INTRODUCTION:

Carbazole derivatives are well known for their pharmacological activities. It is evident from the literature that the derivatives of carbazole moiety possess a wide spectrum of pharmacological activities, such as antibacterial^1–3, antifungal^4,5, anti-tumour, antineoplastic^6–10, anti-convulsant¹¹, anti-oxidant¹², anti-diabetic¹³, anti-psychotic¹⁴ and larvicidal activity¹⁵.

Various hetero-annulated carbazole derivatives have drawn attention because of their natural occurrence and the broad spectrum of biological activity associated with these compounds. The carbazole moiety is a frequent moiety of numerous drugs, such as olivacine, ondansetron, rimcazole, stauroapirone, carbazolol, carvedilol, carprofen, cacotheline, rebaccamycin, ellipticine and various naturally occurring carbazole alkaloids¹⁶.

Carbazole derivatives have documented consistent advances in the design of novel antipsychotic, neuroleptic and anticonvulsant agents¹⁷. Furthermore, various congeners of oxadiazole, thiadiazole, azetidinone and thiazolidinone have also been reported to exhibit potential antimicrobial, anticancer, antipsychotic, antidepressant and anticonvulsant activity^18–21.

In view of broad biological activity of carbazole derivatives, in this study it was planned to synthesize novel carbazole derivatives and by incorporation of new pharmacophores, such as oxadiazole at position 9 of carbazole nucleus, with the hope of obtaining better pharmacologically active drugs as anticancer and antimicrobial agents. In the same direction, a one pot method was developed to synthesize a series of 5-[(9H-carbazol-9-yl) methyl]-N-[(substituted--phenyl) (piperazin-1-yl) methyl]-1,3,4-oxadiazol-2-amine derivatives (4a–o) as Mannich bases.

Experimental:

The purity of all the newly synthesized compounds were checked by TLC on silica gel- G protected aluminium sheets (type 60 F₂₅₄, Merck) and the spots were detected by exposure to iodine vapour and a UV-lamp at λ₂₅₄ nm. The melting points were determined in open capillary tubes and are uncorrected. The infrared (FT-IR) spectra were recorded on a 470S-Shimadzu infrared spectrophotometer using the KBr pressed pellet technique and the result are expressed in cm^-1. The ¹H- and ¹³C-NMR spectra were recorded on a Bruker DRX-300 instrument using CDCl₃ as a solvent. The chemical shifts, δ, are given in ppm downfield from the internal standard tetramethylsilane (TMS). The splitting patterns are designated as follows; s: singlet; d: doublet and m: multiplet. The mass spectra were obtained on a Shimadzu 2010A LC-MS spectrometer. Elemental analyses were realised on an Elemental Vario EL III Carlo Erba 1108 instrument and the obtained values were within ±0.04% of the theoretical values.

Synthesis of ethyl 2-(9H-carbazol-9-yl) acetate (1)

To a solution of carbazole (2.01g, 0.012mol) in 18mL of dry acetone, ethyl chloroacetate (1.472g, 0.012 mol) was added drop wise in the presence of anhydrous potassium carbonate (0.09g) and the resultant mixture refluxed for 22 h. Then the mixture was cooled and the thus obtained solid was filtered, dried and recrystallized from methanol to give compound 1.¹⁷ Yield: 59.44 %; m.p.: 240-241°C.

Synthesis of 1-[2-(9H-carbazol-9-yl) acetyl]semicarbazide (2)

Compound 1 (2.53g, 0.01mol) was dissolved in 90mL of acetone (solution A) and semicarbazide (0.62g, 0.01 mol) was dissolved in 20mL of water (solution B). Solution B was poured into solution A and the mixture was refluxed for 28 h. On cooling, the solid product that separated out was filtered, dried and recrystallized from methanol to give compound 2.¹⁷ Yield: 70.40 %; m.p.: 184–185°C.

Synthesis of 5-[(9H-carbazol-9-yl) methyl]-1, 3, 4-oxadiazol-2-amine (3)

A solution of compound 2 (2.83g, 0.01mol) with 25mL of conc. H₂SO₄ was kept overnight at room temperature, then the reaction mixture was poured into ice-cold water, neutralized with ammonia and extracted with ether. The ethereal solution was distilled off and the product obtained was recrystallized from acetone to give compound 3.17 Yield: 65.9%; m.p.: 161–162°C.

General synthetic procedure for the synthesis of 5-[(9H-carbazol-9-yl) methyl]-N-[(substituted

Phenyl) (piperazin-1-yl) methyl]-1, 3, 4-oxadiazol-2-amines (4a–o)

A mixture of equimolar quantity (0.01mol) of compound (3), derivative of aromatic aldehydes and piperazine along with few drops of glacial acetic acid was refluxed in 15mL of methanol for 8–13 h. The hot solution was poured onto crushed ice and the thus obtained solid mass was filtered, dried and recrystallized from acetone to give compounds 4a–o.

Scheme 1: Synthesis of Carbazole Derivatives 4a-o

Anti-epileptic activity (MES method):

The anti-epileptic activity was carried out by maximal electrical shock induced convulsion method¹⁰. Adult albino mice of either sex weighing of 20 – 30g were used for the study. Mice were treated with newly synthesized carbazoles (20mg/kg, ip) and standard drug, phenobarbitone (20mg/kg, ip). After 30 min, the animals were subjected to electro shock through ear electrodes of 150mA for 0.2 sec by electro convulsiometer and the presence and absence of extensor response was noted and duration of time was analyzed statistically by students “t” test⁹ and expressed in mean±SEM and tabulated in Table 2.

RESULTS AND DISCUSSION:

The reaction sequences leading to different carbazole derivatives are outlined in Scheme 1. The carbazole derivatives were synthesized by incorporating new pharmacophores, such as oxadiazole, at position 9 of the carbazole nucleus by a conventional method in which carbazole was taken as a starting material to produce the corresponding 5-[(9H-carbazol-9-yl)methyl]-N-[(substituted-phenyl)(piperazin-1-yl)methyl]-1,3,4-oxadiazol-2-amines (4a–o). Synthesized compounds were identified based on their physical parameters, i.e., solubility, melting point, chromatographic methods (TLC). The data is given in Table I and spectroscopic methods (UV, IR, ¹H-NMR, MS and elemental analysis are given in the Supplementary material to this paper. The ¹H-NMR spectra showed a peak between δ 6.00–7.00 ppm, which was assigned to the N–H (aliphatic) proton. A peak characteristic of –CH– appeared between δ 4.00–5.00 ppm. The peaks at δ 6.00–8.00 ppm showed the presence of aromatic protons. In the FT-IR spectra, a peak characteristic of N–H (aliphatic) appeared at 3200–3450 cm–1 and a peak for C=N was observed at 1500–1600 cm^–1.

Compound 4b shows highly significant anti-epileptic activity due to the substitution with 2-(2, 3-dimethylphenylamino) benzoic acid. Compounds 4c, 4d, 4e and 4f are devoid from anti-epileptic activity. The maximum anti-nociceptive and anti-epileptic activities were observed in the animals administered with 20 mg/kg body weight of the synthesized compounds as well as in those animals, which received pentazocine (5 mg/kg) and phenytoin (20 mg/kg), respectively.

Table 1: Physical Data of the Synthesized Compounds 4a-o

Compound	R	Yield, %	Reaction time, h	M. p. range, °C	R_fvalue
4a	p-Nitro	82.55	4.0	236-237	0.73
4b	p-Hydroxy	79.69	5.0	208-209	0.62
4c	3,4,5-Trimethoxy	61.5	4.3	184-185	0.69
4d	p-Chloro	73.22	6.0	221-222	0.75
4e	p-(Dimethylamino)	53.78	5.5	202-203	0.59
4f	o-Nitro	63.96	4.4	225-226	0.65
4g	m-Hydroxy	72.71	5.2	213-214	0.56
4h	m-Nitro	69.98	6.8	229-230	0.71
4i	o-Hydroxy	59.33	5.0	218-219	0.60
4j	m-Chloro	67.07	4.4	231-232	0.67
4k	o-Chloro	56.45	6.2	237-238	0.66
4l	m-Methoxy	52.54	4.5	192-193	0.77
4m	p-Methoxy	58.75	7.0	189-190	0.79
4n	p-Fluoro	55.0	6.0	234-235	0.54
4o	H	51.23	5.2	176-177	0.63

Table 2: Anti-epileptic Evaluation of Newly Synthesized Carbazole Derivatives

Treatment	Duration (sec) (Mean ± SEM)			Recovery/ Death
Treatment	Extensor	Clonus	Stupor	Recovery/ Death
Control	31.50 ± 0.9220	22.66 ±1.7638	69.33 ±3.2830	Recovery
Phenytoin (Standard)	16.667 ±1.0541**	9.50 ±0.9574**	24.66 ±2.1551**	Recovery
4a	29.66 ±0.8819	20.33 ±0.9819	42.83 ±1.9221	Recovery
4b	22.66 ±1.1738*	15.33 ±1.1293*	28.00 ±2.7203*	Recovery
4c	26.83 ±0.8724*	15.00 ±0.7746*	34.66 ±2.5517*	Recovery
4d	37.00 ±1.2383	21.33 ±0.8028	38.00 ±2.4631	Recovery
4e	31.50 ±0.7188	22.83 ±1.4701	41.33 ±2.8597	Recovery
4f	17.00 ±2.4766**	17.16 ±1.1377**	35.00 ±2.3523**	Recovery
4g	25.00 ±2.3094*	12.00 ±1.0646*	23.50 ±2.9183*	Recovery
4h	29.83 ±0.8333	21.83 ±1.7208	63.16 ±1.4926	Recovery
4i	36.23 ±1.2083	20.12 ±0.8092	35.00 ±1.4531	Recovery
4j	32.20 ±0.7092	21.15 ±1.1723	42.12 ±1.0892	Recovery
4k	18.12 ±2.1756**	16.12 ±1.1255**	32.25 ±2.1353**	Recovery
4l	24.00 ±2.3094*	12.00 ±1.0646*	23.50 ±2.9183*	Recovery
4m	29.83 ±0.8333	21.83 ±1.7208	63.16 ±1.4926	Recovery
4n	37.00 ±1.2383	21.33 ±0.8028	38.00 ±2.4631	Recovery
4o	31.50 ±0.7188	22.83 ±1.4701	41.33 ±2.8597	Recovery

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Received on 06.02.2022 Modified on 21.02.2022

Asian J. Research Chem. 2022; 15(2):163-165.

DOI: 10.52711/0974-4150.2022.00027