A spirocyclic oxindole analogue: Synthesis and antitumor activities

Article abstract of DOI:10.1016/j.cclet.2011.01.042

A new synthesis of spirocyclic oxindole analogue spiro[piperidine-4, 3′-pyrrolo[2,3-b]pyridin]-2′(1′H)-one 1 is described. The key steps involve dialkylation of arylacetonitrile and cyclization of the azaoxindole ring by an intramolecular Buchwald-Hartwig amidation of carboxylic amide and aryl chloride. A small library was obtained by reductive amination of 1 with various aldehydes and was screened against human lung cancer cell A549, human liver cancer cell BEL7402, and human colon cancer cell HCT-8. The results show that most of the library compounds 2 have some inhibitory activities. 2-(Trifluoromethoxy) benzylic substituted spirocyclic azaoxindole 2e was identified as a nanomolar inhibitor against human lung cancer cell A-549 (IC₅₀ = 50 nmol/L).

Full text of DOI:10.1016/j.cclet.2011.01.042

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1009–1012
www.elsevier.com/locate/cclet
A spirocyclic oxindole analogue:
Synthesis and antitumor activities
*
Hui Hong, Long Jiang Huang, Da Wei Teng
College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, China
Received 6 December 2010
Abstract
A new synthesis of spirocyclic oxindole analogue spiro[piperidine-4,30-pyrrolo[2,3-b]pyridin]-20(10H)-one 1 is described. The
key steps involve dialkylation of arylacetonitrile and cyclization of the azaoxindole ring by an intramolecular Buchwald–Hartwig
amidation of carboxylic amide and aryl chloride. A small library was obtained by reductive amination of 1 with various aldehydes
and was screened against human lung cancer cell A549, human liver cancer cell BEL7402, and human colon cancer cell HCT-8. The
results show that most of the library compounds 2 have some inhibitory activities. 2-(Trifluoromethoxy) benzylic substituted
spirocyclic azaoxindole 2e was identified as a nanomolar inhibitor against human lung cancer cell A-549 (IC50 = 50 nmol/L).
# 2011 Da Wei Teng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Spirocyclic oxindole; Aryl amidation; Dialkylation; Antitumor activity
Spirocyclic oxindoles are important scaffolds in drug discovery [1]. The key structural characteristic of these
compounds is the spiro ring fusion at the C(3)-position of the oxindole core, with varying degrees of substitution
around the pyrrolidine or piperidine ring and oxindole rings. These templates have been shown to exhibit a variety of
interesting biological activities, such as growth hormone secretagogues (Ibutamoren, MK-0677) [2], neurokinin
antagonists [3], oxytocin antagonists [4], monoamine transporter inhibitors [5], bradykinin antagonists [6] and cell
cycle inhibitors [7]. Due to their structure, they interact with a wide range of receptors and this activity has resulted in
significant interest in developing efficient methods to prepare spirocyclic oxindoles and their analogues. We report
herein the efficient synthesis of spiro[piperidine-4,30-pyrrolo[2,3-b]pyridin]-20(10H)-one 1 (Fig. 1) and studies on their
antitumor activities.
The most straightforward routes to the spiro ring fusion at the C(3)-position of the oxindole core involve: (1) C(3)-
dialkylation of oxindoles with the highly toxic and carcinogenic N-methyl-2,20-dichloroethylamine hydrochloride and
subsequent removal of the N-methyl group in a two step procedure involving demethylation with 2,2,2-trichloroethyl
chloroformate and cleavage of the corresponding carbamate with zinc [6,8]. (2) Palladium catalyzed intramolecular a-
arylation of N-protected amide of 20-bromoanilides, and subsequent removal of the N-protective group [9]. (3) C(3)-
dialkylation of oxindoles with cis-1,4-dichloro-2-butene, osmium dihydroxylation of the spirocyclopentene ring,
periodate cleavage of the 1,2-dihydroxycyclopentane ring and subsequent ring recyclization by double reductive
* Corresponding author.
E-mail address: dteng@qust.edu.cn (D.W. Teng).
1001-8417/$ – see front matter # 2011 Da Wei Teng. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.01.042

[(Fig._1)TD$FIG]
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H. Hong et al. / Chinese Chemical Letters 22 (2011) 1009–1012
Fig. 1. The structures of compounds 1 and 2.
amination [10]. As far as we are aware, the relevant methods for the C(3)-dialkylation of N-unsubstituted oxindole lead
to mixtures of compounds, with low yields of the C(3)-dialkylated derivatives [6,8]. All those methods suffer from the
disadvantages of protection and deprotection of N-protective groups and resulted in long steps, low overall yields and
tedious and laborious work up.
1. Results and discussion
We proposed a new and efficient synthesis of spirocyclic oxindole analogue 1. The key steps involve dialkylation of
arylacetonitrile and cyclization of the azaoxindole ring by an intramolecular Buchwald–Hartwig amidation of
carboxylic amide and aryl chloride, which is efficient and general to the derivatives of spirocyclic oxindole analogue 1.
To the best of our knowledge, there are no reported methods to form the spirooxindole or spiroazaoxindole ring with
direct intramolecular arylation of carboxylic amide and arylchloride. The retrosynthetic analysis was outlined in
Scheme 1. The target molecule 1 could be synthesized by Buchwald–Hartwig aryl amidation from N-Boc-4-(2-
chloropyridin-3-yl)piperidine-4-carboxamide 3 which will be obtained by the hydrolysis of the corresponding nitrile
[(Scheme_1)TD$FIG]
Scheme 1. Retrosynthetic analysis.
[(Scheme_2)TD$FIG]
Scheme 2. Reagent and conditions: (a) H2O2, AcOH, toluene, reflux. (b) POCl3, PCl5, reflux. (c) SOCl2, MeOH, 0 8C-rt. (d) NaBH4, CaCl2, THF,
0 8C-rt. (e) SOCl2, CH2Cl2, 0 8C-rt. (f) NaCN, ethanol, rt. (g) N-Boc-bis(2-chloroethylamine), NaH, DMF, 70 8C. (h) H2O2, NaOH, ethanol, 50 8C.
(i) Pd2(dba)3, NaOtBu, dppf, toluene, reflux, 110 8C. (j) HCl/EtOAc, rt.

H. Hong et al. / Chinese Chemical Letters 22 (2011) 1009–1012
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Table 1
Anti-tumor activities of compounds 2a–2m.
Compound
R
IC50 (mg/mL)
A-549
BEL-7402
HCT-8
2a
2b
2c
2d
2e
2f
Isobutyl
6.75
6.11
ꢀ1.32
2.80
3.50
0.60
Cyclohexanemethyl
Benzyl
4.63
1.08
ꢀ1.32
6.43
3-Methoxybenzyl
2-Trifluoromethoxybenzyl
2-Bromobenzyl
ꢀ1.56
0.05
ꢀ8.42
0.81
ꢀ5.20
ꢀ4.78
ꢀ4.88
2.48
ꢀ1.79
7.35
2.36
2g
2h
2i
3-Trifluorobenzyl
4-Methyl-3-fluorobenzyl
(5-Methylfuran-2-yl)methyl
Thiophen-3-ylmethyl
Pyridine-3-methyl
Benzofuran-2-methyl
H
2.30
10.14
ꢀ4.19
0.56
0.97
0.94
10.71
2.00
2j
ꢀ0.13
ꢀ7.07
3.29
2k
2m
1
4.48
ꢀ6.65
1.50
9.42
3.47
ꢀ0.46
1.36
4. The nitrile 4 could be obtained by dialkylation of 2-chloro-3-cyanomethyl pyridine 5 which will be derived from
commercially available 3-nicotinic acid 6.
The synthetic scheme was illustrated in Scheme 2. 3-Nicotinic acid 6 was oxidized and chlorinated to afford 2-chloro-
3-niconitic acid 7 in 90% yield in two steps. Estelle et al. [11] obtained 8 by chlorination of 7 with thionyl chloride and
subsequent reduction with sodium borohydride, unfortunately, it only gave 30% yield in our hand due to the instability of
2-chloronicotinoyl chloride. In our attempts, direct reduction of 7 with borane gave 8 in 66% yield. In a two step reaction
sequence, 7 was methyl esterified and subsequently reduced with lithium aluminum hydride, which only gave theyield of
33% due to the reductive elimination of chloride group. Finally we found that it could be reduced by sodium borohydride
in the presence of calcium chloride [12] and 8 is obtained in 91% yield. Chlorination of 8 with thionyl chloride and
subsequent substitution with sodium cyanide afford 2-chloro-3-cyanomethyl pyridine 5 in 85% yield.
With 5 in hand, we embarked on the dialkylation of 5 with dichloroethylamine. We concerned the
incompatibility of the N-protective group as referenced in the dialkylation of oxindole [8]. After several attempts,
we found that the N-Boc protective group could be used but the alkylation is difficult with various bases such as
sodium alkoxide, lithium hexamethyldisilazide, lithium diisopropylamide and potassium carbonate in various
solvents. The low yields are due to both the reactivity of 2-chloro substituent toward the base nucleophile and the
hindrance of the base or both. Eventually we found that treatment of 5 with sodium hydride in dimethylformamide
and subsequent addition of N-Boc-bis(2-chloroethylmine) gave the desired product 4 in 65% yield under optimized
condition [13].
In compatible with the protective group, hydrolysis of the nitrile 4 was carried out under basic conditions and
carboxylic amide 3 was obtained in 95% yield. Cyclization of 3 was investigated under Ullmann condition [14] and
Buchwald–Hartwig condition [15]. The product 1 was obtained in 30% and 78% yields, separately [16].
A small library 2 was obtained by reductive amination of spirocyclic azaoxindole 1 with various aldehydes and was
screened against human lung cancer cell A549, human liver cancer cell BEL7402, and human colon cancer cell HCT-8
using the MTT method. The results are illustrated in Table 1. It can be seen that most of the library compounds 2 show
some inhibitory activities against human liver cancer cell BEL7402, human colon cancer cell HCT-8 and human lung
cancer cell A549, while some of them promote tumor cell proliferation. The 2-(trifluoromethoxy)benzylic substituted
spirocyclic azoxaindole 2e was identified as a nanomolar inhibitor against human lung cancer cell A-549
(IC50 = 50 nmol/L). The anti tumor activities are still undergoing and the result will be reported later.
2. Conclusion
We developed a new and efficient synthesis of spiro[piperidine-4,30-pyrrolo[2,3-b]pyridin]-20(10H)-one. Initial
biological results show that its derivatives are promising inhibitors against human lung cancer cell A549, human liver
cancer cell BEL7402, and human colon cancer cell HCT-8.

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H. Hong et al. / Chinese Chemical Letters 22 (2011) 1009–1012
Acknowledgments
This research was financially supported by the program of research fund for returning scholars of Ministry of
Education of China (No. 200812) and the Open Foundation of Chemical Engineering Subject (No. 200903), Qingdao
University of Science & Technology, China.
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[13] Procedure of compound 4: To a solution of 2-chloro-3-cyanomethyl pyridine 5 (12 g, 7.9 mmol) in dry dimethylformamide (50 mL) at 0 8C
was added sodium hydride (0.86 g, 36 mmol). The reaction mixture was stirred for 10 min. Tert-butyl bis(2-chloroethyl)carbamate (28.7 g,
10.3 mmol) was added in one portion. The reaction mixture was stirred for 10 min and warmed to room temperature for 5 h. The mixture was
cooled to 0 8C and quenched with saturated ammonium chloride solution. The resulted mixture was diluted with water and extracted with ethyl
acetate, dried over anhydrous sodium sulfate and concentrated. The residue was purified by chromatography on a silica gel column to afford the
title product (16.5 g) as a white solid. 1H NMR (500 MHz, CDCl3): d 8.42 (dd, 1H, J = 5.0 Hz, J = 1.5 Hz), 7.78 (dd, 1H, J = 8.0 Hz,
J = 1.5 Hz), 7.36 (dd, 1H, J = 8.0 Hz, J = 5.0 Hz), 4.33 (m, 2H), 3.29 (m, 2H), 2.48 (m, 2H), 2.02 (m, 2H), 1.51 (t, 9H); 13C NMR (125 MHz,
CDCl3): d 154.3, 150.3, 149.3, 136.1, 132.7, 123.0, 119.0, 80.4, 41.0 (2ꢁ), 40.9, 33.3 (2ꢁ), 28.4 (3ꢁ); LRMS: MS (ES+) m/z = 322.1 (M+1)+.
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[16] Procedure of compound 1: To a solution of 3 (5.0 g, 14.0 mmol) in toluene (100 mL) was added bis(dibenzylideneacetone) palladium (0.65 g,
7.0 mmol), 1,10-bis(diphenylphosphino)ferrocene (0.6 g, 1.05 mmol) and sodium tert-butoxide (1.9 g, 19.6 mmol). The reaction mixture was
stirred at 110 8C for 16 h. The reaction mixture was filtered through a pad of celite, and the filter was washed with ethyl acetate. The filtrate was
concentrated and the residue was purified by chromatography to afford the intermediate as a white crystal (3.48 g). Then it was dissolved in
ethyl acetate (20 mL) and was added saturated solution of hydrogen chloride in ethyl acetate (20 mL). The reaction mixture was stirred for 3 h
at room temperature. The mixture was concentrated and the solid formed was collected by filtration, washed with a small amount of petroleum
1
ether and dried to afford 1 (2.25 g). H NMR (500 MHz, DMSO-d6): d 11.4 (br, 1H), 9.46 (br, 2H), 8.11 (d, 1H, J = 5.0 Hz), 7.79 (d, 1H,
J = 7.0 Hz), 7.07 (t, 1H, J = 6.3 Hz), 3.39 (m, 2H), 3.24 (m, 2H), 2.11 (m, 2H), 1.98 (m, 2H); 13C NMR (125 MHz, DMSO-d6): d 180.1, 155.8,
145.6, 132.3, 128.5, 118.4, 43.5, 39.1 (2ꢁ), 28.5 (2ꢁ); LRMS: MS (ES+) m/z = 204.1 (M+1)+.