Microwave-assisted reductive cyclization: An easy entry to the indoloquinolines and spiro[2H-indole-2,3′-oxindole]

Article abstract of DOI:10.1039/c4ra02814g

Synthesis of two linear indoloquinolines i.e. 6H-indolo[2,3-b]quinoline & 11-hydroxy-10H-indolo[3,2-b]quinoline, and spiro[2H-indole-2,3′- oxindole] from a common intermediate, (2-nitrophenyl)methylidene-dihydro- indolone derivative is described using microwave-mediated reductive cyclization as the key step. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02814g

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Microwave-assisted reductive cyclization: an easy
entry to the indoloquinolines and spiro[2H-indole-
2,30-oxindole]†
Cite this: RSC Adv., 2014, 4, 22481
ab
and Mahesh S. Majika
Received 31st March 2014
Accepted 12th May 2014
*
Prakash T. Parvatkar
DOI: 10.1039/c4ra02814g
www.rsc.org/advances
Synthesis of two linear indoloquinolines i.e. 6H-indolo[2,3-b]quinoline
In our earlier communication,44 we described the synthesis
& 11-hydroxy-10H-indolo[3,2-b]quinoline, and spiro[2H-indole-2,30- of neocryptolepine using isatin and (2-nitrobenzyl)triphenyl-
oxindole] from a common intermediate, (2-nitrophenyl)methylidene- phosphonium bromide as the starting material via intermediate
dihydro-indolone derivative is described using microwave-mediated 16. In the present study, this intermediate 16 is further explored
reductive cyclization as the key step.
for the synthesis of two isomeric linear indoloquinolines and
spiro-oxindole derivative using microwave mediated reductive
cyclization approach.
Introduction
Cryptolepis sanguinolenta (a tropical shrub indigenous to West
Africa) is a rich source of indoloquinoline alkaloids and so far
13 alkaloids containing the indoloquinoline framework have
been isolated from this plant. Indoloquinoline alkaloids have
received prominent attention in recent years due to their
striking biological activities particularly DNA intercalating1 and
antimalarial2,3 properties. In addition, these alkaloids have also
been reported to show antimuscarinic, antibacterial, antiviral,
antimicotic, antihyperglycemic and cytotoxic properties in vitro
and antitumor activity in vivo.4–9 Various alkaloids isolated from
this plant includes cryptolepine10 1, hydroxycryptolepine11 2,
cryptolepinone11–13 3, 11-isocryptolepine14 4, cryptospirolepine12
5, cryptolepicarboline15 6, cryptomisrine16 7, bis-cryptolepine17
8, cryptoquindoline11 9, cryptoheptine11 10, quindoline18 11,
quindolinone13 12, neocryptolepine17,19 13 and iso-
cryptolepine19,20 14 (Fig. 1).
Indoloquinoline alkaloids21 particularly cryptolepine and
neocryptolepine have been the targets of synthetic and medic-
inal chemists during the past decade due to their interesting
structural features and diverse biological activities. Important
reported methods for the synthesis of 10H-indolo[3,2-b]quino-
line and 6H-indolo[2,3-b]quinoline (immediate chemical
precursors of cryptolepine and neocryptolepine) are shown in
Fig. 2 and 3 respectively.
Results and discussion
Our approach for the synthesis of two isomeric linear indolo-
quinolines is based on the retrosynthetic analysis (Scheme 1). It
was anticipated that, linear indoloquinolines 11 and 15 can be
synthesized from a common intermediate 16 via reductive
cyclization reaction. The key intermediate 16 in turn can be
obtained by Wittig reaction of (2-nitrobenzyl)triphenylphos-
phonium bromide with isatin. Hence compound 16 could be
considered as an advance intermediate for preparation of
alkaloids in present work.
Accordingly, intermediate 1644 was prepared by stirring the
mixture of (2-nitrobenzyl)triphenylphosphonium bromide, isa-
tin and Et3N in CHCl3 at room temperature (Scheme 2). The
Wittig olenation product which precipitated out in CHCl3 was
isolated by simple ltration in 92% yield. The Wittig product
was obtained as a mixture of geometrical isomers in 2 : 1 ratio
as evident from 1H NMR data. The stereochemistry of olen was
not of any consequence for further manipulation and hence,
this mixture of Wittig product 16 was subjected to next step
without any further purication. Initially, the Wittig product 16
was subjected to reductive cyclization using Fe powder in
presence of catalytical amount of HCl in reuxing acetic acid to
yield exclusively 6H-indolo[2,3-b]quinoline 15 in 77% yield.44
During this step, four reactions took place in one-pot i.e.
reduction of nitro group, isomerization of mixture of E/Z to Z-
isomer, cyclization and nally dehydration. Regioselective
methylation on quinoline nitrogen of 6H-indolo[2,3-b]quinoline
using reported procedure27 afforded the naturally
occurring alkaloid – neocryptolepine (cryptotackieine) 13.
aBio-organic Chemistry Laboratory, CSIR-National Institute of Oceanography, Dona
Paula, Goa 403 004, India. E-mail: pparvatker@yahoo.com
bInstitute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011,
Japan
† Electronic supplementary information (ESI) available: Copies of 1H and 13C
NMR of all the synthesized compounds. See DOI: 10.1039/c4ra02814g
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Fig. 1 Structures of indoloquinoline alkaloids isolated from Cryptolepis sanguinolenta.
Scheme
1
Retrosynthetic
analysis
of
cryptolepine
and
neocryptolepine.
Fig. 2 Synthetic approaches to 10H-indolo[3,2-b]quinoline.
Scheme 2 Synthesis of indoloquinolines and spiro[2H-indole-2,30-
oxindole].
Cadogan cyclization is used for the synthesis of various N-
heterocycles which was rst reported in 1963 by Cadogan and
Bunyan46 for the preparation of carbazoles. So we attempted the
reductive cyclization (popularly known as Cadogan cyclization)
using triphenyl phosphine (TPP) or triethyl phosphite (TEP) as a
deoxygenating reagent (Table 1) with intension of getting both
Fig. 3 Synthetic approaches to 6H-indolo[2,3-b]quinoline.
6H-Indolo[2,3-b]quinoline 15 itself is a natural product reported the linear indoloquinoline frameworks i.e. 6H-indolo[2,3-b]-
from the leaves of Justica betonica45 and is known as qui- quinoline 15 and 10H-indolo[3,2-b]quinoline 11 in one-pot.
nindoline33 or norcryptotackieine.27 For last several decades, When the reductive cyclization was carried out using TPP in
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Table 1 Optimization of reductive cyclization reaction conditions
the products, some loss during purication by column chro-
matography. To overcome the decomposition of the interme-
diate, reaction was tried at lower temperature i.e. 180–200 ꢀC
(entry 2) but only trace amount of spiro[2H-indole-2,30-oxindole]
19 was observed while most of the starting material remains
unreacted (monitored by TLC) even aer continuing the reac-
tion for 15 hours. Carrying out the reductive cyclization reaction
in reuxing TEP (entry 3), neither improve the yields of the two
products nor it helps in forming the other expected rearranged
product i.e. 10H-indolo[3,2-b]quinoline 11. The low yields in
this case may be due to the formation of N-alkylated side
products50 which are known to form when TEP is used as a
deoxygenating reagent. Never the less, this report represents the
new route to the spiro-derivative of oxindole 19. In recent years,
Yielda [%]
Entry
Reagents and condition
19
15
20
1
2
PPh3, Ph2O, 260–270 ꢀC, 5 h
PPh3, Ph2O, 180–200 ꢀC, 15 h
P(OEt)3, reux, 3 h
25
Trace
19
13
—
10
3
—
—
—
—
3
4b
PPh3, Ph2O, MW, 150 ꢀC, 15 min
and then 180 ꢀC, 10 min
10
5
P(OEt)3, MW, 150 ꢀC, 10 min
33
22
20
a
Isolated yield. b 64% of starting material is recovered.
reuxing diphenyl ether (entry 1), two compounds were the use of microwave energy has become an increasingly
obtained. As expected, one compound was found to be 6H- popular technique in organic synthesis as it not only saves
indolo[2,3-b]quinoline 15 as evident from the NMR and mass energy and time but sometimes also helps in getting the prod-
data while the NMR and mass data of the other compound did ucts which are not obtained by conventional oil-bath synthesis.
not match with the other expected compound i.e. 10H-indolo So, we attempted the reductive cyclization using TPP in
[3,2-b]quinoline 11. Aer careful observation of the NMR and diphenyl ether under microwave irradiation (entry 4). The
mass data, we found that spiro[2H-indole-2,30-oxindole] 19 was microwave reaction was carried out at 150 ꢀC for 15 minutes and
formed instead of 10H-indolo[3,2-b]quinoline 11. Interestingly, then at 180 ꢀC for another 10 minutes. The products 19 and 15
the major compound 19 formed during this reaction contains were obtained in 10% and 3% yields while most of the starting
the spiro-oxindole core which is present in various biologically material remains unreacted. Interestingly, microwave reaction
important alkaloids47 such as horsline, coerulescine, pter- using TEP at 150 ꢀC (entry 5) resulted in the formation of three
opodine and spirotryprostatin-B. Also some of the unnatural products. Two compounds were found to be spiro[2H-indole-
spiro-oxindoles such as spiroimides are known to be potent as 2,30-oxindole] 19 and 6H-indolo[2,3-b]quinoline 15 as expected
aldose reductase inhibitors and antihyperglycemic agents.48,49 while the NMR and mass data of the third compound did not
The low yields of the products may be due to the decomposition match with the other expected possible compound i.e. 10H-
of the intermediate as the reaction was carried out at high indolo[3,2-b]quinoline 11. Aer carefully analyzing the NMR
temperature i.e. 260–270 ꢀC and also due to the low solubility of and mass data, we found that 11-hydroxy-10H-indolo[3,2-b]-
Scheme 3 Proposed mechanism for the formation of indoloquinolines and spiro[2H-indole-2,30-oxindole].
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quinoline 20 is formed during the reaction. It is fascinating to
note that, compound 20 is hydroxyl derivative of compound 11.
Repetition of this cyclization step under MW assisted condition
delivered similar results indicating the reproducibility of this
methodology. In case of entry 4 (Table 1), low yields of the two
products (19 and 15) and no formation of compound 20 may be
due to the temperature (150–180 ꢀC) which is not sufficient for
this substrate to undergo reductive cyclization when TPP is used
as a deoxygenating agent even under microwave irradiation.
This is evident from the recovery of most of the starting material
and also from the entry 2 (Table 1) wherein only trace amount of
compound 19 is observed. Finally, we had completed the
synthesis of spiro[2H-indole-2,30-oxindole] 19, 6H-indolo[2,3-b]-
quinoline 15 and 11-hydroxy-10H-indolo[3,2-b]quinoline 20 in
3-[(2-Nitrophenyl)methylidene]-1,3-dihydro-2H-indol-2-one 16
Yield ¼ 0.50 g, 92%. Experimental details, physical and spec-
troscopic data are same as that of published data.44
6H-Indolo[2,3-b]quinoline 15
Yield ¼ 0.231 g, 77%. Experimental details, physical and spec-
troscopic data are same as that of published data.44
Typical procedure for reductive cyclization using PPh3 or
P(OEt)3
(a)By conventional heating. (i) Using PPh3: mixture of
2-steps with an overall yield of 30%, 20%, and 18% respectively. compound 16 (0.511 g, 1.92 mmol) and PPh3 (1.209 g, 4.61
The probable mechanism for the formation of all the three mmol) in Ph2O (15 mL) was reuxed for 5 h. Aer cooling, the
products is shown in Scheme 3. Nitro compound reacts with reaction mixture was chromatographed on silica gel and
phosphorus reagent to give nitrene intermediate which cyclizes diphenyl ether was removed using hexanes as an eluent. Further
to form aziridine intermediate. Opening of aziridine interme- elution with 10% ethyl acetate in hexanes afforded spiro[2H-
diate is possible in three different ways: (a) case 1 – to give indole-2,30-oxindole] 19 (0.113 g, 25%) and with 20%
directly stable spiro[2H-indole-2,30-oxindole] 19, (b) case 2 – to ethyl acetate in hexanes afforded 6H-indolo[2,3-b]quinoline 15
give labile N-epoxy intermediate which may be reacted with (0.056 g, 13%).
phosphorus reagent to give four-membered intermediate that
(ii) Using P(OEt)3: compound 16 (0.508 g, 1.91 mmol) in
loses triphenyl phosphine oxide to give 6H-indolo[2,3-b]quino- P(OEt)3 (10 mL) was reuxed for 3 h. Aer cooling, excess
line 15 and (c) case 3 – to give epoxy intermediate. We are P(OEt)3 was distilled out under vacuum and the crude reaction
expecting epoxy intermediate to react with phosphorus reagent mixture was chromatographed on silica gel. Elution with 10%
to give four-membered intermediate which then can undergo ethyl acetate in hexanes afforded spiro[2H-indole-2,30-oxindole]
lose of triphenyl phosphine oxide to give 10H-indolo[3,2-b]- 19 (0.085 g, 19%) and with 20% ethyl acetate in hexanes affor-
quinoline 11. But, instead epoxy intermediate opens up to give ded 6H-indolo[2,3-b]quinoline 15 (0.042 g, 10%).
directly 11-hydroxy-10H-indolo[3,2-b]quinoline 20.
(b)By microwave irradiation. Manufacturer – milestone and
model – startsynth.
The reactions were carried out in sealed vessel.
The method of monitoring the reaction mixture temperature
– internal probe.
Conclusion
We have developed a method in which two linear indoloqui-
nolines are formed in the same step along with spiro-oxindole
derivative under microwave irradiation. Use of microwave irra-
diation for carrying out reductive cyclization, not only reduced
energy and time but helps in forming 11-hydroxy-10H-indolo
[3,2-b]quinoline which was not obtained with conventional oil-
bath heating and also increases the yields of the other two
products. Further work to extend this strategy for the synthesis
of derivatives of these compounds for their biological studies
are under progress.
The required temperature is reached in 10 min. and then
maintained for the specied time in each experiment.
(i) Using PPh3: mixture of compound 16 (0.206 g, 0.77 mmol)
and PPh3 (0.488 g, 1.86 mmol) in Ph2O (10 mL) was irradiated in
ꢀ
ꢀ
microwave reactor at 150 C (200 W) for 15 min. and at 180 C
(200 W) for 10 min. Aer cooling, the reaction mixture was
chromatographed on silica gel and diphenyl ether was removed
using hexanes as an eluent. Further elution with 10% ethyl
acetate in hexanes afforded spiro[2H-indole-2,30-oxindole] 19
(0.018 g, 10%), with 20% ethyl acetate in hexanes afforded 6H-
indolo[2,3-b]quinoline 15 (0.005 g, 3%) and with 30%
ethyl acetate in hexanes recovered the starting material (0.132 g,
64%).
Experimental section
General methods
(ii) Using P(OEt)3: compound 16 (0.203 g, 0.76 mmol) in
ꢀ
The reagents were purchased commercially and used without P(OEt)3 (10 mL) was irradiated in microwave reactor at 150 C
further purication. The solvents were distilled prior to use. (200 W) for 10 min. Aer cooling, excess P(OEt)3 was distilled
Column chromatography was performed on silica gel (60–120 out under vacuum and the crude reaction mixture was chro-
mesh). Infrared spectra were recorded with an FTIR spectro- matographed on silica gel. Elution with 10% ethyl acetate in
photometer. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra hexanes afforded spiro[2H-indole-2,30-oxindole] 19 (0.059 g,
were recorded in DMSO-d6. TMS was used as an internal refer- 33%), with 20% ethyl acetate in hexanes afforded 6H-indolo[2,3-
ence. HRMS data were recorded with an ES-QTOF. Melting b]quinoline 15 (0.036 g, 22%) and with 40% ethyl acetate in
points were recorded with a Thieles apparatus and are hexanes afforded 11-hydroy-10H-indolo[3,2-b]quinoline 20
uncorrected.
(0.036 g, 20%).
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Spiro[2H-indole-2,30-oxindole] 19
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6H-Indolo[2,3-b]quinoline 15
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11-Hydroy-10H-indolo[3,2-b]quinoline 20
ꢀ
Mp > 200 C. IR (KBr): gmax ¼ 3215 (–NH), 3160 (–OH), 1637,
1612, 1554, 1456, 1396, 1215, 748 cmꢁ1. 1H NMR (DMSO-d6, 300
MHz) d (ppm):7.23–7.31 (q, J ¼ 7.8 Hz, 2H), 7.34–7.38 (t, J ¼ 7.2
Hz, 1H), 7.48–7.50 (m, 2H), 7.60–7.62 (d, J ¼ 8.1 Hz, 1H), 8.18 (br
m, 2H), 11.41 (br s, –NH), 12.58 (br s, –OH). 13C NMR (DMSO-d6,
75 MHz) d (ppm): 106.8, 112.1, 112.4, 116.5, 121.2, 121.5, 122.1,
122.5, 124.5, 124.8, 129.7, 138.2, 138.3, 141.2, 160.4. HRMS: m/z
[M + H]+ calcd for C15H11N2O, 235.0919; found, 235.0937.
Acknowledgements
Authors thank the OCEAN FINDER and EU-FP7-KBBE-2009-3-
245137 MAREX for nancial assistance and are grateful to CSIR-
NIO for the award of Scientist Fellow-QHS. Special thanks to Mr
H. K. Kadam (Department of Chemistry, Goa University) for his
help during this work.
21 P. T. Parvatkar, P. S. Parameswaran and S. G. Tilve, Curr. Org.
Chem., 2011, 15, 1036.
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