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Ethyl 3-formyl-1H-indol-2-carboxylate (FIC) and 2-(4-methoxyphenyl)-2,4-dihydropyrrolo [3,4-b]indol-3-ol (MPI) were synthesized as indole derivatives. The chemical structures of FIC and MPI were established through analytical and spectroscopic techniques. The inhibitory impacts of FIC and MPI on mild steel (MS) in an acidic environment (0.5 M H2SO4) were investigated by employing methodologies including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP). As the studied indole derivatives adsorbed on the surface of MS, they created a barrier to mass and charge movement, shielding the MS from dangerous ions. It was observed that the inhibitory efficiency (%EF) values increased with the molar concentration of indole derivatives (FIC and MPI). At all concentrations, the two indole derivatives being studied, FIC and MPI, had high efficiency values. The highest efficiencies at 90 ppm were 81.2% with MPI and 76.2% with FIC. The polarization curves also clearly showed that MPI and FIC function as mixed-type inhibitors. Additionally, this study used density functional theory (DFT) and molecular dynamics (MD) simulations to investigate how well the two indole derivatives prevented mild steel corrosion.

Background/Objectives: Cancer remains one of the most significant challenges in modern medicine, requiring the continuous development of novel molecular scaffolds with anticancer potential that act through multiple pathways. Heterocyclic compounds incorporating indole, triazole, oxadiazole, and thiadiazine motifs have attracted considerable attention due to their diverse pharmacological activities. This study aimed to design, synthesize, and evaluate new hybrid heterocyclic systems, including 1,2,4-triazole, 1,3,4-oxadiazole, and thiadiazine motifs, targeting liver and breast cancer. Methods: A series of indolyl-based heterocyclic compounds was synthesized using efficient and environmentally friendly protocols. Indolyl-triazol-thiadiazin-6-ol 5 was prepared via solvent-free fusion of esters 2 and 3 or the corresponding acid 4. Oxadiazole derivatives were produced by reacting hydrazide intermediates with carbon disulfide. Triazole derivatives were synthesized via cylization of thiosemicarbazide 9 in aqueous KOH (4.0 N). Structural characterization was performed using Fourier Transform InfraRed (FTIR), 1H and 13C NMR spectroscopy, and electron impact mass spectrometry (EIMS). Cytotoxic activity was evaluated against liver and breast cancer cell lines, and VEGFR-2 kinase inhibition was assessed for selected derivatives. Results: The synthesized compounds demonstrated notable cytotoxicity activity, with compounds 4, 5, and 9 exhibiting IC50 values in the low micromolar range. Enzymatic assays revealed that compounds 4 and 9 showed strong VEGFR-2 inhibition (97.9% and 96.4%, respectively), indicating apoptosis-inducing effects. Conclusions: The synthesized indolyl-based hybrid heterocycles represent a promising chemotype with in vitro cytotoxic activity and VEGFR-2 inhibitory effects, supporting further investigation, optimization, and mechanistic studies to evaluate their potential lead for anticancer drug development.

According to data provided by the World Health Organization (WHO), a total of 2.3 million women across the globe received a diagnosis of breast cancer in the year 2020, and among these cases, 685,000 resulted in fatalities. As the incidence of breast cancer statistics continues to rise, it is imperative to explore new avenues in the ongoing battle against this disease. Therefore, a number of new indolyl-hydrazones were synthesized by reacting the ethyl 3-formyl-1H-indole-2-carboxylate 1 with thiosemicarbazide, semicarbazide.HCl, 4-nitrophenyl hydrazine, 2,4-dinitrophenyl hydrazine, and 4-amino-5-(1H-indol-2-yl)-1,2,4-triazole-3-thione to afford the new hit compounds, which were assigned chemical structures as thiosemicarbazone 3, bis(hydrazine derivative) 5, semicarbzone 6, Schiff base 8, and the corresponding hydrazones 10 and 12 by NMR, elemental analysis, and X-ray single-crystal analysis. The MTT assay was employed to investigate the compounds’ cytotoxicity against breast cancer cells (MCF-7). Cytotoxicity results disclosed potent IC50 values against MCF-7, especially compounds 5, 8, and 12, with IC50 values of 2.73 ± 0.14, 4.38 ± 0.23, and 7.03 ± 0.37 μM, respectively, compared to staurosproine (IC50 = 8.32 ± 0.43 μM). Consequently, the activities of compounds 5, 8, and 12 in relation to cell migration were investigated using the wound-healing test. The findings revealed notable wound-healing efficacy, with respective percentages of wound closure measured at 48.8%, 60.7%, and 51.8%. The impact of the hit compounds on cell proliferation was assessed by examining their apoptosis-inducing properties. Intriguingly, compound 5 exhibited a significant enhancement in cell death within MCF-7 cells, registering a notable increase of 39.26% in comparison to the untreated control group, which demonstrated only 1.27% cell death. Furthermore, the mechanism of action of compound 5 was scrutinized through testing against kinase receptors. The results revealed significant kinase inhibition, particularly against PI3K-α, PI3K-β, PI3K-δ, CDK2, AKT-1, and EGFR, showcasing promising activity, compared to standard drugs targeting these receptors. In the conclusive phase, through in vivo assay, compound 5 demonstrated a substantial reduction in tumor volume, decreasing from 106 mm³ in the untreated control to 56.4 mm³. Moreover, it significantly attenuated tumor proliferation by 46.9%. In view of these findings, the identified leads exhibit promises for potential development into future medications for the treatment of breast cancer, as they effectively hinder both cell migration and proliferation.

A multicomponent synthesis was empolyed for the synthesis of ethyl 2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-3-carboxylate 1. An interesting cyclization was obtained when the amino-ester 1 reacted with ethyl isothiocyanate to give the benzo[4,5]thieno[2,3-d][1,3]thiazin-4-one 3. Acylation of the amino-ester 1 with chloroacetyl chloride in DCM and Et3N afforded the acylated ester 4. The amino-ester 1 was cyclized to benzo[4,5]thieno[2,3-d]pyrimidin-4(3H)-one 8, which was reacted with some alkylating agents leading to alkylation at nitrogen 9–13. Hydrazide 14 was utilized as a synthon for the synthesis of the derivatives 15–19. Chloro-thieno[2,3-d]pyrimidine 20 was synthesized and reacted with the hydrazine hydrate to afford the hydrazino derivative 21, which was used as a scaffold for getting the derivatives 22–28. Nucleophilic substitution reactions were used for getting the compounds 29–35 from chloro-thieno[2,3-d]pyrimidine 20. In the way of anticancer therapeutics development, the requisite compounds were assessed for their cytotoxicity in vitro against MCF-7 and HepG-2 cancer cell lines. Twelve compounds showed an interesting antiproliferative potential with IC50 from 23.2 to 95.9 µM. The flow cytometric analysis results showed that hit 4 induces the apoptosis in MCF-7 cells with a significant 26.86% reduction in cell viability. The in vivo study revealed a significant decrease in the solid tumor mass (26.6%) upon treatment with compound 4. Moreover, in silico study as an agonist for inhibitors of JAK2 and prediction study determined their binding energies and predicted their physicochemical properties and drug-likeness scores.

The hit compound 1,2,4-triazolo[4’,3’:2,3]pyridazino[4,5-b]indole 3 was synthesized from the reflux of 4-amino-5-indolyl-1,2,4-triazole-3-thione 1 with 4′-bromoacetophenone 2 in methanol catalyzed by concentrated HCl and the desired final molecule was obtained by recrystallization from methanol. The suggested structures of compounds 1 and 3 based on the spectral characterizations were confirmed by X-ray single crystal diffraction analysis. Compound 3 crystallized in the triclinic crystal system and P-1 space group with a = 5.9308(2) Å, b = 10.9695(3) Å, c = 14.7966(4) Å, α = 100.5010(10)°, β = 98.6180(10)°, and γ = 103.8180(10)°. On the other hand, the crystal system of 1 is monoclinic, where a = 6.23510(10) Å, b = 26.0156(4) Å, c = 12.4864(2) Å, β = 93.243(2)° and the space group is P21. The triazole and indole rings are found twisted from each other in both compounds. The twist angle is higher in 3 (12.65°) than 1 (4.94–7.22°). In the case of the former, the H…H (39.6%), H…C (22.0%), N…H (12.8%) and Br…H (13.2%) contacts are the most dominant while the C…C, C…H, Br…H, N…H and S…S contacts have the characteristics of strong interactions. In the latter, the C…H, N…H, S…H, S…S, and C…C contacts are the most important. In this case, the percentages of the H…H, C…H, N…H and S…H contacts are in the range of 34.9–37.4, 20.5–24.0, 12.2–13.6, 14.0–15.8, respectively. In both systems, the shape index and curvedness of surfaces confirmed the presence of π–π stacking interactions.

The regiospecific S-benzylation/allylation of two 4-aryl-5-indolyl-1,2,4-triazole-3-thione precursors was carried out using Et3N as a base. Allyl group migration from exocyclic sulfur to the triazole nitrogen (N3) was successfully achieved in a short time via thermal fusion without the need for any catalyst. The allylation of indole nitrogen, along with exocyclic sulfur or triazole nitrogen (N3), was carried out using K2CO3 as stronger base. S,N-Diallylated products were converted to N,N-diallylated analogues using a simple fusion approach. Structural analyses of the two newly synthesized hybrids 2b and 5b investigated via the X-ray diffraction of a single crystal combined with Hirshfeld calculations. The compound 5b was crystallized in a monoclinic crystal system and the P21/c space group, whereas in compound 2b, the crystal system comprises the less symmetric triclinic and P − 1 space group. The asymmetric unit contains one and two molecules of 5b and 2b, respectively, while the unit cell contains four molecules in both cases. Hirshfeld analysis was performed in both systems to analyze the non-covalent interactions that control molecular packing. For 5b, C…H, N…H, S…H, Cl…N and H…H interactions are the most significant. Their percentages are 23.7, 8.8, 4.5, 1.2 and 48.2, respectively. In the case of 2b, the Cl…C, S…N, C…H, H…H and N…H interactions have the upper hand in molecular packing. In one unit, the percentages of these contacts are 2.3, 0.9, 26.8, 38.7 and 9.3%, while in the other unit, the corresponding values are 4.4, 1.3, 22.1, 43.6 and 9.0%, respectively.

The synthesis of 4-methyl/phenyl-1,2,5,6-tetraazafluoranthen-3(2H)-one 4 and 7 has been reported with ninhydrin via a reaction first with ethyl acetoacetate or ethyl benzoylacetate and then a reaction of the resultant esters with hydrazine hydrate. The mechanism of hydrazinolysis and cyclization to obtain tetraazafluoranthen-3(2H)-ones is ambiguous, and the previously proposed mechanism was not based on facts because the actual intermediates were not isolated. Herein, the important intermediates involved in the hydrazinolysis–cyclization mechanistic pathway were isolated and characterized using NMR and X-ray single-crystal analysis. The intermediates demonstrate that the reaction carried out via two hydrazinolysis–cyclization reactions, the first of which includes the condensation of one hydrazine molecule with two ketone groups and the second of which includes the reaction of another hydrazine molecule with the ester and then condensation with the other ketone group. The stability of hydrazide 11 enabled the hydrazine to reduce the carbonyl of the ketone group to form 12 via a Wolff–Kishner-like reduction. The structure of the three intermediates was confirmed using X-ray crystallographic analysis. It was found that the three fused ring systems deviated from planarity to different extents, with their deviation from being coplanar reaching up to 5.3°. The possible non-covalent interactions which control the molecular packing of these intermediates were elucidated with the aid of Hirshfeld analysis.

The present synthetic strategy involves the synthesis of indolyl-triazolo-thiadiazole heterocyclic ring systems 8–13 from the condensation of 4-amino-5-(1H-indol-2-yl)-3H-1,2,4-triazole-3-thione 1 with the aromatic carboxylic acid derivatives 2–7 in presence of POCl3 for 1 h. All compounds were obtained in very good yields and have been well-characterized using spectroscopic techniques. Exclusively, good quality crystals from the target organic hybrid 8-(1H-indol-2-yl)-5-(p-tolyl)-[1,2,4]triazolo [3,4-b][1,3,4]thiadiazole 9 were obtained and found suitable for X-ray single crystal diffraction measurement, which is used to confirm and analyze the molecular and supramolecular structure aspects of 9. The solid-state structure of the synthesized molecule 9 agrees very well with other characterizations. The packing of 9 is dominated by the N…H, S…H, C…C and S…C non-covalent interactions, which agree with the Hirshfeld surface analysis. The percentages of these contacts are calculated to be 20.3%, 5.4%, 9.4% and 4.3%, respectively.

The novel hydrazone-containing thieno[2,3-d]pyrimidine, namely, N′-(1,3-dioxo-1,3-dihydro-2H-inden-2-ylidene)-2-(4-oxo-5,6,7,8-tetrahydrobenzo[4,5]thieno[2,3-d]pyrimidin-3(4H)-yl)acetohydrazide 4 was synthesized in a very good yield from the reaction of the triketoester 1 or ninhydrin 2 with the exocyclic acetohydrazide 3 in methanol. Good-quality crystals of 4 were obtained by recrystallization of the compound from the DMF/MeOH solvent mixture. The target product 4 crystallized in the triclinic crystal system and P-1 space group. The topology analysis of molecular packing indicated that the H…H (30.4%), O…H (22.0%) and H…C (17.0%) contacts are the most dominant intermolecular interactions in the crystal of 4, while the O…H, N…H, H…C, N…C, O…C, C…C and O…O are the only contacts which have shorter interaction distances than the vdWs radii sum of the interacting atoms. The structure of 4 is optimized and the calculated structure showed good agreement with the experimental one. Additionally, MEP, HOMO, LUMO and the reactivity descriptors were calculated.

A new series of nitrogen and sulfur heterocyclic systems were efficiently synthesized by linking the following four rings: indole; 1,2,4-triazole; pyridazine; and quinoxaline hybrids. The strength of the acid that catalyzes the condensation of 4-amino-5-(1H-indol-2-yl)-2,4-dihydro-3H-1,2,4-triazole-3-thione 1 with aromatic aldehydes controlled the final product. Reflux in glacial acetic acid yielded Schiff bases 2–6, whereas concentrated HCl in ethanol resulted in a cyclization product at C-3 of the indole ring to create indolo-triazolo-pyridazinethiones 7–16. This fascinating cyclization approach was applicable with a wide range of aromatic aldehydes to create the target cyclized compounds in excellent yield. Additionally, the coupling of the new indolo-triazolo-pyridazinethiones 7–13 with 2,3-bis(bromomethyl)quinoxaline, as a linker in acetone and K2CO3, yielded 2,3-bis((5,6-dihydro-14H-indolo[2,3-d]-6-aryl-[1,2,4-triazolo][4,3-b]pyridazin-3 ylsulfanyl)methyl)quinoxalines 19–25 in a high yield. The formation of this new class of heterocyclic compounds in high yields warrants their use for further research. The new compounds were characterized using nuclear magnetic resonance (NMR) and mass spectral analysis. Compound 6 was further confirmed by the single crystal X-ray diffraction technique.