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. 2020 Jan 8;25(2):255. doi: 10.3390/molecules25020255

One-Pot Multicomponent Synthesis and Cytotoxic Evaluation of Novel 7-Substituted-5-(1H-Indol-3-yl)Tetrazolo[1,5-a] Pyrimidine-6-Carbonitrile

Mohamed A A Radwan 1,2,*, Fahad M Alminderej 1, Hanem M Awad 3
Editors: David Díez, María Ángeles Castro
PMCID: PMC7024365  PMID: 31936309

Abstract

A series of novel 7-substituted-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile was synthesized via a one-pot, three-multicomponent reaction of appropriate aldehydes, 1H-tetrazole-5-amine and 3-cyanoacetyl indole in catalytic triethylamine. The cytotoxic activity of the new synthesized tetrazolopyrimidine-6-carbonitrile compounds was investigated against HCT-116, MCF-7, MDA-MB-231, A549 human cancer cell lines and one human healthy normal cell line (RPE-1) using the MTT cytotoxicity assay. Compounds 4h, 4b, 4c, 4i and 4a showed potent anticancer activities against human colon cancer. Additionally, all the compounds showed potent anticancer activities on human lung cancer.

Keywords: indole, tetrazole, pyrimidine, multicomponent reaction (MCRs), cytotoxicity, HCT-116, MCF-7, MDA-MB-231, A549

1. Introduction

Indole Scaffold is one of the most extensively distributed heterocycles in nature which are widely used in drug development. The indole ring is a subunit of many natural products and medicines with extensive pharmaceutical applications [1]. Various indole-constructed heterocyclic compounds gained much attention owing to their miscellaneous biological activities, including antimicrobial, antiviral and anticancer pharmacological properties [2,3,4,5]. Within such a scaffold, tetrazole [6] and pyrimidine [7] moieties, are well-explored as a key substructure motif of several medicinal molecules as they display many biological activities such as antimicrobial [8,9], antifungal [10,11], antimalarial [12,13], anticancer [14,15], antihypertensive [16] and anti-tuberculosis. [17,18,19,20,21]. The current investigation is directed to both scaffolds, that is, tetrazole and pyrimidine moieties which could have a potential synergistic effect. This event is conducted through the general synthetic framework called Multicomponent Reactions (MCRs). In these multicomponent reactions (MCRs), three components or more interact and convert into many target products by the one-pot method [22,23,24,25,26,27]. This method is advantageous relative to mainstream synthetic frameworks in easy set-up, cost-effectiveness and high yield. Recently, several MCRs have been used effectively to build nitrogen-heterocyclic compounds especially the indole substructure that possesses distinct pharmacological and biological activities [28,29,30,31,32,33,34,35,36,37,38,39].

3-Substituted indole is a nitrogen-heterocyclic compound that is predominant, and is very vital in therapeutic interactions [30,40]. Molecules with these constructions have many bioactivities, as anti-tumor [31], anti-inflammatory, reducing hypoglycemic, analgesic and antipyretic effects [30]. In continuation with our previous investigation that focused on the synthesis of bioactive indolyl-azines and azoles [41,42,43,44,45,46] (Figure 1), here we report the synthesis and cytotoxic evaluation of a series of new 7-substituted-5-(1H-indol-3-yl)tetrazolo[1,5-a] pyrimidine-6-carbonitrile in respectable products through MCRs reaction of 1H-tetrazole-5-amine, appropriate aldehydes and 3-cyanoacetyl indole (Scheme 1).

Figure 1.

Figure 1

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Activity of meridianin D analogs against MCF7.

Scheme 1.

Scheme 1

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Synthesis of 7-substituted-5-(1H-indol-3-yl)tetrazolopyrimidine-6-carbonitrile.

2. Results and Discussion

2.1. Chemistry

In the current submission, a series of novel 7-substituted-5-(1H-indol-3-yl)tetrazolopyrimidine-6-carbonitrile was established via the multicomponent reaction of appropriate aldehydes 1, 1H-tetrazole-5-amine 2 and 3-cyanoacetyl indole 3 as preliminary compounds (Scheme 1 and Figure 2).

Figure 2.

Figure 2

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Multi-component synthesis of 7-substituted-5-(1H-indol-3-yl)tetrazolopyrimidine-6-carbonitrile. Reaction conditions: aldehyde 1 (1 mmol), 1H-tetrazole-5-amine 2 (1 mmol), 3-cyanoacetyl indole 3 (1 mmol), Et3N (0.25 mmol), in dimethylformamide (DMF) at 120 °C, and about 10 h reaction time.

To improve the reaction condition, the classical MCRs were carried out in different solvents and alkali at an appropriate temperature. The MCRs were mainly performed in ethanol with no catalyst, which furnishes no products, even though it was refluxed for 48 h (entry 1, Table 1). Next, we have done the MCRs in catalytic Et3N in ethanol, acetonitrile, 1,4-dioxane, toluene and dimethylformamide (DMF) (entries 2–6, Table 1). Concerning the classification of the solvent, the greatest products are attained through DMF. Subsequently, we examined the MCRs in the existence of basic catalysts, for example, piperidine, 4-(dimethylamino)pyridine (DMAP), pyridine, KOH, NaOH and K2CO3 (entries 7–12, Table 1).

Table 1.

Optimization of the multicomponent reactions (MCRs) between 4-choloro benzaldehyde, 1H-tetrazole-5-amine, and 3-cyanoacetyl indole.

Entry Base (mmol) Solvent Temp. (°C) Time (h) Yield (%)
1 - EtOH 80 10 -
2 Et3N EtOH 80 10 Trace
3 Et3N CH3CN 80 10 12
4 Et3N 1,4-Dioxane 80 10 15
5 Et3N Toluene 120 10 -
6 Et3N DMF 120 10 76
7 Piperidine 0.5 DMF 120 10 65
8 DMAP 0.2 DMF 120 10 40
9 Pyridine DMF 120 10 35
10 KOH CH3CN 80 8 22
11 NaOH CH3CN 80 8 27
12 K2CO3 CH3CN 80 8 Trace
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Products exhibited that organic bases had a respectable catalytic result, but inorganic strong alkali (KOH, NaOH) released the less catalytic outcome. However, the inorganic weak base (K2CO3) could not supply target products. Finally, the best situation was found in this technique in DMF as a solvent, and 25 mol % triethylamine as a catalyst at 120 °C, for 10 h (Table 1). The results were recorded in Table 1.

The construction of targets 4a–i were recognized by the study of spectral data and mass analysis (Experimental part) as characterized for structure 4a: the IR of 4a unveiled absorptions at 3397 and 2223 cm−1 representative –NH– and cyano groups, respectively, but the amino (–NH2) and carbonyl (–C=O) signals are lost. Our 1H NMR chart revealed a broad singlet at δ 12.23 of –NH groups (D2O exchangeable) and aromatic signals at δ 7.21–8.43. The 13C NMR spectrum and mass spectra also supported the structural assignment (m/z 371.80, M+).

From the above results, a probable mechanism is suggested for the production of 7-substituted-5-(1H-indol-3-yl)tetrazolopyrimidine-6-carbonitrile in the presence of triethylamine (TEA). Firstly, TEA initiates aldehydes 1 through hydrogen binding to start the nucleophilic addition of 3-cyanoacetyl indole 3. Activation of the starting aldehydes by hydrogen bonding increases the electrophilicity of the aldehyde and supports the production of the corresponding intermediate A (Knoevenagel product) [47] with compound 3. This adduct undergoes a Michael type addition reaction with 1H-tetrazol-5-amine 2 to yield an adduct B intermediate. After that, intermediate B underwent intramolecular cyclization leading to the C–N bond formation followed by the auto-oxidation leading to the formation product 4, Scheme 2.

Scheme 2.

Scheme 2

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Suggested reaction mechanism.

2.2. Cytotoxicity Screening

Nine compounds 4a–i were screened in vitro on HCT-116, MCF-7, MDA-MB-231 and A549 human cancer cells, as well as RPE-1 human normal cells via the MTT assay. The measurements of viable cells were considered and compared to the control. Activities of these compounds on the five human cell lines were compared to that of doxorubicin or 5-fluorouracil as reference drugs. Compounds 4a–i suppressed the five human cell types in a dose-dependent manner (Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7). Regarding HCT-116, both Figure 3 and Table 2 display that three compounds (4b, 4c and 4h, respectively) had more potent cytotoxic activities compared to that of doxorubicin.

Figure 3.

Figure 3

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Dose dependent cytotoxic activities of new compounds on HCT-116 cancer cells according to the MTT assay.

Figure 4.

Figure 4

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Dose dependent cytotoxic activities of new compounds on MCF-7 cancer cells according to the MTT assay.

Figure 5.

Figure 5

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Dose dependent cytotoxic activities of new compounds on MDA-MB-231 cancer cells according to the MTT assay.

Figure 6.

Figure 6

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Dependent cytotoxic activities of new compounds on A549 cancer cells according to the MTT assay.

Figure 7.

Figure 7

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Dose dependent cytotoxic activities of new compounds on RPE-1 human normal cells according to the MTT assay.

Table 2.

IC50 of the new compounds against the four human cancer cells as well as one human normal cell types according to the MTT assay.

Compound Code IC50 (µM) ± SD
HCT-116 MCF-7 MDA-MB-231 A549 RPE-1
4a 20.4 ± 2.1 32.5 ± 2.4 21.1 ± 2.1 20.9 ± 2.1 61.5 ± 2.5
4b 14.1 ± 1.5 28.1 ± 2.1 22.5 ± 2.9 22.4 ± 1.8 56.3 ± 3.1
4c 14.2 ± 1.7 22.0 ± 1.8 19.4 ± 3.1 24.7 ± 2.3 42.4 ± 2.8
4d 23.1 ± 2.2 25.7 ± 2.1 16.2 ± 2.3 22.7 ± 2.1 31.8 ± 3.1
4e 26.6 ± 2.1 17.6 ± 1.5 19.7 ± 2.8 25.6 ± 1.9 50.9 ± 2.9
4f 31.9 ± 2.6 23.9 ± 1.9 30.6 ± 2.2 25.9 ± 2.1 62.4 ± 2.8
4g 28.8 ± 1.9 26.3 ± 2.5 31.1 ± 2.9 26.0 ± 2.3 60.4 ± 3.4
4h 14.6 ± 1.5 26.0 ± 2.1 26.3 ± 1.8 27.5 ± 2.5 67.5 ± 3.9
4i 19.9± 2.1 24.8 ± 2.4 27.4 ± 2.3 26.6 ± 2.1 77.2 ± 3.8
Doxorubicin 17.2 ± 1.5 17.5 ± 2.1 - 27.0 ± 2.4 66.1 ± 3.5
5-Fluorouracil - - 3.8 ± 0.2 - -
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In addition, two compounds (4i and 4a, respectively) had comparable cytotoxic activities and the rest of the compounds had less cytotoxic activities compared to doxorubicin. In the case of MCF-7 cells, two tested compounds (4e and 4c, respectively) had slightly less anticancer activities compared to that of doxorubicin (Figure 4 and Table 2); the rest of the compounds had significantly less cytotoxic effect on MCF-7 cancer cells compared to that of the reference drug. In case of MDA-MB-231, all investigated compounds had significantly less anticancer effects compared to that of the reference drug 5-fluorouracil (Figure 5 and Table 2). In the case of A549 cancer cells, six compounds (4a, 4b, 4d, 4c, 4e and 4f, respectively) had more potent cytotoxic activities; three compounds (4g, 4i and 4h, respectively) had comparable cytotoxic effect compared to that of doxorubicin (Figure 6 and Table 2). In the case of RPE-1 human normal cells, four compounds (4d, 4c, 4e and 4b, respectively) showed more cytotoxic effect; four compounds (4g, 4a, 4f and 4h, respectively) had comparable cytotoxic effect, and one compound (4i) had significantly less cytotoxic activities compared to doxorubicin. The IC50s indicated in Table 2 reveal that all the nine compounds have strong cytotoxic effect on cancer cells rather than on normal cells.

Furthermore, From the above-mentioned results, one can conclude that, compounds 4b, 4c, 4h, 4i and 4a are potent anticancer candidate drugs on the human colon cancer, respectively; all the nine compounds are potent anticancer candidate drugs on the human lung cancer; compounds 4c, 4e and 4h are specifically good anticancer candidate drugs on hormone-dependent human breast cancer rather than on the hormone-independent human breast cancer.

3. Structure–Activity Relationship

Generally, tetrazolopyrimidine unit have interesting biological activities. All synthesized compounds contain a tetrazolopyrimidine unit based on the indole ring at the 5-position. Compound 4b substituted with 4-nitrophenyl at 7-position has the best anticancer activities against human colon cancer followed by compound 4h which contains the pyrrole ring at the 7-position. In the case of MCF-7 cells, both compound 4e substituted with 2-nitrophenyl at the 7-position and 4c which contains a 4-bromophenyl ring at 7- had slightly less anticancer activities compared to that of doxorubicin. Compound 4a substituted with 4-chlorophenyl at the 7-position has the best anticancer activities against A549 cancer cells. In the case of RPE-1 human normal cells, compound 4d substituted with 2-hydroxyphenyl, followed by compound 4c substituted with 4-bromophenyl, respectively) showed more cytotoxic effect.

4. Experimental

4.1. General Methods

All chemicals were purchased from the Merck and Sigma-Aldrich chemical companies (Muskegon, MI, USA) and used without further purification. Melting points were determined on XT-5 microscopic melting-point apparatus (Veb Analytik, Dresden, Germany) and were uncorrected. Infrared (IR) spectra were recorded on the FT Bruker Tensor 27 spectrometer ((Jasco, Easton, PA, USA). Proton nuclear magnetic resonance (1H NMR) and carbon-13 (C13) nuclear magnetic resonance (13C NMR) spectra were obtained from solution in dimethyl sulfoxide (DMSO)-d6 using a Bruker-400 spectrometer (Bruker, Karlsruhe, Germany). Mass spectra were recorded on AccuTOF LC-Plus from (Jeol, Tokyo, Japan). All melting points were determined using open capillaries on an Electrothermal-9100 (Veb, Analytik, Dresden, Germany) instrument and are uncorrected.

4.2. General Procedure for the Synthesis of 7-Substituted-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4a–i

The mixture of appropriate aldehydes (1 mmol), 1H-tetrazol-5-amine (1 mmol) and 3-(1H-indol-3-yl)-3-oxopropanenitrile (1 mmol) in dimethylformamide (DMF) (10 mL), Et3N (0.25 mmol) was put into a reaction flask and refluxed at 120 °C about 10 h (monitored by thin-layer chromatography (TLC)). After the completion of the reaction, the reaction mixture was cooled to room temperature and the products was recrystallized from DMF or ethanol (EtOH).

4.2.1. 7-(4-Chlorophenyl)-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4a

Yellow crystals; m.p.: 276–278 °C (DMF), yield (0.282 g, 76%). IR (KBr, cm−1): 3211 (NH), 2220 (CN), 1658, 1569, 1494, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 7.21–7.46 (m, 4H, Ar-H), 7.48–7.91 (m, 4H, indole-H), 8.43 (s, 1H, indole-H2), 12.26 (br s, 1H, NH). 13C-NMR (400 MHz, DMSO-d6) δ (ppm): 150.58, 148.31, 136.80, 136.72, 136.30, 131.34, 129.23, 126.04, 123.60, 122.46, 121.30, 117.38, 113.48, 112.49, 112.09. HRMS (ESI-TOF) m/z (%) 371 (M+, 4), 369 (11), 270 (37), 171 (54), 146 (45), 136 (18), 117 (19), 102 (49), 86 (10), 61 (12). Anal. Calcd for C19H10ClN7: C, 61.38; H, 2.71; Cl, 9.54; N, 26.37. Found: C, 61.47; H, 2.66; Cl, 9.44; N, 26.23.

4.2.2. 5-(1H-Indol-3-yl)-7-(4-nitrophenyl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4b

Yellow crystals; m.p.: 286–288 °C (ethanol), yield (0.273 g): 71%. IR (KBr, cm−1): 3232 (NH), 2223 (CN), 1655, 1561, 1550, 1463, 1430, 1332, 1243, 1127, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 7.23–7.46 (m, 4H, Ar-H), 7.49–7.97 (m, 4H, indole-H), 8.48 (s, 1H, indole-H2), 12.33 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 149.32, 148.68, 138.64, 136.88, 136.80, 131.12,130.11, 125.96, 124.01, 122.60, 121.30, 116.77, 115.13, 113.40, 112.55. HRMS (ESI-TOF) m/z (%) 384 (M+ + 2, 3), 367 (5), 318 (41), 171 (64), 160 (9), 132 (8), 102 (59), 86 (100), 65 (16). Anal. Calcd for C19H10N8O2: C, 59.69; H, 2.64; N, 29.31; O, 8.37. Found: C, 59.76; H, 2.46; N, 29.21; O, 8.41.

4.2.3. 7-(4-Bromophenyl)-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4c

Yellow crystals; m.p.: 279–281 °C (ethanol), yield (0.31 g): 74%. IR (KBr, cm−1): 3286 (NH), 2204 (CN), 1650, 1570, 1494, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm):7.21–7.42 (m, 4H, Ar-H), 7.44–7.90 (m, 4H, indole-H), 8.42 (s, 1H, indole-H2), 12.27 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 150.70, 148.14, 136.72, 136.28, 132.17, 131.98, 131.66, 126.03, 125.84, 123.60, 122.46, 121.31, 117.38, 112.50, 112.15. HRMS (ESI-TOF) m/z (%) 415 (M+ − 1, 4), 371 (2), 351 (19), 284 (16), 136 (18), 117 (100), 75 (31). Anal. Calcd for C19H10BrN7: C, 54.83; H, 2.42; Br, 19.20; N, 23.56. Found: C, 55.09; H, 2.34; Br, 19.11; N, 23.23.

4.2.4. 7-(2-Hydroxyphenyl)-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4d

Yellow crystals; m.p.: 271–273 °C (ethanol), yield (0.235 g): 67%. IR (KBr, cm−1): 32103324– (OH, and NH), 2210 (CN), 1620, 1560, 1492, 1433, 1334, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm):7.22–7.41 (m, 4H, Ar-H), 7.45–7.93 (m, 4H, indole-H), 8.30 (s, 1H, indole-H2), 12.19 (br s, 1H, OH), 12.20 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 158.32, 154.14, 152.37, 142.20, 137.68, 136.93, 135.44, 133.12, 131.22, 126.04, 123.60, 122.46, 121.30, 117.38, 113.48, 112.49, 112.09. HRMS (ESI-TOF) m/z (%) 354 (M+ + 1, 2), 320 (19), 261 (15), 187 (10), 146 (45), 136 (18), 117 (19), 102 (100), 86 (10), 71 (6). Anal. Calcd for C19H11N7O: C, 64.59; H, 3.14; N, 27.75; O, 4.53. Found: C, 61.58; H, 2.45; Cl, 9.21; N, 26.09.

4.2.5. 5-(1H-Indol-3-yl)-7-(2-nitrophenyl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4e

Yellow crystals; m.p.: 291–293 °C (ethanol), yield (0.27 g): 70%. IR (KBr, cm−1): 3202 (NH), 2206 (CN), 1632, 1554, 1494, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 7.20–7.39 (m, 4H, Ar-H), 7.43–7.92 (m, 4H, indole-H), 8.40 (s, 1H, indole-H2), 12.37 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 151.28, 147.80, 147.65, 137.64, 136.98, 136.91, 136.30, 131.34, 129.23, 126.04, 123.60, 122.46, 121.30, 117.38, 113.48, 113.12, 112.94. HRMS (ESI-TOF) m/z (%) 384 (M+ + 2, 20), 371 (8), 340 (49), 175 (21), 160 (11), 136 (18), 116 (100), 75(7). Anal. Calcd for C19H10N8O2: C, 59.69; H, 2.64; N, 29.31; O, 8.37. Found: C, 59.72; H, 2.39; N, 29.03; O, 8.41.

4.2.6. 5-(1H-Indol-3-yl)-7-(thiophen-2-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4f

Yellow crystals; m.p.: 266–268 °C (ethanol), yield (0.22 g): 64%. IR (KBr, cm−1): 3245 (NH), 2213 (CN), 1625, 1545, 1492, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm):7.20–7.43 (m, 3H, thiophene-H), 7.42–7.90 (m, 4H, indole-H), 8.50 (s, 1H, indole-H2), 12.20 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 145.42, 138.78, 136.51, 135.36, 135.20, 128.54, 126.18, 123.48, 122.30, 121.40, 118.16, 113.71, 112.43, 106.64. HRMS (ESI-TOF) m/z (%) 343 (M+, 3), 324 (3), 300 (20), 256 (24), 195 (18), 117 (100), 102 (70), 76 (19). Anal. Calcd for C17H9N7S: C, 59.47; H, 2.64; N, 28.56; S, 9.34. Found: C, 59.65; H, 2.49; N, 28.39; S, 9.21.

4.2.7. 7-(Furan-2-yl)-5-(1H-indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4g

Yellow crystals; m.p.: 269–271 °C (ethanol), yield (0.212 g): 65%. IR (KBr, cm−1): 3232 (NH), 2214 (CN), 1636, 1548, 1466, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm):7.19–7.41 (m, 3H, furan-H), 7.42–7.96 (m, 4H, indole-H), 8.44 (s, 1H, indole-H2),7.19–8.44 (m, 8H, Ar-H), 12.19 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 148.82, 148.76, 137.47, 135.13, 126.19, 123.48, 122.38, 122.30, 121.43, 117.75, 113.99, 113.67, 112.43, 105.70. HRMS (ESI-TOF) m/z (%) 327 (M+, 1), 263 (100), 223 (7), 185 (15), 160 (21), 93 (18), 86 (13). Anal. Calcd for C17H9N7O: C, 62.57; H, 3.09; N, 34.34. Found C, 62.69; H, 3.01; N, 34.76.

4.2.8. 5-(1H-Indol-3-yl)-7-(1H-pyrrol-2-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4h

Yellow crystals; m.p.: 258–261 °C (DMF), yield (0.21 g): 64%. IR (KBr, cm−1): 3231 (NH), 3221 (NH), 2216 (CN), 1658, 1569, 1494, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 6.817–.41 (m, 3H, pyrrole-H), 7.42–7.90 (m, 4H, indole-H), 8.33 (s, 1H, indole-H2), 6.81–8.33 (m, 8H, Ar-H), 12.11 (br s, 1H, NH)11.64 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 146.98, 145.33, 136.88, 135.10, 126.12, 123.41, 122.28, 122.10, 121.12, 117.05, 113.34, 112.87, 112.21, 104.98. HRMS (ESI-TOF) m/z (%) 328 (M+ + 2, 1), 306 (9), 262 (11), 185 (100) (18), 159 (8), 143 (12), 82 (4). Anal. Calcd for C17H10N8: C, 66.45; H, 3.41; N, 30.14. Found: C, 66.59; H, 3.32; N, 30.07.

4.2.9. 5,7-di(1H-Indol-3-yl)tetrazolo[1,5-a]pyrimidine-6-carbonitrile 4i

Yellow crystals; m.p.: 296–298 °C (DMF), yield (0.23 g): 61%. IR (KBr, cm−1): 32363265– (2NH), 2206(CN), 1656, 1560, 1454, 1436, 1335, 1241, 1129, 808, 741, 684. 1H-NMR (400 MHz, DMSO-d6) δ (ppm): 7.17–7.94 (m, 8, di-indole-H), 8.31, 8.33 (two s, 2H, indole-H2), 12.01 (br s, 1H, NH), 12.23 (br s, 1H, NH). 13C NMR (400 MHz, DMSO-d6) δ (ppm): 151.04, 150.54 136.80, 136.72, 136.30, 131.34, 130.11, 129.23, 126.04, 123.96, 123.60, 122.46, 121.30, 117.38, 113.48, 111.22, 110.12. HRMS (ESI-TOF) m/z (%) 376 (M+, 4), 351 (11), 270 (37), 171 (54), 146 (45), 136 (18), 117 (19), 102 (49), 86 (10), 61 (12). Anal. Calcd for C21H12N8: C, 67.01; H, 3.21; N, 29.77. Found: C, 67.34; H, 3.12; N, 29.52.

4.3. Materials and Methods

4.3.1. In-Vitro Cytotoxic Activity

Cell cultures of human colorectal carcinoma (HCT)-116, MCF-7 (hormone-dependent human breast adenocarcinoma), MDA-MB-231 (hormone-independent human breast adenocarcinoma), A549 (human lung carcinoma) and human normal Retina pigmented epithelium (RPE)-1 cell lines were purchased from the American Type Culture Collection (Rockville, MD, USA) and maintained in Dulbecco’s modified Eagle medium (DMEM) medium which was supplemented with 10% heat-inactivated fetal bovine serum (FBS), 100 U/mL penicillin and 100 U/mL streptomycin. The cells were grown at 37 °C in a humidified atmosphere of 5% CO2.

4.3.2. MTT Cytotoxicity Assay

The cytotoxicity activity on HCT-116, MCF-7, MDA-MB-231 and A549 human cancer cell lines as well as RPE-1 human normal cells was estimated using the 3-[4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, which is based on the reduction of the tetrazolium salt by mitochondrial dehydrogenases in viable cells [48,49,50]. Cells were dispensed in 96-well sterile microplates (5 × 104 cells/well), and incubated at 37 °C with a series of different concentrations, in DMSO, of each tested compound or Doxorubicin (positive control) for 48 h in a serum free medium prior to the MTT assay. After incubation, media were carefully removed, 40 µL of MTT (2.5 mg/mL) were added to each well and then incubated for an additional 4 h. The purple formazan dye crystals were solubilized by the addition of 200 µL of DMSO. The absorbance was measured at 570 nm using a Spectra Max Paradigm Multi-Mode microplate reader. The relative cell viability was expressed as the mean percentage of viable cells compared to the untreated control cells. The relative cell anti-proliferative was measured according to the following equation: % cytotoxicity = (1 − As/Ab) × 100. Where; As = Absorbance of each sample and Ab = Absorbance of the blank. All experiments were conducted in triplicate and repeated on three different days. All the values were represented as mean ± SD. IC50s were determined by probit analysis by SPSS Inc probit analysis (IBM Corp., Armonk, NY, USA).

5. Conclusions

Briefly, we have a conventional three-constituent reaction to the construction of a completely substituted new series of tetrazolopyrimidine-6-carbonitrile based on indole moiety in the existence of triethylamine as a catalyst and DMF as a solvent. The significance of this process is important over the additional usual ones, by short times, respectable yields, trivial conditions, cost-effective and relaxed management. Cytotoxic evaluation of novel series of 7-substituted-5-(1H-indol-3-yl)tetrazolopyrimidine-6-carbonitrile was investigated against HCT-116, MCF-7, MDA-MB-231 and A549 human cancer cell lines using the MTT test. From the results indicated in this work, one can conclude that compounds 4h, 4b, 4c, 4i and 4a had potent anticancer activities against human colon cancer, respectively; all the nine compounds had potent anticancer activities on human lung cancer as well.

Acknowledgments

The authors gratefully acknowledge Qassim University, represented by the Deanship of Scientific Research, on the material support for this research under the number (cos-2018-1-14-S-5060) during the academic year 1439 AH/2018AD.

Author Contributions

The listed authors contributed to this work as described in the following: M.A.A.R. planned the research idea, applied the synthesis and characterization of novel compounds, and contributed to the data interpretation. F.M.A. and. M.A.A.R. contributed to discuss the results, writing the original draft manuscript, and revisions. H.M.A. (Department of Tanning Materials and Leather Technology, National Research Centre) conducted the biological assays and provided the experimental procedures and results of biological part. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Qassim University, represented by the Deanship of Scientific Research, grant number (cos-2018-1-14-S-5060) during the academic year 1439 AH/2018AD.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

Sample Availability: Samples of the compounds are available from the authors.

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