Benzimidazole condensed ring systems. XI. Synthesis of some substituted cycloalkyl pyrido[1,2-a]benzimidazoles with anticipated antineoplastic activity

Article abstract of DOI:10.1016/S0223-5234(00)80036-7

As part of a research project on the synthesis of a number of pyrido[1,2-a]benzimidazole derivatives with possible antineoplastic activity, and as a result of the interesting antineoplastic activity recorded for one such compound (NSC 649900), some new cycloalkylpyrido[1,2-a]benzimidazoles were prepared and evaluated for such activity. Compound (7c, NSC 682011) exhibited a good in vitro antineoplastic activity especially against most of the leukaemia cell lines. This compound has been selected by the NCI for further testing in a new in vivo anticancer hollow fibre assay.

Full text of DOI:10.1016/S0223-5234(00)80036-7

Eur. J. Med. Chem. 34 (1999) 663−667
© 1999 Éditions scientifiques et médicales Elsevier SAS. All rights reserved
663
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Benzimidazole condensed ring systems. XI. Synthesis of some substituted
cycloalkyl pyrido[1,2-a]benzimidazoles with anticipated antineoplastic activity#
El-Sayed A.M. Badaweya*, Thomas Kappeb
aDepartment of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Alexandria, Alexandria AR, Egypt
bInstitute für Organische Chemie, Karl Franzens Universität, Graz, Austria
(Received 30 March 1998; revised 27 July 1998; accepted 7 October 1998)
Abstract – As part of a research project on the synthesis of a number of pyrido[1,2-a]benzimidazole derivatives with possible antineoplastic
activity, and as a result of the interesting antineoplastic activity recorded for one such compound (NSC 649900), some new
cycloalkylpyrido[1,2-a]benzimidazoles were prepared and evaluated for such activity. Compound (7c, NSC 682011) exhibited a good in vitro
antineoplastic activity especially against most of the leukaemia cell lines. This compound has been selected by the NCI for further testing in
a new in vivo anticancer hollow fibre assay. © 1999 Éditions scientifiques et médicales Elsevier SAS
cycloalkylpyrido[1,2-a]benzimidazoles / antineoplastic agents
1. Introduction
order to assess their potential as anticancer agents. The
11-chloro compound 5 was of particular interest because
of its structural similarity to NSC649900, however, its
antineoplastic profile did not reveal any improvement in
activity. Interestingly, the 5-(p-fluorophenylamino) de-
rivative (7c, NSC 682011) showed promising in vitro
antineoplastic activity and was selected by the NCI for
further testing in a new in vivo anticancer hollow fibre
assay. The synthesis of the target compounds is outlined
in figure 2.
Several years ago we started a research plan to synthe-
size several pyrido[1,2-a]benzimidazoles (PBIs) as a
class of compounds which is inadequately studied for
antineoplastic activity [1–4]. In a recent investigation, we
found that 1-chloro-2-(2-chloroethyl)-3-methylpyrido-
[1,2-a]benzimidazole-4-carbonitrile (NSC 649900, fig-
ure 1) exhibited good in vitro activity and subpanel
disease selectivity especially against leukaemic cell lines;
however, it revealed only weak activity against P388
murine leukaemia in vivo [5]. In a search for a cytotoxic
candidate with an improved antineoplastic profile, we
have replaced the tricyclic PBI ring system in NSC
649900 with other N-heterocycles having a nearly similar
substitution pattern; however, the designed compounds
were less active [6–8]. This suggested that the PBI ring
system may be partly essential for the biological activity.
For this reason and for further information concerning the
SAR study, we undertook in this report the synthesis
of some new cycloalkylpyrido[1,2-a]benzimidazoles in
2. Chemistry
Previously, we have described a facile one step syn-
thesis of cyclopentapyrido[1,2-a]benzimidazole, as a new
tetracyclic ring system, by fusing 1H-benzimidazole-2-
acetonitrile (1) with ethyl cylopentanone-2-carboxylate
ester at 140 °C in the presence of ammonium acetate [9].
We have now utilized this reaction condition for the
synthesis of some cycloalkyl-pyrido[1,2-a]benzimi-
dazoles (2–4), namely, the cyclopentapyrido[1,2-
a]benzimidazoles (2 and 3) and the benzimidazo[1,2-
b]isoquinoline (4). Thus, reacting 1 with an equimolar
quantity of diethyl cyclopentanone-2,5-dicarboxylate in
the presence of two equivalents of ammonium acetate at
*Correspondence and reprints
#This work was partly presented in the 6th Ibn Sina Internatio-
nal Conference on pure and applied Heterocyclic Chemistry,
December, 1997. For part X, see ref. [4]

664
Figure 1. Structures of NSC 649900 and NSC 682011.
140 °C furnished the corresponding ethyl cyclo-
pentapyrido[1,2-a]benzimidazole-3-carboxylate (2) in
12% yield. Similarly, the corresponding 1-methyl car-
boxylate ester (3) was obtained in a nearly similar yield
by reacting 1 with dimethyl cyclopentanone-2,3-dicar-
boxylate. Whereas, the benzimidazo[1,2-b] isoqui-
noline (4) was prepared in 67% yield from 1 and ethyl
cyclohexanone-2-carboxylate. Next, the N-butyl deriva-
tive 6 was obtained by refluxing 4 with tributyl phosphate
in the presence of potassium carbonate, while its treat-
ment with phosphorus oxychloride gave the 11-chloro
derivative (5). Reacting this compound with the selected
p-substituted-anilines in dimethylformamide at 80 °C
yielded the respective 11-arylamino- derivatives (7a–c) in
70–85% yield. Reaction of 5 with sodium azide at room
temperature resulted in the corresponding 11-azido com-
Figure 2. 7a: R = CH3, 7b: R = CI, 7c: R = F.

665
pound 8 which, upon treatment with triphenylphosphine,
gave the corresponding 11-triphenyl-phosphoranyli-
deneamino derivative (9). A literature survey revealed
that compounds 4 and 5 were prepared by Russell in
1995 [10]. However, it is worth mentioning that, although
these compounds were prepared according to our previ-
ously reported procedure [9], the author did not refer to
our leading reference. Surprisingly, we found that some
cyclopentapyrido[1,2-a]benzimidazoles which are ana-
logues to compounds 4 and 5 and were reported by us in
1989 [9] were also included in this article.
Elmer 298 spectrophotometer using samples in potassium
bromide discs. The H-NMR spectra were recorded on a
Varian Gemini 200 at 200 MHz using DMSO-d6, unless
otherwise stated, with tetramethylsilane as the internal
standard. Microanalyses were performed on a Carlo Erba
1106 analyser and are within ± 0.4 of the theoretical
percentages.
1
4.1.1. Ethyl 4-Cyano-2,3,5,11-tetrahydro-11-oxo-1H-
cyclopenta[4,5]pyrido[1,2-a]benzimidazole-3-carboxy-
late 2
A mixture of 1H-benzimidazole-2-acetonitrile
1
(1.57 g, 10 mmol), diethyl cyclopentanone-2,5-dicarb-
oxylate (2.5 g, 11 mmol) and ammonium acetate (1.7 g,
22 mmol) were heated in an oil bath at 140–150 °C for
30 min. During this period, ethanol and ammonia were
liberated, and the reaction mixture was gradually solidi-
fied. After cooling, the product was suspended in aceto-
nitrile, filtered and dried; yield: 0.39 g (12%); m.p. >
300 °C (DMF/H2O); IR ν cm–1: 3 300–2 500 bm, 2 210 s,
1 750 s, 1 670 m, 1 610 w, 1 590 w. 1H-NMR
(CF3COOH): δ 1.4 (t, CH3CH2O-), 2.5 (m, 2 H at C-2),
3.3 (2 t, 1 H at C-1 + 2 H at C-3), 4.5 (q, CH3CH2O-), 7.8
(bs, 3 ArH), 8.7 (d, 1 ArH at C-6); MS: m/z 321.2, 292,
248.7 (base peak M/E), 220.2. Anal. (C18H15N3O3) C, H,
N.
3. Antineoplastic activity
The prepared compounds were evaluated for their in
vitro antineoplastic activity against 60 human cell lines
derived from seven clinically isolated cancer types (lung,
colon, melanoma, renal, ovarian, brain, and leukaemia)
according to the NCI standard protocol [11]. The test
results indicated that only the 5-arylamino-tetra-
hydrobenzimidazo[1,2-b]isoquinolines (7a–c) showed
significant activity, particularly against the leukaemic cell
lines. The p-fluorophenylamio derivative (7c, NSC
682011) was the most active candidate in this series,
especially against K-562, Molt-4 and RPMI-8226 leu-
kaemic cell lines, KM12 colon cell line, M14 melanoma
cell line, ACHN renal cell line and the T-47D breast cell
line. As a result of the promising activity recorded for 7c
and as it belongs to a new class which is not adequately
studied for antineoplastic activity, this compound was
selected by the NCI for further testing in a new in vivo
anticancer hollow fibre assay. We can conclude from this
result, and other investigations [4–8] that the pyrido[1,2-
a]benzimidazole structure is partly essential for the bio-
logical activity, and by proper substitution, good cyto-
toxic candidates can be obtained (e.g., NSC 649900 and
NSC 682011, figure 1). For this reason, we have synthe-
sized several new pyrido[1,2-a]benzimidazoles which are
related to 7c (NSC 682011) and have similar p-fluoro-
phenylamino substituents and p-fluorophenylamino-
methylene or p-fluorophenyazo moieties at position 1 or
2. The test results are underway, and they will be the
subject of the next investigation.
4.1.2. Methyl 4-Cyano-2,3,5,11-tetrahydro-11-oxo-1H-
cyclopenta[4,5]pyrido[1,2-a]benzimidazole-1-carboxy-
late 3
This was prepared, similar to 2, from 1 (1.57 g,
10 mmol), dimethyl cyclopentanone-2,3-dicarboxylate
(2.2 g, 11 mmol) and ammonium acetate (1.7 g,
22 mmol); yield: 0.4 g (13%); m.p. = 295–300 °C
(DMF); IR ν cm-1: 3 300–2 500 bm, 2 210 s, 1 740 s,
1 670 s, 1 610 w, 1 590 w. 1H-NMR (CF3COOH): δ
3.3–3.7 (m + t, 4 H at C-1,2 + 1 H at C-3), 3.9 (s, CH3O-),
7.7 (bs, 3 ArH), 8.7 (d, 1 ArH at C-6). Anal.
(C17H13N3O3) C, H, N.
4.1.3. 5,7,8,9,10,11-Hexahydro-11-oxo-benzimidazo[1,2-
b]isoquinoline-6-carbonitrile 4
This was prepared, similar to 2, from 1 (7.85 g,
50 mmol), ethyl cyclohexanone-2-carboxylate (9.4 g,
55 mmol) and ammonium acetate (8.5 g, 110 mmol);
yield: 8.82 g (67%); m.p. > 300 °C (DMF); IR ν cm-1:
3 250–2 500 bm, 2 210 s, 1 660 s, 1 615 w. 1H-NMR
(360 MHz): δ 1.7 (m, 4H, -CH2(CH2)2CH2-), 2.5 & 2.7
(2 t, 4 H, -CH2(CH2)2CH2-), 7.3 (m, 2 ArH at C-8,9), 7.5
(d, 1 ArH at C-10), 8.6 (d, 1 ArH at C-7), 13.3 (s, NH).
Anal. (C16H13N3O) C, H, N.
4. Results
4.1. Chemistry
Melting points were determined in open-glass capillar-
ies on a Gallenkamp melting point apparatus and are
uncorrected. The IR spectra were recorded on a Perkin-

666
4.1.4. 11-Chloro-7,8,9,10-tetrahydrobenzimidazo[1,2-b]-
isoquinoline-6-carbonitrile 5
4.1.8. 11-(4-Fluorophenylamino)-7,8,9,10-tetrahydroben-
zimidazo[1,2-b]isoquinoline-6-carbonitrile 7c
This was prepared, similar to 7a, from 5 (1.4 g,
5 mmol) and p-fluoroaniline (1.11 g, 10 mmol); yield:
1.25 g (70%); m.p. = 238–239 °C (EtOH); IR ν cm-1:
3 400 bw, 2 210 s, 1 625 s, 1 600 s. 1H-NMR: δ 1.8 (m, 4
Compound 4 (2.63 g, 10 mmol) was refluxed with
phosphorus oxychloride (15 mL) for 2 h. The excess
phosphorus oxychloride was removed under vacuum, and
the residue was treated with ice-water, and neutralized
with sodium carbonate. The insoluble was then filtered,
washed with water, and dried; yield: 2.7 g (96%); m.p. =
229–231 °C (DMF); IR ν cm-1: 2 210 s, 1 625 m, 1 600 s.
1H-NMR (CF3COOH): δ 2.2 (m, 4 H, -CH2(CH2)2CH2-),
3.3 & 3.5 (2 t, 4 H, -CH2(CH2)2CH2-), 7.8–8.0 (m, 3
ArH), 9.0 (d, 1 ArH at C-7). Anal. (C16H12ClN3) C, H, N.
H, -CH2(CH2)2CH2-), 2.7
&
3.1 (2 t,
4
H,
-CH2(CH2)2CH2-), 6.7 & 7.1 (dd, 4 ArH, aryl at C-5), 7.2
& 7.5 (2 t, 2 ArH at C-8,9), 7.8 (d, 1 ArH at C-10), 7.9 (d,
1 ArH at C-7), 9.1 (s, NH). Anal. (C22H17FN4) C, H, N.
4.1.9. 11-Azido-7,8,9,10-tetrahydrobenzimidazo[1,2-b]-
isoquinoline-6-carbonitrile 8
The chloro compound 5 (1.4 g, 5 mmol) was treated
with sodium azide (0.40 g, 6 mmol) in dimethylforma-
mide (15 mL) and stirred at room temperature for 30 min.
After addition of water the insoluble was filtered and
dried to yield: 1.3 g (90%) of 8; m.p. = 155 °C dec.
(EtOH); IR ν cm-1: 2 210 s, 2 150 s, 1 630 s, 1 600 s.
Anal. (C16H12N6) C, H, N.
4.1.5. 5-Butyl-5,7,8,9,10,11-Hexahydro-11-oxo-benzimi-
dazo[1,2-b]isoquinoline-6-carbonitrile 6
Compound 4 (1.32 g, 5 mmol) was refluxed with
tributyl phosphate (10 mL) for 1 h in the presence of
anhydrous potassium carbonate (0.2 g). After cooling and
addition of water, the product was filtered, and dried;
yield: 1.3 g (81%); m.p. = 154–157 °C (EtOH/H2O); IR ν
1
cm-1: 3 000–2 800 bw, 2 200 s, 1 660 s, 1 610 w. H-
4.1.10. 11-Triphenylphosphoranylideneamino-7,8,9,10-
tetrahydrobenzimidazo[1,2-b]isoquinoline-6-carbonitrile
9
NMR (CDCl3): δ 1.0 (t, 3 H, CH3(CH2)2CH2-N), 1.2–1.9
(m, 8 H, CH3(CH2)2CH2-N and CH2(CH2)2CH2-), 2.5 &
2.7 (2 t, 4 H, -CH2(CH2)2CH2-), 4.4 (t, 2 H,
CH3(CH2)2CH2-N), 7.0–7.4 (m, 3 ArH), 8.7 (d, 1 ArH at
C-7). Anal. (C20H21N3O) C, H, N.
To a stirred suspension of 8 (2.3 g, 8 mmol) in benzene
(20 mL), a solution of triphenylphosphine (2.62 g,
10 mmol) in benzene (15 mL) was added at room tem-
perature. An immediate clear solution was formed with
evolution of nitrogen gas and separation of an orange-red
product. After stirring for 1 h, the solid was filtered,
washed with benzene, and dried; yield: 4.0 g (96%); m.p.
= 265 °C dec. (EtOH); IR ν cm-1: 2 900 w, 2 200 s,
1 620 s, 1 570 s. 1H-NMR: δ 1.2 & 1.5 (2 m, 4 H,
4.1.6. 11-(4-Methylphenylamino)-7,8,9,10-tetrahydroben-
zimidazo[1,2-b]isoquinoline-6-carbonitrile 7a
This was prepared by stirring a solution of 5 (1.4 g,
5 mmol) and p-toluidine (1.07 g, 10 mmol) in dimethyl-
formamide (10 mL) for 4 h at 80 °C. After cooling and
addition of water, the precipitate was filtered and dried;
yield: 1.5 g (86%); m.p.: 220–221 °C (EtOH); IR ν cm-1:
3 300 bs, 2 210 s, 1 620 w, 1 600 w. 1H-NMR: δ 1.8 (m,
4 H, -CH2(CH2)2CH2-), 2.2 (s, CH3), 2.7 & 3.1 (2 t, 4 H,
-CH2(CH2)2CH2-), 6.6 & 7.0 (dd, 4 ArH, aryl at C-5), 7.2
& 7.5 (2 t, 2 ArH at C-8,9), 7.8 (d, 1 ArH at C-10), 7.9 (d,
1 ArH at C-7), 9.0 (s, NH). Anal. (C23H20N4) C, H, N.
-CH2(CH2)2CH2-), 2.0
&
2.9 (2 t,
4
H,
-CH2(CH2)2CH2-), 6.7 & 7.3 (2 t, 2 ArH at C-8,9),
7.6–7.9 (m, 15 ArH of (C6H5)3P = N- + 1 ArH at C-10),
8.1 (d, 1 ArH at C-7). Anal. (C34H27N4P) C, H, N.
Acknowledgements
We thank the staff of the National Cancer Institute,
Bethesda MD, USA, for the anticancer test reports.
4.1.7. 11-(4-Chlorophenylamino)-7,8,9,10-tetrahydroben-
zimidazo[1,2-b]isoquinoline-6-carbonitrile 7b
This was prepared, similar to 7a, from 5 (1.4 g,
5 mmol) and p-chloroaniline (1.13 g, 10 mmol); yield:
1.4 g (75%); m.p. = 240–242 °C (EtOH); IR ν cm-1:
3 600 bm, 2 210 s, 1 630 m, 1 590 s. 1H-NMR: δ 1.8 (m,
4 H, -CH2(CH2)2CH2-), 2.7 & 3.2 (2 t, 4 H,
-CH2(CH2)2CH2-), 6.7 & 7.3 (dd, 4 ArH, aryl at C-5), 7.2
& 7.5 (2 t, 2 ArH at C-8,9), 7.8 (d, 1 ArH at C-10), 7.9 (d,
1 ArH at C-7), 9.3 (s, NH). Anal. (C22H17ClN4) C, H, N.
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667
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