Synthesis and biological evaluation of novel carbazolyl glyoxamides as anticancer and antibacterial agents

Abstract

A new library of 24 carbazolyl glyoxamides 14a–x were designed and synthesized from glyoxalic acids and arylamines in the presence of HATU as a coupling reagent under MW irradiation. The synthesized carbazolyl glyoxamides were evaluated for their in vitro anticancer and antibacterial activities. Of the synthesized carbazolyl glyoxamides, compounds 14l and 14q exhibited the most potent cytotoxicity towards a breast cancer cell line with IC₅₀ values of 9.3 and 9.8 μM, respectively. Further, caspase-3 assay for carbazolyl glyoxamides indicated that these compounds induced apoptotic cell death in Jurkat cells. Furthermore, some of the synthesized carbazolyl glyoxamides 14g, 14k, 14l and 14n exhibited comparable or even better antibacterial activity (MIC = 8–16 μg mL⁻¹) than chloramphenicol against the selected bacterial strains.

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Introduction

Indole alkaloids are an important class of natural products and medicinally important molecules, which elicit a wide range of biological activities through diverse mechanisms.^1,2 Among the indole containing heterocycles, the benzo[b]indole system is a carbazole motif that is widely present in a variety of compounds obtained from natural and commercial sources with anticancer and/or antibacterial properties (Fig. 1).^3–6 For example, Fujita et al. investigated carbazole-based hydrazone, HND-007 (1) and related compounds for their in vivo antitumor activity suppressing the growth of various cancer cell lines (IC₅₀ ∼ 1.3–4.6 μM).⁷ Pyridine fused carbazole derivative, S16020 (2) exhibited potent cytotoxic effects (IC₅₀ = 27.5 nM) by virtue of its DNA intercalative and topoisomerase inhibition properties.^8–10 Multidrug resistant tumor cell lines are sensitive to S16020 and it is currently being evaluated in the clinical stages.^11,12 Naturally occurring carbazole derivative, granulatimide (3) was isolated from the ascidian Didemnum granulatum and found to display anticancer and antibacterial activities.¹³ Caulfield and co-workers had patented carbazolyl chalcone 4 for its potential as a tubulin polymerization inhibitor (IC₅₀ = 2 μM).¹⁴ Further, carbazole sulfonamide (5), exhibited significant cytotoxicity against leukemia cells (IC₅₀ = 19 nM).¹⁵ Nakamura and co-workers reported the isolation of the carbazomycins A–F (6) from Streptoverticillium ehimense H 1051-MY 10 and these carbazole-based congeners were found to possess promising antibacterial and antifungal activities.^16–18 The current interest in carbazoles for clinical applications arises mainly due to their high efficiencies against several types of diseases, limited toxic side effects, and complete lack of hematological toxicity.¹⁹ In addition to interesting and useful biological applications, carbazole derivatives are also used as organic materials due to their photorefractive, photoconductive, whole transporting and light-emitting properties.²⁰

Fig. 1 Carbazole analogues as anticancer and antibacterial agents.

Glyoxamide is an important structural unit found in many biologically active compounds and synthetic drug candidates,^21,22 especially those with anticancer and antibacterial properties. For example, indibulin (7) destabilizes tubulin polymerization by arresting tumor cell growth at the G2/M phase. Indibulin is also active in multidrug resistant tumor cell lines and its oral formulation is currently being examined in clinical trials.²³ Structurally similar, conscinamides A–C (8) containing an indolic enamide fragment, were isolated from marine sponge Coscinoderma sp and found to display antitumor activity against a human prostate cancer cell line (IC₅₀ = 7.6 μg mL⁻¹)²⁴ and partial cytoprotection against HIV.²⁵ More recently, Singh et al. have described the synthesis of different bis(indole)glyoxamides 9 as potent antibacterial candidates.²⁶ In our continued efforts to find out biologically active indole-based molecules, recently, we have prepared α-cyano bis(indolyl)chalcones,²⁷ 2-arylamino-5-(3′-indolyl)-1,3,4-oxadi-azoles,²⁸ 5-(3′-indolyl)-1,3,4-thiadiazoles,²⁹ 2-arylamino-5-(3′-indolyl)-1,3,4-thiadiazoles³⁰ and indolyl-1,2,4-triazoles³¹ as potent anticancer agents. Very recently, we have identified 2-(3′-indolyl)-N-arylthiazole-4-carboxamides as antibacterial and anticancer agents.³² Encouraging anticancer and antibacterial activities of glyoxamides and pivotal roles of carbazole scaffold in bioactive compounds prompted us to investigate their new analogues. In this paper, we report a series of 24 carbazolyl glyoxamides 14a–x by incorporating important scaffolds, glyoxamide and carbazole in a single molecule (Fig. 2).

Fig. 2 Design of carbazolyl glyoxamides 14a–x.

Results and discussions

Synthesis and characterization

Carbazolyl glyoxamides 14a–x were synthesized from the reaction of carbazole 10 with ethyl chlorooxoacetate in the presence of anhydrous AlCl₃ followed by ester hydrolysis using LiOH to afford glyoxalic acid 13 in good yield (Scheme 1). For the coupling of 13 with aryl/heteroaryl amines, we optimized the reaction conditions by varying temperatures, solvents, and reagents as illustrated in Table 1. Initial efforts by using thionyl chloride or oxalyl chloride failed to produce 14a (Table 1, entries 1–2). Subsequent efforts by employing well known carbodiimides including DCC, CDI and EDCI·HCl as coupling reagents under conventional as well as microwave (MW) irradiation conditions generated 14a in low yield (40%, Table 1 entries 3–6). Finally, the reaction of 13a and aryl/heteroarylamines in the presence of O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexafluorophosphate (HATU) at 70 °C under MW irradiation led to 14a in 70% yield. Scope and generality of this developed protocol was further demonstrated by coupling glyoxalic acid 13 with various arylamines and a series of carbazolyl glyoxamides 14a–x was prepared in 70–91% yields. Structures of the newly synthesized glyoxamides were well characterized by using spectroscopic techniques including IR, NMR (¹H and ¹³C) and HRMS. HPLC analysis of synthesized carbazolyl glyoxamides 14a–x indicated the purity of all the compounds was greater than 97%. In IR spectra of 14a–x, characteristic peaks at ∼1690 and 1655 cm⁻¹ were assigned to the carbonyl and amide functionalities, respectively. Further, in ¹³C NMR spectra of 14a–x, the carbons of carbonyl and amide functionalities were resonated at ∼180 (CO) and ∼160 ppm (NHCO).

Scheme 1 Reagents and conditions: (a) CH₃I/C₂H₅I/4-ClC₆H₄CH₂Cl, KOH, DMF, rt, 13 h; (b) Cl(CO)₂OEt, AlCl₃, DCM, 0 °C to rt, 3 h; (c) LiOH, THF : H₂O (1 : 1), rt, 2 h; (d) R¹NH₂, HATU, DIPEA, DMF, 70 °C, 45 min, MW.

Table 1 Optimization of reaction conditions for the synthesis of carbazolyl glyoxamide 14a^a

Entry	Reagent	Solvent	Base	Temperature (°C)	Conventional heating		Microwave irradiation
					Time (h)	Yield (%)	Time (min)	Yield (%)
a RT: room temperature; NA: not attempted.
1	SOCl2	DCM	TEA	RT	12	Trace	NA	NA
2	(COCl)2	DCM	TEA	RT	15	Trace	NA	NA
3	DCC	THF	TEA	RT	30	Trace	NA	NA
4	CDI	THF	TEA	RT	20	Trace	NA	NA
5	EDCI·HCl/HOBt	THF	DIPEA	RT	14	30	60	40
6	EDCI·HCl/HOBt	DMF	TEA	70	12	Trace	60	Trace
7	HATU	THF	DIPEA	60	15	Trace	60	Trace
8	HATU	DMF	DIPEA	70	12	40	45	70
9	HATU	DMF	TEA	70	12	20	45	40

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Anticancer activity

Twenty four synthesized glyoxamides were evaluated for their anticancer activities towards human T lymphocyte (Jurkat), histiocytic lymphoma (U937), and breast (MCF-7 and MDA-MB-231) cancer cell lines (Table 2). Structure–activity relationship (SAR) studies of carbazolyl glyoxamides 14a–x were demonstrated by varying aryl/heteroarylamines and substitution at 9-NH position of carbazoles. Initial screening of compounds 14a–x at 10 μM concentration indicated that compounds 14i–m and 14q displayed about 50% cell survival (Table 2). N-Methyl and N-ethyl congeners of the carbazole were reported to possess significant cytotoxicity, therefore, derivatives 14r–x were synthesized.^7,15 Unfortunately, analogs 14r–x exhibited low activity against the tested tumor cell lines. The IC₅₀ values of selected carbazolyl glyoxamides 14i–m and 14q are summarized in Table 3. Compound 14i, bearing 4-chlorobenzyl and N,N′-dimethylaminophenyl moieties was found to show moderate activity. Cytotoxicity was retained by the replacement of a phenyl ring in 14a with heteroaryl moieties such as 6-quinolyl (14j) and 2-(5-methyl)thiazolyl (14k).

Table 2 In vitro cytotoxicity of carbazolyl glyoxamides 14a–x^a

Percentage cell survival (@10 μM)
Compound	R	R^1	Jurkat	U937	MCF-7	MDA-MB-231
a The activity data represent mean values ± SD of experiments conducted in triplicates at three independent times.
14a	4-ClC6H4CH2	C6H5	71.5 ± 10.7	88.5 ± 5.2	69.5 ± 7.8	82.3 ± 7.5
14b	4-ClC6H4CH2	4-CH3C6H4	70.2 ± 0.8	79.5 ± 3.4	67.1 ± 7.4	83.5 ± 6.8
14c	4-ClC6H4CH2	4-CH3OC6H4	71.3 ± 26.7	82.1 ± 2.2	74.6 ± 0.9	80.5 ± 5.5
14d	4-ClC6H4CH2	3-CH3OC6H4	71.3 ± 14.4	67.2 ± 8.5	70.9 ± 10.4	78.6 ± 6.8
14e	4-ClC6H4CH2	3,4-(CH3O)2C6H3	69.8 ± 14.7	85.5 ± 5.3	67.3 ± 4.6	72.5 ± 2.2
14f	4-ClC6H4CH2	3,4,5-(CH3O)3C6H2	64.5 ± 1.4	98.8 ± 2.9	73.7 ± 19.9	70.9 ± 5.2
14g	4-ClC6H4CH2	4-FC6H4	118.4 ± 26.4	83.2 ± 6.7	93.3 ± 5.6	88.9 ± 6.4
14h	4-ClC6H4CH2	4-Pyridyl	83.1 ± 19.6	59.1 ± 14.8	66.3 ± 5.4	90.2 ± 2.2
14i	4-ClC6H4CH2	4-(CH3)2N,N-C6H4	56.5 ± 6.9	78.8 ± 1.2	70.5 ± 5.8	62.8 ± 3.5
14j	4-ClC6H4CH2	6-Quinolyl	56.5 ± 1.6	60.3 ± 17.4	68.0 ± 7.0	63.5 ± 2.9
14k	4-ClC6H4CH2	2-(5-Methyl)thiazolyl	51.3 ± 0.5	62.2 ± 5.8	58.4 ± 6.6	64.8 ± 1.9
14l	H	C6H5	83.4 ± 41.0	64.4 ± 8.7	48.5 ± 4.0	75.2 ± 2.4
14m	H	3,4,5-(CH3O)3C6H2	52.4 ± 5.9	67.4 ± 5.8	59.1 ± 13.3	61.3 ± 2.0
14n	H	3-CH3OC6H4	60.7 ± 0.7	72.1 ± 6.9	59.8 ± 19.9	72.6 ± 8.3
14o	H	4-CH3OC6H4	77.9 ± 9.4	85.6 ± 6.5	89.4 ± 17.3	84.9 ± 10.5
14p	H	4-Pyridyl	94.9 ± 6.3	92.8 ± 5.2	60.5 ± 10.6	98.2 ± 5.4
14q	H	3,4-(CH3O)2C6H3	60.5 ± 6.2	65.8 ± 8.5	50.0 ± 7.3	65.2 ± 4.5
14r	CH3	C6H5	95.31 ± 7.2	109.7 ± 10.4	104.2 ± 6.7	94.7 ± 6.2
14s	CH3	3,4,5-(CH3O)3C6H2	99.77 ± 6.6	107.5 ± 9.3	72.1 ± 5.4	92.3 ± 3.9
14t	C2H5	C6H5	93.7 ± 5.3	91.3 ± 10.9	114.2 ± 11.3	94.5 ± 5.2
14u	C2H5	3,4-(CH3O)2C6H3	92.3 ± 2.2	104.7 ± 9.5	106.9 ± 12.1	96.5 ± 6.6
14v	C2H5	3,4,5-(CH3O)3C6H2	100.4 ± 10.8	106.9 ± 7.4	78.1 ± 8.8	88.8 ± 7.1
14w	C2H5	4-(CH3)2N,N-C6H4	102.3 ± 9.7	93.6 ± 4.2	79.7 ± 7.6	89.9 ± 9.2
14x	C2H5	6-Quinolyl	100.4 ± 10.8	95.7 ± 7.2	73.8 ± 8.5	83.9 ± 8.9
Control (negative)			100 ± 5.9	101.5 ± 9.5	100 ± 3.2	100 ± 2.2
Doxorubicin (positive)			21.2 ± 5.6	31.8 ± 1.1	38.2 ± 10.6	40.2 ± 3.5

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Table 3 IC₅₀ values (μM) of selected carbazolyl glyoxamides^a

S. No	Jurkat	U937	MCF-7	MDA-MB-231
a Bold value indicates IC50 > 10 μM; DX-1 = doxorubicin.
14i	10.5 ± 2.1	29.2 ± 3.9	23.5 ± 8.2	17.9 ± 5.7
14j	11.3 ± 3.6	15.1 ± 4.8	18.9 ± 3.8	18.7 ± 7.7
14k	10.2 ± 2.9	17.5 ± 8.2	12.2 ± 5.4	20.1 ± 8.4
14l	11.8 ± 3.3	18.3 ± 7.8	9.3 ± 4.3	31.2 ± 11.4
14m	12.1 ± 1.8	23.3 ± 10	11.5 ± 5.5	14.7 ± 5.4
14q	17.8 ± 3.2	29.2 ± 9.2	9.8 ± 2.8	18.5 ± 5.4
DX-1	0.25 ± 0.11	0.15 ± 0.05	0.35 ± 0.15	0.5 ± 0.20

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Compound 14l with N–H free carbazole and N-phenyl glyoxamide was identified as the most active member of the series with IC₅₀ values between 9.3 to 31.2 μM. Also, compound 14l was found to be 2–3 fold more cytotoxic towards MCF-7 (IC₅₀ = 9.3 μM) and Jurkat (IC₅₀ = 11.8 μM) cells. No significant change in activity was observed by the replacement of a phenyl group in compound 14l with a trimethoxyphenyl (14m) and dimethoxyphenyl (14q) groups except for 14q (IC₅₀ = 9.8 μM; MCF-7). However, compound 14q was found to be equipotent (compounds 14l vs. 14q) against MCF-7 cell line (IC₅₀ = 9.8 μM). From the structural variation it was realized that carbazole–N–H with its appended glyoxamide bearing phenyl, dimethoxyphenyl and trimethoxyphenyl units increases cytotoxicity.

To further characterize the mode of cellular death by carbazolyl glyoxamides, apoptosis induction studies for the selected compounds 14i–m and 14q were performed on Jurkat cells by the caspases 3/7 activation method. Caspases belonging to a family of cysteine proteases are known to play an essential role in apoptosis.³³ Out of these caspases, caspase-3 is an effector caspase that cleaves multiple proteins in cells leading to apoptotic cell death. Therefore, activation of caspase 3 pathway is a hallmark of apoptosis and can be used in cellular assay to quantify activator. Of the carbazolyl glyoxamides tested, compounds 14i, 14k, 14l and 14q showed 4–5-fold enhancement in caspase level compared to the control (Fig. 3). These results imply that carbazolyl glyoxamides induced apoptosis in Jurkat cells via caspase-3-dependent pathway.

Fig. 3 Carbazolyl glyoxamides 14i–m and 14q induced caspase activation in Jurkat cells.

Antibacterial activity

In light of interesting antibacterial activities of many carbazole containing natural and synthetic compounds, the newly synthesized carbazolyl glyoxamides 14a–x were screened for their antibacterial activity.²⁷ All the compounds were tested for their in vitro antibacterial activities against Gram-positive bacteria including Staphylococcus aureus (MTCC 96) and Bacillus subtilis (MTCC 121), and Gram-negative bacteria including Escherichia coli (MTCC 1652) and Pseudomonas putida (MTCC102) with respect to chloramphenicol, a standard drug. The Minimum Inhibitory Concentrations (MICs) and zone of inhibition (ZOI) for compounds 14a–x were determined in vitro by the modified broth micro-dilution values method as given in Table 4. Compounds containing N-chlorobenzyl carbazole and C₆H₅ (14a), CH₃C₆H₄ (14b) CH₃OC₆H₄ (14c), (CH₃O)₃C₆H₂ (14f) and N,N′-(CH₃)₂C₆H₄ (14i) substituents in the glyoxamide fragment were found to display moderate activity against the tested bacterial strains. Interestingly, introduction of an electron-withdrawing fluoro group in the phenyl ring resulted in 14g endowed with potent antibacterial activity against all tested bacterial strains with MIC values ranging between 8 and 16 μg mL⁻¹. Replacement of a N-phenyl ring in 14a with heteroaryl groups such as 4-pyridyl (14h) and 6-quinolyl (14j) led to inactive derivatives; except 14k bearing 2-(5-methyl)thiazolyl moiety exhibited comparable antibacterial activity against B. cereus and E. coli bacterial strains (ZOI = 16–19 mm; MIC = 16 μg mL⁻¹). Compound 14l with carbazole N–H and a phenyl moiety on the amide part displayed improved activity when compared to the corresponding N-substituted carbazole 14a. Replacement of a phenyl ring in 14l by methoxyphenyl (14m–o and 14q) or 4-pyridyl (14p) moiety led to a decrease in the activity except for 14n (MIC = 8–16 μg mL⁻¹). Alkylations of carbazole N–H resulted in compounds 14r–x with moderate activity against the tested bacterial strains.

Table 4 In vitro antibacterial activities of carbazolyl glyoxamides 14a–x^a

Compound	R	R^1	Gram-positive bacteria				Gram-negative bacteria
			B. cereus		S. aureus		E. coli		P. putida
			ZOI	MIC	ZOI	MIC	ZOI	MIC	ZOI	MIC
a ZOI (in mm) and MIC (in μg mL^−1) values, bold values indicate comparable or even better antibacterial activity than chloramphenicol.
14a	4-ClC6H4CH2	C6H5	16	32	16	>32	14	64	15	32
14b	4-ClC6H4CH2	4-CH3C6H4	15	>32	—	—	16	64	—	—
14c	4-ClC6H4CH2	4-CH3OC6H4	16	32	15	32	17	>16	15	>16
14d	4-ClC6H4CH2	3-CH3OC6H4	17	16	—	—	14	>16	—	—
14e	4-ClC6H4CH2	3,4-(CH3O)2C6H3	15	>64	17	64	15	32	16	>16
14f	4-ClC6H4CH2	3,4,5-(CH3O)3C6H2	15	>32	17	>64	18	32	16	32
14g	4-ClC6H4CH2	4-FC6H4	18	16	18	>8	20	8	19	16
14h	4-ClC6H4CH2	4-pyridyl	15	128	17	>32	16	32	16	32
14i	4-ClC6H4CH2	4-(CH3)2N,N-C6H4	16	64	18	>32	14	32	16	64
14j	4-ClC6H4CH2	6-Quinolyl	15	>32	16	>128	15	32	18	64
14k	4-ClC6H4CH2	2-(5-Methyl)thiazolyl	16	16	19	>8	19	16	18	>16
14l	H	C6H5	17	16	18	>8	19	>8	17	8
14m	H	3,4,5-(CH3O)3C6H2	15	>16	17	>16	16	32	16	64
14n	H	3-CH3OC6H4	18	>8	19	16	16	16	18	8
14o	H	4-CH3OC6H4	14	64	14	>64	17	128	15	>64
14p	H	4-Pyridyl	15	32	16	>64	18	32	16	>32
14q	H	3,4-(CH3O)2C6H3	16	128	18	>32	17	>16	14	>64
14r	CH3	C6H5	13	>32	13	64	15	32	14	32
14s	CH3	3,4,5-(CH3O)3C6H2	14	64	13	32	17	32	15	>32
14t	C2H5	C6H5	15	>32	14	64	16	>16	17	>16
14u	C2H5	3,4-(CH3O)2C6H3	14	64	—	—	16	>32	16	>16
14v	C2H5	3,4,5-(CH3O)3C6H2	15	>16	15	>32	14	>64	—	—
14w	C2H5	4-(CH3)2N,N-C6H4	15	>32	14	>64	16	32	16	32
14x	C2H5	6-Quinolyl	12	32	14	32	13	>32	14	64
Chloramphenicol			21	32	21	16	22	16	21	16

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Cell viability assay

Although antibacterial (ZOI & MIC) assays might show a potential to kill pathogenic micro-organisms, concentration vs. time curves provide more insight about the rate of antibacterial activity. To determine the rate of bactericidal activity of the two most active compounds 14g and 14l, time-kill studies were performed. The interactive time and the change in the number of microorganisms for compounds 14g and 14l is presented in Fig. 4A–D.

Fig. 4 Cell viability assay (A–D) of 14g and 14l against selected bacteria.

The percent population reduction at different time interval was calculated to demonstrate the change of the population of microorganism respect to concentration dependent dose. The compound 14g showed high bacteriostatic effect against E. coli in the first hours of incubation.

Moreover, the highest reduction (85%) in E. coli population was observed at 8 h of incubation with compound 14g (Fig. 4A). Compound 14g was also showed about >70% reduction in viable cells of S. aureus (Fig. 4B). Compound 14l with free carbazole N–H and a phenyl moiety in amidic part led to significant inhibition of bacterial growth after 4 h of incubation. It is evident that within 8 h, compound 14l exhibited almost 80% reduction in the viability of P. putida (Fig. 4C). Similarly, bacteriostatic effect of compound 14l against S. aureus reveals about 70% reduction of viable cell at 8 h of incubation (Fig. 4D). Thus the results of present study revealed that 14g and 14l were capable of inhibiting the bacterial growth within few hours of initial interactions.

The toxicity of potent compounds 14i–m and 14q was evaluated using LDH (lactate dehydrogenase) assay. The LDH activity shows that all the tested compounds 14i–m and 14q exhibited lower toxicity than the standard drug, doxorubicin which justifies the potential use of these compounds as anti-bacterial agents (Fig. 5).

Fig. 5 Cytotoxicity induced by carbazolyl glyoxamides in terms of LDH release.

From the structure–activity relationship (SAR) studies of carbazolyl glyoxamides it implies that a combination of carbazole N–H and glyoxamide unit possessing p-fluorophenyl and methoxyphenyl substituents are beneficial for the activity (Fig. 6). All the synthesized compounds demonstrated well to moderate cytotoxicity against a panel of cancer cell lines and excellent to moderate antibacterial activity towards tested bacterial strains. Particularly, compound 14l with carbazole N–H and phenyl moiety in glyoxamide part exhibited potent cytotoxicity and antibacterial activities. Exclusively, with carbazole N–H and dimethoxyphenyl moieties, compound 14q was found to be the most active against the tested cancer cell lines and less potent towards tested bacterial strains. However, compound 14g with N-chlorobenzylcarbazole and fluorophenyl units, and analogue 14n having carbazole N–H and 3-methoxyphenyl moieties, were the most active carbazolyl glyoxamides against the tested bacteria but exhibited low cytotoxicity towards tested cancer cell lines.

Fig. 6 Structure–activity relationship of carbazolyl glyoxamides 14a–x.

Conclusions

In summary, we have synthesized various carbazolyl glyoxamides from readily available glyoxalic acids 13 and arylamines by employing HATU as a coupling reagent. Synthesized glyoxamides were assessed for their cytotoxicity which enabled us to identify 14l and 14q as the most potent compounds against MCF-7 cells with IC₅₀ values of 9.3 μM and 9.8 μM, respectively. Preliminary mechanism of action studies indicated that carbazolyl glyoxamides induced apoptosis in Jurkat cells via caspase-3 and -7 activation. In addition, antibacterial activity evaluation led us to compounds 14g, 14k, l and 14n with significant potency against Gram-positive and Gram-negative bacteria (MIC = 8–16 μg mL⁻¹ and ZOI = 16–20 mm). Antibacterial activities of potent compounds 14g, 14k, l and 14n were found to be comparable to the reference drug, chloramphenicol. Cell viability assay revealed that analogues 14g and 14l were capable of inhibiting the bacterial growth within few hours of initial interactions. Interesting activity results indicate that the identified potent carbazolyl glyoxamides 14g and 14l can be exploited further to develop either highly specific or potent antibacterial/anticancer agents, or if required both of these properties could be incorporated in the same molecule.

Experimental

General procedure for the synthesis of carbazolyl glyoxamides (14a–x)

To a 10 mL microwave tube was added carbazole glyoxalic acid 13 (0.275 mmol), HATU (0.12 g, 0.317 mmol), N,N-diisopropylethyl-amine (0.09 g, 0.687 mmol) and an appropriate aryl/heteroarylamine (0.303 mmol) in DMF (2 mL). The tube was sealed with a pressure cap and placed in the microwave cavity. The sample was irradiated for 45 min at 70 °C and then allowed to cool at room temperature. The residue was poured into ice-cold water (30 mL) and stirred for 20 min at room temperature. The solid so obtained was filtered, dried and purified by column chromatography on silica gel using ethylacetate : hexane (3 : 7) as eluent to give pure 14a–g, 14j–o and 14q–x in excellent yields. Some of the compounds (14h, i and 14p) were crystallized from acetone to obtain pure products in 70–91% yields.

2-(9-(4-Chlorobenzyl)-9H-carbazol-3-yl)-2-oxo-N-phenyl-acetamide (14a)

Yellow solid; yield 70%; mp: 191–193 °C; ¹H NMR (400 MHz, CDCl₃) δ 9.47 (d, J = 1.3 Hz, 1H), 9.15 (s, 1H), 8.58 (dd, J = 8.8, 1.7 Hz, 1H), 8.25 (dd, J = 7.6, 1.1 Hz, 1H), 7.78 (dd, J = 8.6, 1.0 Hz, 2H), 7.54–7.50 (m, 1H), 7.45 (dd, J = 8.3, 7.5 Hz, 3H), 7.41–7.36 (m, 3H), 7.26 (t, J = 2.6 Hz, 2H), 7.08 (d, J = 8.6 Hz, 2H), 5.55 (s, 2H); ¹³C NMR (101 MHz, CDCl₃) δ 185.9, 160.0, 144.2, 141.2, 136.9, 134.6, 133.7, 129.8, 129.3, 129.2, 127.7, 127.0, 126.2, 125.2, 125.0, 123.6, 123.1, 121.1, 121.1, 120.0, 109.5, 108.8, 46.24; IR (KBr, ν, cm⁻¹): 3335, 3090, 3052, 2916, 1682, 1653, 1589, 1520, 1435, 1307, 1250, 1134, 1011, 825, 795; anal. RP-HPLC t_R = 4.641 min, purity 98.55%; HRMS (ESI⁺) calculated for C₂₇H₂₀ClN₂O₂ [M + H]⁺, 439.1213; found 439.1208 and 461.1025 [M + Na]⁺.

2-(9-(4-Chlorobenzyl)-9H-carbazol-3-yl)-2-oxo-N-(p-tolyl) acetamide (14b)

Yellow solid; yield 72%; mp: 172–173 °C; ¹H NMR (400 MHz, CDCl₃) δ 9.48 (d, J = 1.6 Hz, 1H), 9.09 (s, 1H), 8.58 (dd, J = 8.8, 1.7 Hz, 1H), 8.27–8.23 (m, 1H), 7.67–7.64 (m, 2H), 7.53–7.49 (m, 1H), 7.44–7.35 (m, 3H), 7.26 (t, J = 2.2 Hz, 3H), 7.24 (s, 1H), 7.09 (d, J = 8.6 Hz, 2H), 5.55 (s, 2H), 2.39 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 186.0, 159.9, 144.2, 141.1, 134.9, 134.6, 134.3, 133.7, 129.9, 129.7, 129.2, 127.7, 126.9, 126.1, 125.1, 123.6, 123.1, 121.1, 121.0, 119.9, 109.5, 108.7, 46.2, 21.0; IR (KBr, ν, cm⁻¹): 3325, 3094, 3055, 2916, 1682, 1651, 1620, 1582, 1520, 1443, 1327, 1265, 1149; anal. RP-HPLC t_R = 5.317 min, purity 98.10%; HRMS (ESI⁺) calculated for C₂₈H₂₁ClN₂O₂ [M + H]⁺, 453.1369; found 453.1365 and 475.1183 [M + Na]⁺.

2-(9-(4-Chlorobenzyl)-9H-carbazol-3-yl)-N-(4-methoxyphenyl)-2-oxoacetamide (14c)

Yellow solid; yield 72%; mp: 168–170 °C; ¹H NMR (400 MHz, CDCl₃) δ 9.45 (d, J = 1.5 Hz, 1H), 9.04 (s, 1H), 8.56 (dd, J = 8.8, 1.7 Hz, 1H), 8.22 (d, J = 7.7 Hz, 1H), 7.69–7.65 (m, 2H), 7.51–7.47 (m, 1H), 7.42–7.32 (m, 4H), 7.25–7.23 (m, 1H), 7.06 (d, J = 8.5 Hz, 2H), 6.97–6.94 (m, 2H), 5.52 (s, 2H), 3.84 (s, 3H); ¹³C NMR (101 MHz, CDCl₃) δ 186.1, 159.8, 157.0, 144.2, 141.1, 134.6, 133.7, 130.1, 129.7, 129.2, 127.7, 126.9, 126.1, 125.1, 123.6, 123.1, 121.6, 121.1, 121.0, 114.4, 109.5, 108.7, 55.5, 46.2; IR (KBr, ν, cm⁻¹): 3348, 3055, 2924, 1682, 1643, 1620, 1582, 1528, 1443, 1327, 1250, 1149; anal. RP-HPLC t_R = 4.368 min, purity 98.67%; HRMS (ESI⁺) calculated for C₂₈H₂₂ClN₂O₃ [M + H]⁺, 469.1319; found 469.1310 and 491.1129 [M + Na]⁺.

Acknowledgements

The authors acknowledge the financial support received from DBT (No. BCIL/NER-BPMC/2012/1549) New Delhi, India. Dr Rachna Sadana thanks to University of Houston-Downtown (UHD) for providing organized research and creative activity (ORCA) funds.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27175d

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