Efficient synthetic route toward biologically active γ-carboline derivatives

Article abstract of DOI:10.1080/00397911.2015.1103874

An efficient and short route was established for the synthesis of anti-bovine viral diarrhea virus agents, namely 4-methyl-γ-carboline (SK4M) 1, 3-methyl-γ-carboline (SK3M) 2, 5-methyl-γ-carboline (SK5M) 3, and a new γ-carboline derivative 4, using therma

Full text of DOI:10.1080/00397911.2015.1103874

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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An Efficient Synthetic Route toward Biologically
Active γ-Carboline Derivatives
Dattatray G. Hingane, Nikita P. Parekh, Ayesha Khan & Radhika S. Kusurkar
To cite this article: Dattatray G. Hingane, Nikita P. Parekh, Ayesha Khan & Radhika S. Kusurkar
(2015): An Efficient Synthetic Route toward Biologically Active γ-Carboline Derivatives, Synthetic
Communications, DOI: 10.1080/00397911.2015.1103874
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Date: 29 December 2015, At: 09:33

An Efficient Synthetic Route Toward Biologically Active γ-Carboline Derivatives
Dattatray G. Hingane1, Nikita P. Parekh1, Ayesha Khan1, Radhika S. Kusurkar1
1Department of Chemistry, Savitribai Phule Pune University, Pune, Maharashtra, India
Address correspondence to Radhika S. Kusurkar, Department of Chemistry, Savitribai
Phule Pune University, Pune-411007, Maharashtra, India. E-mail:
rsk@chem.unipune.ac.in
Abstract
An efficient and short route was established for the synthesis of anti-BVDV agents
namely 4-methyl-γ-carboline (SK4M) 1, 3-methyl-γ-carboline (SK3M) 2, 5-methyl-γ-
carboline (SK5M) 3 and a new γ-carboline derivative 4 using thermal electrocyclization
reaction as a key step. The evaluation of cytotoxicity of compound 4 against human
cervical cancer cell line HeLa and Leukemic cell line HL-60 furnished CC50 value of 19.5
and 18.8 μM respectively.
KEYWORDS: 4-Methyl-γ-carboline (SK4M), 3-methyl-γ-carboline (SK3M), 5-methyl-
γ-carboline (SK5M), electrocyclization
INTRODUCTION
γ-Carbolines (pyrido[4,3-b]indoles) are a class of annelated derivatives of indole
which are commonly found in natural products.[1,2] As compared to β-carbolines these
compounds are less studied however they are known to exhibit[3,4] various biological
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activities. Due to the medicinal importance of γ-carboline alkaloids, these are interesting
synthetic targets and several methods are reported[5-7] for their synthesis.
Literature survey,[8a, 9] revealed that γ-carboline derivatives like SK4M (1), SK3M
(2) and SK5M (3) (Fig.1) exhibit anti-BVDV activity as well as cytotoxicity. Amongst
these, SK5M showed most potent anti-BVDV activity (EC50 = 0.26 µM).
In continuation with our previous work [10] and considering the biological activity
of the above mentioned γ-carboline derivatives, it was planned to devise a synthetic
strategy for these compounds using electrocyclization reaction. Earlier, only two
synthetic methods are reported[8a,b, 9] for these compounds by using palladium-catalyzed
amination and arylation reactions.
We herein, demonstrate the synthesis of anti-cancer and anti-BVDV agents 1
(SK4M), 2 (SK3M), 3 (SK5M) and an unnatural new γ-carboline derivative 4. The
cytotoxicity of compound 4 was also evaluated.
RESULTS AND DISCUSSION
Chemistry
The retrosynthetic analysis for the above target molecules starting from simple
compound like indole, using thermal electrocyclization strategy is shown in scheme 1.
2

Initially, 1-phenylsulfonylindole 5 (Scheme 2) was treated with LDA and acetone
or propionaldehyde was added as an electrophile to furnish alcohol 6 or 7 respectively.
Further, deprotection of the phenylsulfonyl group and dehydration reaction using
tetrabutylammonium fluoride (TBAF) gave compound 9 and 10 respectively.
Simultaneously, N-methyl-2-vinylindole 11 was prepared by Wittig reaction of N-
methylindole-2-carbaldehyde.[11] Subsequently, 3-position of dienes 9, 10 and 11 was
formylated using Vilsmeier Haack reaction to produce unreported aldehydes 13, 14 and
15 respectively in good yield. All these products were characterized using spectral and
analytical data. Further, oximination of aldehydes 13, 14 and 15 with hydroxylamine
hydrochloride and refluxing in dioxane separately furnished target compounds 1, 2 and 3
respectively in good yield. Formation of compounds 1, 2 and 3 can be explained by initial
oxime formation (Scheme 1) and further thermal electrocyclization of 1-aza-1,3,5-triene
system in the oxime along with dehydration.
After achieving successfully a short, high yielding and efficient route for
compounds 1, 2 and 3, it was thought to use the same reaction sequence for the synthesis
of a new and unnatural γ-carboline derivative 4. Thus, 1-phenylsulfonylindole 5 (Scheme
2) was treated with LDA and cyclopentanone was added as an electrophile to furnish [12]
alcohol 8. Further, deprotection of the phenylsulfonyl group and dehydration reaction
using TBAF gave diene 12. Vilsmeier Haack formylation of N-(benzenesulfonyl)-2-(1-
cyclopentenyl)indole (12) was carried out to produce unreported aldehyde 2-(cyclopent-
1-en-1-yl)-1H-indole-3-carbaldehyde 16 in 88% yield which was characterized using
spectral and analytical data. Oximination of 16 with hydroxylamine hydrochloride and
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electrocyclization by refluxing in dioxane furnished target compound 4. The structure of
compound 4 was confirmed by the presence of a downfield singlet of C5H at 9.08 in 1H
NMR, three singlets for methylene carbons in 13C NMR spectrum and DEPT experiment
(see experimental). As the activity of the compounds 1, 2 and 3 was already reported, the
evaluation of the cytotoxic activity of product 4 was undertaken.
BIOLOGICAL EVALUATION
Anticancer Activity of Compound 4
The cytotoxicity of the compound 4 was tested against human cancer cell lines
viz. HeLa (cervical cancer) and promyelocytic leukemic cell line HL-60 after respective
incubation period ranging from 24 to 72h. The cell viability was assessed by the MTT
assay.[13] The resulting concentration–effect curves obtained with continuous exposure for
various time points are depicted in figure 2. Significant cell death was observed after the
tumor cells were incubated in the presence of 25 μM of compound 4 for 24h. The cell
viability decreased linearly for all the cell lines in a time dependant manner i.e. upto 72h.
The compound 4 showed apoptotic behavior in all the cell lines and did not show
any significant changes in the morphology (see supplementary material). Compound 4
was found to execute maximum cell death in the HL-60 (~56%) cell lines (72h) similar to
HeLa (~59%). Thus, the evaluated CC50 (µM) value for compound 4 was 18 ± 2. The
reported values for compounds 1, 2 and 3 are 21 ± 3, 70 ± 0 and 29 ± 3 respectively.
Indole -3-carbinol (I3C) was selected as a positive control in the cytotoxicity assay.14 The
CC50 (µM) for I3C was found to be 325± 3 (data not shown). From this data, compound 4
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was shown to exhibit better activity than all others. Compound 4 should be further
explored for several other human cancer cell lines to potentiate its activity.
EXPERIMENTAL
General Procedure for the Synthesis of Compound 1, 2, 3 and 4
A mixture of aldehyde 13/14/15/16 (1mmol), hydroxylamine hydrochloride (2
mmol), and sodium acetate (2 mmol) in 10 mL dioxane was refluxed under stirring for 24
hours. The reaction was quenched by the 20 mL of water. The product was extracted with
ethyl acetate (20 mL x 3) and the combined organic layer was washed with brine.
Evaporation of the solvent, followed by column chromatography (CHCl3-MeOH) gave
corresponding compound 1/2/3/4.
4-Methyl-5H-Pyrido[4,3-B]Indole (1)
Yield: 72% ; M.p. 198-200oC; 1H NMR (300 MHz, CDCl3 + DMSO-d6) δ : 2.63 (s, 3H),
7.32 (t, J = 7.6Hz, 1H), 7.52 (t, J = 7.2Hz, 1H), 7.6 (d, J = 8.1Hz, 1H), 8.1 (d, J = 7.7Hz,
1H), 8.2 (s, 1H), 9.1 (bs, 1H), 12.0 (s, 1H); 13C NMR (75 MHz, CDCl3 ) δ : 13.97,
111.63, 120.33, 120.42, 120.77, 126.80, 137.44, 139.64, 140.37, 144.13; FT-IR (KBr cm-
1): 3417.98, 2960.83, 1026.16.
3-Methyl-5H-Pyrido[4,3-B]Indole (2)
Yield: 74%; M.p. 220-222oC (lit. M. p. 222-225 oC)[8]; 1H NMR (300 MHz, DMSO-d6) δ:
2.47 (s, 3H), 7.2 (t, J = 6.7Hz, 1H), 7.4-7.6 (m, 3H), 8.1 (d, J =7.6Hz, 1H), 9.1 (s, 1H),
11.8 (s, 1H); 13C NMR (75 MHz, CDCl3 ) δ : 18.79, 108.40, 112.046, 119. 57, 120.09,
5

121.25, 121.91, 128.56, 129.29, 132.39, 141.49, 145.82; FT-IR(KBr cm-) : 3223.99,
2924.22,1455.0
5-Methyl-5H-Pyrido[4,3-B]Indole (3)
Yield: 76%; M.p. 87-89oC (lit. M. p. 88-90 oC)[8]; 1H NMR (300 MHz, DMSO-d6) δ :
3.89 (s, 3H), 7.31 (dd, J = 7.9, 8.1 Hz, 1H), 7.55 (dd, J = 7.9, 8.1 Hz, 1H), 7.61 (d, J = 5.6
Hz, 1H), 7.67 (d, J = 8.1 Hz, 1H), 8.25 (d, J = 8.1 Hz, 1H), 8.48 (d, J = 5.6 Hz, 1H), 9.34
(s,1H); 13C NMR (75 MHz, CDCl3 ) δ : 29.04, 108.39, 116.2, 117.0, 118.80, 120.28,
122.75, 125.64, 140.98. FT-IR (KBr cm-1): 2919.58, 1456.99, 711.20.
1 2,3,10-Tetrahydrocyclopenta[5,6]Pyrido[4,3-B]Indole (4)
Yield: 76%; M.p. 276-278oC; 1H NMR (300 MHz, DMSO-d6): 2.16 (m, 2H), 3.06(m,
4H), 7.2 (t, J= 7.0 Hz, 1H), 7.4 (t, J= 7.0 Hz, 1H), 7.5 (t, J= 8.2 Hz, 1H), 8.15 (d, J = 8.2
Hz, 1H), 9.08 (s, 1H), 11.60 (s, 1H); 13C NMR (75 MHz, CDCl3 + DMSO-d6): 22.54*,
27.94*, 33.96*, 111.34, 117.34, 117.8, 119.79, 120.33, 121.42, 126.02, 139.75, 140.6,
141.24, 160.04; IR (Nujol): v 3440 cm-1; GC–MS (DIP)= 208 (M+); calcd for C14H12N2:
C, 81.76; H, 5.76; N, 13.46; Found : C, 80.68; H, 5.70; N, 13.64.
*confirmed as CH2 in DEPT.
CONCLUSION
An efficient and short route was established for biologically active γ-carbolines
SK4M (1), SK3M (2), SK5M (3) and for a new γ-carboline derivative 4 using thermal
6

electrocyclization strategy with good overall yield of 42.96%, 39.66% 59.12% and
41.68% respectively. The anti-cancer activity was evaluated as 4 > 1 > 3 > 2. The
addition of a 5-membered saturated ring D could be a plausible reason for increased
activity in compound 4 and the mechanism is under investigation.
ACKNOWLEDGEMENTS
We are grateful to Mrs. J. P. Chaudhari for NMR spectra and Mr. Shishupal for
GCMS and IR spectra.
FUNDING
D.G.H. is thankful to UGC for FIP. N.P.P is thankful to CSIR (New Delhi) for
fellowship.
SUPPLEMENTAL MATERIAL
Supplemental data for this article and the cytotoxicity evaluation of compound 4 can be
accessed on the publisher’s website.
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Scheme 1. Retrosynthetic strategy
9

Scheme 2. Reagent and conditions: (a) LDA, THF, -780C, acetone/ propionaldehyde/
cyclopentanone, 4h; (b) TBAF, THF, reflux 2h; (c) methyltriphenyl phosphonium salt,
1.6M n-BuLi, THF, 0oC, 2h; (d) DMF, POCl3, rt, 1h; (e) NH2OH, HCl, sodium acetate,
dioxane, reflux, 24h.
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Figure 1. γ-carboline derivatives.
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Figure 2. Percentage cell viability of compounds 4 on Hela and HL-60 cancer cells
12