquinolines and
pyridines,11 N-fused imidazoles,12 3-acylated indolizines,13 tetrahy-
droisoquinolines,14 aminoindolizines,15 chromenoquinoxalines,16
dihydroquinolines and indoles,17 indolequinone,18 benzo[b]fur-
ans,19 pyrano[4,3-b]quinolines,20 3-halochalco-genophene[3,2-
c]chromene,21 and azaindoles.22 The azaanthraquinone skeleton
has been constructed by the intramolecular cyclization of N-propa-
rgylaminoquinones.23 In continuation of our interest in the assem-
bly of functionalized novel heterocycles by tandem, multi-
component, and green transformations,24 herein we describe the
synthesis of N-propargylaminonaphthoquinones, from the sequen-
tial reaction of readily available starting materials, viz. 2-hydroxy-
naphthalene-1,4-dione, substituted anilines and propargyl
bromide/3-ethylpropargyl bromide, and their subsequent intramo-
lecular cyclization furnishing 1,2-dihydrobenzo[g]quinoline-5,
10-diones (Scheme 1).
R4
O
R3
Dielsiquinone, R1, R2 = H; R3 = Me; R4, R5 = H
Marcanine A, R1, R2 = H; R3 = Me; R4, R5 = H
Alkynes serve as important synthons, besides being the subunits
of myriad organic compounds. In particular, recently several reports
R2
O
Marcanine B, R1 = H; R2 = OMe; R3 = Me; R4, R5 = H
Marcanine C, R1 = H; R2 = OMe; R3 = CH2OH; R4, R5 = H
Marcanine D, R1 = H; R2 = OMe; R3 = Me; R4 = OH; R5 = H
Marcanine E, R1 = Me; R2 = OMe; R3 = Me; R4 = H; R5 = OH
N
R5
O
R1
⇑
Corresponding author. Tel./fax: +91 452 2459845.
E-mail address: subbu.perum@gmail.com (S. Perumal).
Figure 1. Structure of naturally occurring 1,2-dihydrobenzo[g]quinoline-5,10-
diones.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.125

3736
B. Devi Bala et al. / Tetrahedron Letters 54 (2013) 3735–3739
NH2
R
R
O
O
O
O
R
N
O
O
O
O
Br
R1
3
2
Cu(OTf)2
MW, 120W,
aq.NaOH
TBAB
Toluene, reflux
NH
N
OH
100 oC, 3 min
1 h
Toluene, rt
1-2 h
6
4
5
1
solvent-free
R1
R1
R1
R= H, Et
Scheme 1. Synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5.
Table 1
Solvent- and base-screen for the synthesis of N-propargylaminonaphthoquinone 4a
Entry
Solvent and base
Molar ratio
Time (h)
Yield of 4aa (%)
3/base/TBAB
b
1
2
3
4
5
6
7
8
9
DMF, K2CO3
1:2:0
1:2:0
1:2:0
1:0.5:0
1:1:0
1:0.5:0
1:0.5:0.2
1:1:0.2
1:2:1
12
10
12
9
9
9
3
1
12
10
—
—
—
b
Acetone, K2CO3
DMSO, K2CO3
DMF, Cs2CO3
DMF, Cs2CO3
DMSO, Cs2CO3
Toluene, NaOH
Toluene, NaOH
Dioxane, NaOH
NaH, THF
b
43
43
38
53
74
b
—
—
b
10
1:1:0
a
Yields after purification by flash column chromatography.
Reaction failed to occur.
b
Table 2
Synthesis of N-propargylaminonaphthoquinones 4
Entry
Compd
R
R0
Time (h)
Yield of 4aa (%)
mp (°C)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
H
H
H
H
H
H
H
H
H
H
H
H
H
Et
4-Me
4-Br
4-Cl
4-F
4-CF3
3-Cl
3-OMe
2-Me
2-Cl
2,4-(Me)2
2,4-Cl2
2,5-Cl2
H
1.0
1.0
1.0
1.0
1.0
1.5
1.0
1.5
1.0
2.0
1.5
1.0
1.0
1.0
74
78
79
72
73
76
70
72
74
70
71
68
71
70
99
147
131
104
165
111
97
107
116
103
171
138
94
Figure 2. ORTEP diagram of 4k.
Table 3
Screening of catalysts and solvents for the synthesis of 1,2-dihydrobenzo[g]quinoline-
5,10-dione 5a
O
O
4-Cl
97
a
Yields after purification by flash column chromatography.
catalyst
solvent
N
N
O
O
We first investigated the model three-component reaction of 2-
hydroxynaphthalene-1,4-dione 1, p-toluidine 2a, and propargyl
bromide 3 (Scheme 1) in a sequential fashion. In this procedure,
initially the model reaction of 1 and 2a was performed under
microwave irradiation at 120 W, 100 °C and 1 bar pressure for
3 min in the absence of any solvent. This reaction of 1 and 2a led
to the exclusive formation of 2-(p-tolylamino)naphthalene-1,4-
dione 6a in almost quantitative yield. Then, employing different
anilines in the above reaction, a series of 2-arylaminonaphtha-
lene-1,4-diones 6 were obtained. Among the deactivated anilines,
viz. p-trifluoromethyl- and p-nitroanilines used in this reaction,
the former afforded a good yield of the product, while the latter
did not react. Incidentally, this green protocol for the synthesis of
6 is found to be advantageous than most of the methods reported
earlier,25 which suffer from one or more disadvantages such as
long reaction time, use of organic solvents, and moderate yields.
It is also pertinent to note that the reaction of 2-hydroxynaphtha-
lene-1,4-dione and aniline when performed in a 1:2 molar ratio,
did not afford the product arising from double Michael addition
Me
4a
Me
5a
S. No.
Solvent
Catalysta
Reaction time (h)
Yieldb (%)
c
1
2
3
4
5
6
7
8
9
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CH3CN
CuI
CuCl
8
4
2
5
6
5
1
6
6
8
8
2
—
20
57
19
27
31
71
CuCl2ꢀ2H2O
CuBr2
Cu(OAc)2ꢀH2O
Cu(NO3)2ꢀ3H2O
Cu(OTf)2
c
CuSO4ꢀ5H2O
—
Cu(OTf)2
11
21
10
11
12
DMF
DMSO
Diaxone
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
c
—
64
a
b
c
All reactions performed with 10 mol % of the catalyst.
Yields after filtration through a pad of silica gel.
Reaction failed to occur.

B. Devi Bala et al. / Tetrahedron Letters 54 (2013) 3735–3739
3737
Table 4
Synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5a via Cu(OTf)2 catalyzed 6-endo-dig cyclization
O
O
O
O
O
N
N
F
N
N
O
O
O
Cl
Br
Me
5a (71%)
5b (79%)
5c (74%)
5d (76%)
O
O
O
O
N
N
N
N
O
O
O
O
Me
OMe
Cl
CF3
5h (70%)
5e (75%)
5f (72%)
5g (71%)
O
O
O
O
N
O
N
O
N
O
N
Cl
Me
Cl
O
Cl
Cl
Me
Cl
5i (75%)
5j (71%)
5k (73%)
5l (70%)
O
O
N
O
N
O
Cl
5m (73%)
5n (70%)
a
Yield after filtration through a pad of silica gel.
elimination sequence and this reaction also furnished 2-arylami-
nonaphthalene-1,4-diones exclusively.
reaction furnishing 4a was allowed to go to completion at ambient
temperature. Then product 4a was isolated and purified by flash
column chromatography. With the above optimized reaction con-
ditions, the scope of this transformation leading to a series of 4
was then investigated using a series of 2-arylaminonaphthoqui-
nones 6 and propargyl/3-ethylpropargyl bromides (Table 2).26
The structure of 4 is in accord with elemental analysis and 1H
and 13C NMR spectroscopic data as illustrated with a representa-
tive example 4c (vide Supplementary data). The X-ray crystallo-
The optimization of the reaction conditions for the second step
of the reaction, viz. the propargylation of 2-arylaminonaphthalene-
1,4-diones 6 was investigated taking 2-(p-tolylamino)naphtha-
lene-1,4-dione 6a as the representative example. Thus, the reaction
of 2-(p-tolylamino)naphthalene-1,4-dione 6a with propargyl bro-
mide 3 to obtain 4a was investigated employing different base-sol-
vent combinations (Table 1). This reaction failed to occur, when
DMF/K2CO3, acetone/K2CO3, DMSO/K2CO3, dioxane/NaOH, and
THF/NaH were used, while with DMF or DMSO in the presence of
Cs2CO3, this reaction, even after prolonged reaction time, furnished
4a in poor yields, 43% and 38%, respectively (Table 1, entries 4 and
6). Interestingly, N-propargylaminonaphthoquinone 4a was ob-
tained in 74% yield in toluene in the presence of NaOH and the
phase transfer catalyst, tetrabutylammonium bromide (TBAB).
Typically, to a solution of 2-(p-tolylamino)naphthalene-1,4-dione
6a, generated in situ from the reaction of 2-hydroxynaphthalene-
1,4-dione and p-toluidine (2 mmol each) in toluene (10 ml), prop-
argyl bromide (2.6 mmol), a catalytic amount (20 mol %) of tetra-
butylammonium bromide (TBAB), and an aqueous solution of
sodium hydroxide (2 mmol in 1 ml of water) were added and the
6.92, dt, 1H, J = 9.6, 1.8 Hz
120.4
O
H-6&9:7.86-7.89, m, 1H
6
4
8.10-8.13 m, 1H
126.0, 126.4
5.70, dt, 1H, J = 9.6, 4.2 Hz
3
2
5
10
7
119.3
8
N
4.54, dd, 2H, J = 4.2, 1.8 Hz
1
9
1'
53.1
O
H-7&8:7.61, td, 1H, J = 7.5, 1.5 Hz
2'
3'
6'
5'
132.7,
7.70, td, 1H, J = 7.5, 1.5 Hz
133.8
H-2'&6':7.03, d, 2H, J = 8.7 Hz
125.1
4'
Cl
H-3'&5':7.31, d, 2H, J = 8.7 Hz
129.1
Figure 3. NMR characterization of compound 5c.

3738
B. Devi Bala et al. / Tetrahedron Letters 54 (2013) 3735–3739
NH2
R
O
O
2
O
O
O
R
Br
NH
R1
3
N
OH
- H2O
substituion
addition-
O
4
elimination
1
R1
6
R1
Cu(OTf) 2
activation of
alkyne
R
O
O
Cu(OTf)2
O
R
R
5-exo-dig
6-endo-dig
annulation
N
N
N
O
O
O
5'
5
4
R1
R1
R1
Not formed
R= H, Et
Scheme 2. Plausible mechanism for the synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5.
graphic study of 4k (Fig. 2)27 confirms the structure of 4 deduced
from NMR spectroscopic data.
the activation of the C–C triple bond by Cu(OTf)2. Presumably, the
presence of three sp2 carbons and the nitrogen, which also could
prefer sp2 hybridization due to conjugation with the quinone ring
system, imposes strain to the five-membered ring of 50, which
could preclude its formation. Relatively, the six-membered ring
system in 1,2-dihydrobenzo[g]quinoline-5,10-diones could be less
strained facilitating the regioselectivity of the annulation process
in its favour.
In conclusion, we have developed an efficient synthesis of 1,2-
dihydrobenzo[g]quinoline-5,10-diones in good yields employing
simple starting materials through (i) a facile synthesis of N-propa-
rgylaminonaphthoquinones and (ii) their copper(II) triflate-cata-
lyzed 6-endo-dig cyclization. The synthetic utility of these 1,2-
dihydrobenzo[g]quinoline-5,10-diones is under further investiga-
tion by our research group to obtain more complex heterocycles
of biological relevance.
We then explored the intramolecular cyclization of N-propa-
rgylaminonaphthoquinone 4a in toluene in the presence of differ-
ent copper catalysts. When the reaction was performed in the
presence of 10 mol % of CuI and CuSO4, the cyclization failed to oc-
cur. In the presence of 10 mol % of CuCl, CuCl2, CuBr2, Cu(OAc)2, and
Cu(NO3)2, the cyclized product was obtained in 20%, 57%, 19%, 27%,
and 31% yields respectively (Table 3). To our delight, we found that
a higher yield of 71% of 5a could be obtained by performing the
reaction in the presence of 10 mol % of Cu(OTf)2 in toluene at
100 °C for 1 h. The reaction in the presence of Cu(OTf)2 in DMSO
failed to occur, while in CH3CN, DMF and dioxane, diminished yield
of the product was obtained. Presumably, the heteroatoms present
in DMSO, CH3CN, and DMF quenched the electrophilicity of
Cu(OTf)2 by coordination and diminished its catalytic efficacy in
this transformation. These results disclose that Cu(OTf)2–toluene
pair is the ideal catalyst–solvent pair for this reaction and the
scope of the transformation of a series of 4 to obtain 5 was inves-
tigated under the optimized reaction conditions (Table 4).26
The structure of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5
was deduced using one- and two-dimensional NMR spectroscopic
data (Fig. 3), mass spectrum, and elemental analysis. As a represen-
tative case, the structural assignment of 5c is described below. The
1H NMR spectrum of 5c has a doublet of doublets at 4.54 ppm
(J = 4.2, 1.8 Hz) for two protons assignable to the methylene hydro-
gens of the dihydropyridine ring. This signal shows a H,H-COSY
correlation with the doublet of triplets at 5.70 ppm (J = 9.6,
4.2 Hz) due to H-3. Further, H-3 shows a H,H-COSY correlation
with the doublet of triplets at 6.92 ppm (J = 9.6, 1.8 Hz), which
arises from H-4. The very small coupling constant of 1.8 Hz pre-
sumably arises from the long range homoallylic coupling between
H-2 and H-4. The multiplets occurring at 7.86–7.89 and 8.08–
8.11 ppm are assigned to H-6 and H-9. The two triplets of doublets
at 7.61 and 7.70 (1H, J = 7.5, 1.5 Hz) are due to H-7 and H-8.
A plausible mechanism for the formation of 1,2-dihydro-
benzo[g]quinoline-5,10-diones 5 is depicted in Scheme 2. Presum-
ably, the intermediate, 2-(arylamino)naphthalene-1,4-dione 6 was
formed from the reaction of 2-hydroxynaphthalene-1,4-dione and
substituted anilines by Michael addition-elimination reactions.
Then compound 6 upon reaction with propargyl bromide 3 gives
the corresponding N-propargylaminonaphthoquinone 4. Ulti-
mately, the intramolecular 6-endo-dig annulation of N-propargyla-
minonaphthoquinones furnishing the final product 5 is enabled by
Acknowledgments
S.P. thanks CSIR and DST, New Delhi, for Major Research Pro-
jects 01(2433)/10/EMR-II and SR/S1/OC-50/2011 respectively and
(ii) DST-IRHPA program for funds for the purchase of a high reso-
lution NMR spectrometer. B.D.B. and S.M. thank UGC, New Delhi
for the award of Senior Research Fellowships respectively.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.04.
125.
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26. General procedure for the synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5
i. Synthesis of N-propargylaminonaphthoquinones 4
A mixture of 2-hydroxynaphthalene-1,4-dione (1) and substituted aniline (2)
(2 mmol each) was irradiated under microwave irradiation at 120 W, 100 °C
and 1 bar pressure for 3 min. The resulting product was dissolved in toluene
(10 ml), into which propargyl or 3-ethylpropargyl bromide (3) (2.6 mmol) and
tetrabutylammonium bromide (TBAB) (20 mol %) were added and the solution
was stirred well. Then an aqueous solution of sodium hydroxide (2 mmol;
1 ml) was slowly added and stirring continued at ambient temperature for 1–
2 h till the reaction went to completion (TLC). Then, the reaction mixture was
washed with water (2 ꢁ 20 ml), the organic layer separated and dried over
anhydrous sodium sulfate and the solvent was removed under reduced
pressure. The resulting crude product was purified by flash column
chromatography using petroleum ether–ethyl acetate (19:1 v/v) mixture.
ii. Synthesis of 1,2-dihydrobenzo[g]quinoline-5,10-diones 5
To
a solution of N-propargylaminonaphthoquinones (1 mmol) in toluene
(10 ml), copper(II) triflate (0. 1 mmol) was added and the flask heated to
100 °C in an oil bath for 1 h. After completion of the reaction (TLC), the solvent
was removed under reduced pressure and the resulting crude product was
purified by filtration through a pad of silica gel using petroleum ether–ethyl
acetate (49:1 v/v) to afford pure 5
27. Crystallographic data for the derivative 4k in this manuscript have been
deposited with the Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 918355. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 (0)1223 336033 or e-mail: deposit@ccdc.cam.ac.uk).