Novel convenient one-pot method for the synthesis of indoloquinolines

Article abstract of DOI:10.1007/s11172-019-2493-4

A novel one-pot method based on the sequence of the Fisher reaction between nitroacetophenone and phenylhydrazines, the reduction with metallic tin directly in polyphosphoric acid, and acylation has been developed for the synthesis of indoloquinolines. Th

Full text of DOI:10.1007/s11172-019-2493-4

Russian Chemical Bulletin, International Edition, Vol. 68, No. 4, pp. 836—840, April, 2019
836
Novel convenient one-pot method for the synthesis of indoloquinolines*
N. A. Aksenov,a A. Z. Gasanova,a G. M. Abakarov,b I. V. Aksenova,a and A. V. Aksenova
aNorth Caucasus Federal University,
1A ul. Pushkina, 355009 Stavropol, Russian Federation.
E-mail: radioanimation@rambler.ru
bDagestan State Technical University,
70 ul. Imama Shamilya, 367010 Makhachkala, Russian Federation
A novel one-pot method based on the sequence of the Fisher reaction between nitroaceto-
phenone and phenylhydrazines, the reduction with metallic tin directly in polyphosphoric acid,
and acylation has been developed for the synthesis of indoloquinolines. The obtained indolo-
[3,2-c]quinolines are precursors of the analogs of isocryptolepine alkaloid.
Key words: Fischer indole synthesis, isocryptolepine, reduction, one-pot synthesis, indole,
Cryptolepis Sanguinolenta.
The indole cycle system is an important moiety of many
Usually, the synthesis of indolo[3,2-c]quinolines in-
cludes a final assembly of one of the cores, either quino-
line8—13 or indole14—22 one, while the second core was
already present in the substrate. Many of these methods
involve expensive starting compounds and include many
steps with the isolation of intermediate compounds. The
assembly of both cores was also reported,23—28 including
our recent work,28 wherein a one-pot synthesis of indolo-
[3,2-c]quinolines was revealed as a sequence of the Fisher
reaction between phenylhydrazine 1 and 2-aminoaceto-
phenone 2, the acylation of resulting 2-(2-aminophenyl)-
indoles 3, and the cyclization into target product 4.
Obtained indoloquinolines 4 either already contain a methyl
group at the position 5 or can be easily converted into the
corresponding isocryptolepine derivatives by the treatment
with dimethyl sulfate or methyl iodide (Scheme 1).
Despite the high product yields in this reaction, the
major drawback of that method is the utilization of
2-aminoacetophenones as the starting compounds. We
believed that the replacement of 2-aminoacetophenones 2
by their precursors, 2-nitroacetophenones 5, is an interest-
ing idea. To implement this approach, it was necessary to
add a step of the reduction in polyphosphoric acid, but
there were no reported examples of such transformations
in the literature. We have previously investigated a number
of interesting conversions proceeding in polyphosphoric
acid,29—31 including the opportunity to reduce aliphatic
nitro compounds into amides of carboxylic acid using
phosphorus trichloride.32 We assumed that this transfor-
mation could be carried out similarly to the reduction by
metals dissolving in the presence of acids, such as HCl,
H3PO4, and AcOH. For example, some works25,26 re-
ported on the reduction of 2-(2-nitrophenyl)indoles by
Fe—HCl system in ethanol.
drugs and several thousands of alkaloids exhibiting various
types of biological activity. It is known that indole deriva-
tives are more than a quarter of all the known alkaloids.1
More than a dozen of alkaloids were extracted from roots
of the South African plant Cryptolepis Sanguinolenta, and
many of which demonstrated antimalarial and antitumor
activities.2—6 Alkaloids of this plant are mainly present by
tetracyclic systems containing an indole core, e.g., indolo-
[3,2-b]quinoline, indolo[3,2-c]quinoline, indolo[2,3-b]-
quinoline, and indolo[3,2-b][1]benzazepine, whereas
isocryptolepine, neocryptolepine, and cryptolepine can be
distinguished among them. Their analogs, e.g., 5-amino-
pyrido[3´,2´:4,5]thieno[3,2-c]isoquinolines reported pre-
viously,7 are not of less interest from the point of view of
the biological activity.
* Based on the materials of the V All-Russian Organic Chemistry
Conference (ROCC-V) (September 10—14, 2018, Vladikavkaz,
Russia).
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 0836—0840, April, 2019.
1066-5285/19/6804-0836 © 2019 Springer Science+Business Media, Inc.

One-pot synthesis of indoloquinolines
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 4, April, 2019
837
Scheme 1
Reagents and conditions: i. 1) 120 °C, 2) polyphosphoric acid (PPA), 100 °C; ii. PPA.
Scheme 2
Reagents and conditions: i. PPA, 60 °C; ii. PPA.
In order to verify this hypothesis, we prepared 2-(2-nitro-
phenyl)indole 6a via the Fisher reaction in PPA medium
at 60 °C for 2 h (Scheme 2). Zinc dust (1 equiv.) was added
in portions every 1 h to the resulting reaction mixture
until the nitro compound disappeared in the reaction
mixture. Acetic acid was then added, the mixture was
heated for another 30 min, and the product was isolated.
The reaction yield was about 5% (Table 1). We explain this
result by the high instability of 2-(2-nitrophenyl)indoles
6 under the reaction conditions, which are rapidly poly-
merized at the temperatures above 80 °C. In addition, the
zinc dust is insoluble in polyphosphoric acid. To acceler-
ate the reduction process, zinc was replaced by tin, which
should produce Sn2+ salts soluble in PPA. At this end, Sn0
worked slightly better than Sn2+. Yields of the obtained
products are given in Table 1.
a nature of the acylating agent should not significantly
affect the reaction yield. To verify this hypothesis, we have
varied the acylating reagents and also the substituents at
the position 8 of indoloquinoline (Scheme 3). The results
are given in Table 2.
Therefore, we have developed a convenient method for
the synthesis of 11H-indolo[3,2-c]quinolines, which ex-
cludes the utilization of o-aminoacetophenones that are
inaccessible and unstable under the reaction conditions.
The obtained compounds can be converted into the
Table 1. Optimization of conditions for the synthesis of com-
pound 4b
Concentration of
P2O5 in PPA (%)
Reducing agent
(equiv.)
T/°C
Yield of 4b
(%)
We have earlier demonstrated28 that corresponding
11H-indolo[3,2-c]quinolines 4 can be obtained in the
yields of 75—86%, while the step of acylation of 2-(2-amino-
phenyl)indole 3 proceeded in a quantitative yield. As one
can see from the optimization table, the main reason of
the decreased yield is the thermal decomposition of
2-(2-nitrophenyl)indole 6 during the reaction. The faster
reduction resulted in the higher yield. We suggested that
76
80
86
86
86
86
86
Sn (2)
Sn (2)
60
60
60
70
80
60
60
10
10
25
15
10
20
15
Sn (2)
Sn (2)
Sn (2)
SnCl2•2 H2O (2)
Zn (3)

838
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 4, April, 2019
Aksenov et al.
Scheme 3
Reagents and conditions: i. PPA, 60 °C; ii. PPA, Sn, 60 °C; iii. A, PPA, 90—120 °C.
Table 2. Structures and yields of compounds 4a—e
ance of the starting reagents (~1.5 h, TLC control). Metallic Sn
(1 mmol) was added, the mixture was stored for another 2 h, and
the acylating agent (1.2 mmol) was added. The reaction mixture
was then heated to 130 °C (1,3,5-triazine was used as the syn-
thetic equivalent of HCOOH at 100 °C) and stored until the
intermediate product disappeared completely (about 2 h). Next,
The reaction mixture was cooled to room temperature, diluted
with water (40 mL), and neutralized with 20% aqueous ammonia
(~7 mL) until basic pH. The resulting mixture was extracted with
ethyl acetate (4×15 mL), evaporated, and purified by column
chromatography using an acetone—hexane mixture (from 1 : 5
to 1 : 1) as the eluent.
Reagents
Product
R
R´
Yield
1
A
(%)
a
b
c
d
e
1,3,5-Triazine
AcOH
PhCOOH
1,3,5-Triazine
PhCOOH
4a
4b
4c
4d
4e
H
H
H
Me
Ph
H
25
25
24
22
26
H
Me
Pri
Ph
11H-Indolo[3,2-c]quinoline (4a). Colorless crystals, the yield
of 27 mg (25%), m.p. 340—341 °C (cf. Ref. 28: m.p. 340—341 °C).
IR, ν/cm–1: 3047, 2775, 1571, 1519, 1462, 1373, 1341, 1242, 1158,
933, 771, 740. 1H NMR (DMSO-d6), δ: 7.34 (t, 1 H, H(8),
J = 7.3 Hz); 7.55—7.45 (m, 1 H, H(9)); 7.78—7.65 (m, 3 H, H(2),
H(3), H(10)); 8.14 (d, 1 H, H(7), J = 7.9 Hz); 8.32 (d, 1 H, H(1),
J = 7.8 Hz); 8.53 (dd, 1 H, H(4), J = 8.0 Hz, J = 1.1 Hz); 9.60
(s, 1 H, H(6)); 12.73 (s, 1 H, NH). 13C NMR (DMSO-d6), δ:
111.8 (C(10)), 114.3 (C(6a)), 117.1 (C(6b)), 120.1 (C(9)), 120.6
(C(7)), 121.9 (C11b), 122.1 (C(8)), 125.5 (C(2)), 125.7 (C(3)),
128.0 (C(1)), 129.5 (C(4)), 138.8 (C(10a)), 139.8 (C(11a)), 144.8
(C(6)), 145.4 (C(C4a)). MS (ESI—TOF): found: m/z 219.0917
[M + H]+; C15H11N2; calculated: M = 219.0917. Rf = 0.19 (ethyl
acetate), Rf = 0.5 (hexane—acetone, 1 : 1).
6-Methyl-11H-indolo[3,2-c]quinoline (4b). Colorless crystals,
the yield of 29 mg (25%), m.p. 208—210 °C (cf. Ref. 28: m.p.
208—210 °C. IR, ν/cm–1: 3054, 2932, 2853, 1599, 1555, 1452,
1359, 1114. 1H NMR (DMSO-d6), δ: 3.10 (s, 3 H, C(6)Me); 7.37
(ddd, 1 H, H(8), J = 8.0 Hz, J = 7.0 Hz, J = 0.6 Hz); 7.52 (ddd,
1.H, H(9), J = 8.1 Hz, J = 7.0 Hz, J = 0.8 Hz); 7.65 (ddd, 1 H,
H(2), J = 7.9 Hz, J = 6.8 Hz, J = 0.9 Hz); 7.76—7.70 (m, 2 H,
H(3), H(10)); 8.06 (d, 1 H, H(7), J = 8.1 Hz); 8.22 (d, 1 H, H(1),
J = 8.0 Hz); 8.52 (dd, 1 H, H(4), J = 8.0 Hz, J = 0.9 Hz); 12.90
(s, 1 H, NH). 13C NMR (DMSO-d6), δ: 22.5 (Me), 112.0 (C(10)),
113.0(C(9)), 116.0 (C(7)), 121.0 (C(6a)), 121.6 (C(8)), 122.1
(C(2)), 122.3 (C(3)), 125.3 (C(6b)), 125.4 (C(1)), 127.3 (C(4)),
128.6 (C(11b)), 138.9 (C(10a)), 140.3 (C(11a)), 143.3 (C(4a)),
154.0 (C(6)). MS (ESI—TOF): found: m/z 233.1074 [M + H]+;
isocryptolepine derivatives by their treatment with methyl
iodide according to the known procedure.33
Experimental
1H NMR spectra were recorded on a Bruker AVANCE III
HD instrument (the operating frequency of 400.40 MHz) in
DMSO-d6 or CDCl3 using the residual solvent signals as the
internal standard. Mass spectra of the compounds were obtained
on a Bruker Maxis impact spectrometer using the direct injection
system, the ionization method was ESI, and the calibrant was
HCO2Na—HCO2H.
IR spectra were recorded on a Shimadzu IRAffinity-1S FTIR
spectrometer equipped with an ATR device (ATR means attenu-
ated total reflection).
The purity of compounds was controlled by TLC on Silufol
UV-254 plates, the eluent was acetone—hexane mixture (from
1 : 5 to 1 : 1), and the plates were visualized by UV irradiation.
Melting points were measured on a Stuart SMP30 instrument.
Polyphosphoric acids were obtained either according to the
standard procedure34 or by dissolving P2O5 in H3PO4 (85%).35
Synthesis of 11H-indolo[3,2-c]quinolines (general procedure).
A mixture of 2-nitroacetophenone 5 (82 mg, 0.5 mmol), corre-
sponding phenylhydrazine 1 (0.5 mmol), and polyphosphoric
acid (1.0 g, the P2O5 content of 86%) was stirred at room
temperature for 5 min. The process was strongly exothermic. The
mixture was then stirred at 60 °C until the complete disappear-

One-pot synthesis of indoloquinolines
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 4, April, 2019
839
C16H13N2; calculated: M = 233.1073. Rf = 0.5 (ethyl acetate),
Rf = 0.72 (hexane—acetone, 1 : 1).
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6-Phenyl-11H-indolo[3,2-c]quinoline (4c). Colorless crystals,
the yield of 35 mg (24%), m.p. 248—250 °C (cf. Ref. 28: m.p.
248—250 °C). IR, ν/cm–1: 3172, 3054, 2917, 2843, 1560, 1530,
1501, 1452, 1359, 1320, 1241, 1222. 1H NMR (DMSO-d6), δ:
7.14 (ddd, 1 H, H(8), J = 8.0 Hz, J = 7.0 Hz, J = 0.9 Hz),
7.50—7.33 (m, 6 H, H(9), Ph); 7.62—7.57 (m, 2 H, H(2), H(3));
7.97—7.90 (m, 2 H, H(10), H(7)); 8.24 (dd, 1 H, H(1) , J = 8.2 Hz,
J = 0.8 Hz); 8.33 (d, 1 H, H(4), J = 8.3 Hz); 12.24 (s, 1 H, NH).
13C NMR (DMSO-d6), δ: 111.8 (C(10)), 113.4 (C(6a)), 116.8
(C(6b)), 120.8 (C(9)), 121.8 (C(7)), 121.9 (C(8)), 122.8 (C(11b)),
125.6 (C(2)), 125.6 (C(3)), 128.6 (2 C, C(3)6-Ph, C(5)6-Ph), 128.7
(C(1)), 128.9 (C(4)), 129.0 (2 C, C(2)6-Ph, C(6)6-Ph), 129.1
(C(4)6-Ph), 139.4 (C(1)6-Ph), 140.3 (C(10a)), 141.8 (C(11a)),
145.3 (C(4a)), 156.9 (C(6)). MS (ESI—TOF): found: m/z
295.1237 [M + H]+; C21H15N2; calculated: M = 295.1230.
Rf = 0.30 (hexane—ethyl acetate, 1 : 1), Rf = 0.75 (hexane—ac-
etone, 1 : 1).
8-Methyl-11H-indolo[3,2-c]quinoline (4d). Colorless crystals,
the yield of 26 mg (22%), m.p. 305—311 °C (cf. Ref. 28: m.p.
305—311 °C). IR, ν/cm–1: 3042, 2773, 2366, 1570, 1363, 1239.
1H NMR (DMSO-d6), δ: 2.52 (s, 3 H, H(8)Me); 7.32 (dd, 1 H,
H(9), J = 8.3 Hz, J = 1.3 Hz); 7.61 (d, 1 H, H(10), J = 8.3 Hz);
7.77—7.64 (m, 2 H, H(2), H(3)); 8.14—8.09 (m, 2 H, H(1),
H(7)); 8.50 (dd, 1 H, H(4), J = 8.0 Hz, J = 1.1 Hz); 9.54 (s, 1 H,
H(6)); 12.59 (s, 1 H, NH). 13C NMR (DMSO-d6), δ: 21.2
(C(8)Me), 111.5 (C(10)), 114.1 (C(6a)), 117.2 (C(6b)), 119.8
(C(9)), 122.0 (C(7)), 122.1 (C(11b)), 125.6 (C(2)), 126.9 (C(3)),
127.9 (C(1)), 129.4 (C(8)), 129.5 (C(4)), 137.0 (10a), 139.8 (11a),
144.7 (6), 145.4 (C(4a)). MS (ESI—TOF): found: m/z 233.1076
[M + H]+; C16H13N2; calculated: M = 233.1073. Rf = 0.24 (hex-
ane—ethyl acetate, 4 : 1), Rf = 0.44 (hexane—acetone, 1 : 1).
8-Isopropyl-6-phenyl-11H-indolo[3,2-c]quinoline (4e). Color-
less crystals, the yield of 44 mg (26%), m.p. 129.0—132.6 °C
(cf. Ref. 27: m.p. 129—132 °C). IR, ν/cm–1: 3056, 2920, 2366,
1615, 1590, 1557, 1516, 1490, 1443. 1H NMR (DMSO-d6),
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δ: 1.14 (d, 6 H, H(8)Pri, J = 6.9 Hz); 2.87 (sept, 1 H, H(8)
,
i
J = 6.9 Hz); 7.34 (m, 2 H, H(7), H(9)); 7.66—7.60 (m, 4 PHr ,
H(3)5-Ph, H(4)5-Ph, H(5)5-Ph, H(10)); 7.71—7.67 (m, 1 H, H(2));
7.75 (t, 1 H, H(3), J = 7.2 Hz); 7.83 (dd, 2 H, H6-Ph(2), H(6)6-Ph
,
J = 7.1 Hz, J = 1.8 Hz); 8.12 (d, 1 H, H(1), J = 8.2 Hz); 8.55
(d, 1 H, H(4), J = 8.0 Hz); 12.77 (s, 1 H, NH). 13C NMR
(DMSO-d6), δ: 24.4 (2 C, C(8)Me), 33.4 (C(8)CH), 111.6
(C(10)), 112.0 (C(6a)), 116.4 (C(6b)), 118.0 (C(9)), 121.7
(C(11b)), 121.9 (C(7)), 124.6 (C(2)), 125.6 (C(3)), 128.3 (2 C,
C(3)6-Ph, C(5)6-Ph), 128.4 (C(1)), 128.9 (C(4)), 129.0 (2 C,
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(C(8)), 140.7 (C(10a)), 141.1 (C(11a)), 144.9 (C(4a)), 155.5
(C(6)). MS (ESI—TOF): found: m/z 337.1703 [M + H]+;
C24H21N2; calculated: M = 337.1699. Rf = 0.71 (hexane—ethyl
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This work was financially supported by the Russian
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Received October 31, 2018;
in revised form December 25, 2018;
accepted December 26, 2018