A Comprehensive on Synthesis and Antimicrobial Evaluation of Substituted-Arylideneamino-5-(5-Chlorobenzofuran-2-yl)-1, 2, 4-Triazole-3-Thiol Derivatives/ Schiff Bases

Schiff bases are well known and popular classes of organic compounds containing imine (R2C = NH) group that are widely used as catalysts and intermediates in numerous organic transformations. Schiff bases are medicinally very important because they exhibit antimicrobial like antibacterial, antiviral and anticancer activities. Benzofuran based Schiff bases have been found as interesting scaffolds for the synthesis and design of biologically active agents. Moreover, they possess a wide range of biological activities against fungal, bacterial, malarial, inflammatory and viral diseases. In this reviw, substituted-arylideneamino-5-(5-chlorobenzofuran-2-yl)-1,2,4-triazole-3-thiols have been synthesized by using efficient synthetic protocols. The synthesized derivatives are also evaluated against different bacterial strains.

Developing a fast and catalyst-free protocol to form C=N double bond with high functional group tolerance

The carbon-nitrogen double bond (C=N) is a fundamentally important functional group in organic chemistry. This is largely due to the fact that C=N acts as electrophilic synthon to give nitrogen-containing compounds. Here, we report the condensation of primary amine or hydrazine with very electron-deficient aldehyde to form C=N bond in the absence of any catalysts (metals and acids). The protocol performs at room temperature and applies water as co-solvent. Two hundred examples are presented here. With its intrinsic advantages of wide substrate scopes, excellent efficiency (high yields and short reaction time), operational simplicity, mild condition (room temperature as reaction temperature, no catalysts, no additions, water as co-solvent and opening to air) and available starting materials, the protocol can be compatible with various drugs, prodrugs, dyes and pharmacophores containing primary amino group. In addition, we also successfully apply this protocol to rapidly synthesize the core scaffolds of bioactive molecules.

Ease to Challenges in Achieving Successful Synthesized Schiff Base, Chirality, and Application as Antibacterial Agent

This study reports how to overcome the challenges experienced in achieving successful synthesized Schiff base via types of Schiff base (chiral and achiral), synthesis, nature of products, and its antibacterial applications. Schiff base is a versatile ligand which is useful in asymmetric reactions to prepare chiral catalysts. It is also used in symmetric reactions to prepare achiral compounds. In line with the achiral compounds, conventional (room temperature and refluxing) and microwave irradiation methods are the two main types of methods to synthesize achiral Schiff base as reported in this review. Among various experimental approaches, this study supports the green chemistry microwave approach to synthesize Schiff base because of its benefits environmental sustainability. Problems relating to the nature of products formed from the synthesized Schiff bases were examined and resolved. Herein, the products could either be solid (crystals, powder, and precipitate), oily, or viscous (sticky) products. Some familiar characterization techniques used to identify and confirm the successful syntheses of Schiff bases, such as solubility test, melting point (MP), Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis), and nuclear magnetic resonance (NMR, 1H NMR, and 13C NMR), were discussed. In addition, the antibacterial studies on Schiff base and corresponding metal complexes confirmed their biological relevance to the human.

Manganese Schiff Base Complexes, Crystallographic Studies, Anticancer Activities, and Molecular Docking

Choice of ligands is significant to successful synthesis of metal complexes (coordination compounds). This study reports the use of Schiff base as the right ligand to control the poor bioavailability and neurodegenerative toxicity challenges of manganese ion. In line with this study, document analysis was used as the methodological approach to evaluate the significance of Schiff base ligands in easing these manganese’s challenges and aligning the resultant coordination compounds (manganese Schiff base complexes) as therapeutic agents in anticancer studies. Report also involves crystallographic studies where single crystal X-ray crystallography was used as a chemical characterization technique. In addition, molecular docking studies, MOE2008, and AutoDock software were used to reveal the mode of interaction between the Schiff base and the manganese(II) and (III) ions, as well as scrutinizing the biological efficacy of the manganese(II) and manganese(III) Schiff bases coordination compounds as anticancer agents against some anticancer cell lines. Conclusion drawn was that manganese(II) and manganese(III) Schiff bases coordination compounds gave more active and potent activities than the corresponding Schiff bases. As a result, challenges of neurodegenerative toxicity and poor bioavailability of manganese ion were overcome, and the chelation therapy was fulfilled. Results from single crystal X-ray crystallography confirmed the successful synthesis of manganese(II) and manganese(III) Schiff bases coordination compounds and revealed the mechanism of reaction, while the molecular docking buttressed the biological activities of the Schiff base ligand and manganese Schiff base coordination compounds by portraying the structure activity relationship (SAR) between either Schiff base or the manganese Schiff base coordination compounds and the virtual cancer cell line (receptor protein), where hits were obtained for lead optimizations.

Low-Temperature Infrared Spectra and Ultraviolet-Induced Rotamerization of 5-Chlorosalicylaldehyde

5-Chlorosalicylaldehyde (abbreviated as 5CSA) is an important chemical used in the synthesis of fragrances, dyes, and pharmaceuticals. In this investigation, 5CSA isolated in solid N₂, at 10 K, and in its neat amorphous and crystalline phases, at 50 and 190 K, respectively, were investigated by infrared spectroscopy and DFT(B3LYP)/6-311++G(d,p) calculations. The systematic theoretical analysis of the 5CSA conformational landscape showed that the compound exhibits four different conformers, which were structurally characterized in detail. In the as-deposited low-temperature matrices of 5CSA, only the most stable conformer, the intramolecularly hydrogen-bonded form I, was found. The same was observed in the case of the investigated low-temperature amorphous and crystalline phases of 5CSA. Conformer I was successfully converted into a higher-energy conformer(II), where both aldehyde and hydroxyl groups are rotated by 180° relative to their position in the initial conformer, through narrowband ultraviolet (UV) (λ = 308 nm) in situ irradiation of the as-deposited N₂ matrix of 5CSA. The infrared spectra of both matrix-isolated conformers, as well as those of the neat amorphous and crystalline phases of 5CSA, were assigned and interpreted in comparative terms, allowing us to elucidate structurally and vibrationally relevant effects of the main intra- and intermolecular interactions operating in the different studied phases. Very interestingly, the observed UV-induced I → II rotamerization was found to take place in an exclusive basis, with no other photochemical processes being observed to occur upon UV irradiation, under the experimental conditions used in the present investigation.

Synthesis (Z) vs (E) Selectivity, Antifungal Activity against Fusarium oxysporum, and Structure-Based Virtual Screening of Novel Schiff Bases Derived from l-Tryptophan

Schiff bases are widely used molecules due to their potential biological activity. In this manuscript, we presented the synthesis and NMR study of new enamine Schiff bases derived from l-tryptophan, showing that the Z-form of the enamine is the main tautomeric form for aliphatic precursors. The DFT-B3LYP methodology at the 6-311+G**(d,p) level suggested that the tautomeric imine forms are less stable than the corresponding enamine forms. Their isomerism depends on the formation of intramolecular hydrogen bonds and steric factors associated with the starting carbonyl precursors. The in vitro biological activity tests against Fusarium oxysporum revealed that acetylacetone derivatives are the most active agents (IC₅₀ < 0.9 mM); however, the antifungal activity could be disfavored by bulky groups on ester and enamine moieties. Finally, the structure-based virtual screening through molecular docking and MM-GBSA rescoring revealed that Schiff bases 3e, 3g, and 3j behave putatively as binders for target proteins involved in the life processes of F. oxysporum. In this sense, molecular dynamics analysis showed that the ligand-protein complexes have good stability with root-mean-square deviation (RMSD) values within the allowed range. Therefore, the present study paves the way for designing new antifungal compounds based on l-tryptophan-derived Schiff bases.

Base-Free Synthesis and Photophysical Properties of New Schiff Bases Containing Indole Moiety

Schiff bases represent an essential class in organic chemistry with antitumor, antiviral, antifungal, and antibacterial activities. The synthesis of Schiff bases requires the presence of an organic base as a catalyst such as piperidine. Base-free synthesis of organic compounds using a heterogeneous catalyst has recently attracted more interest due to the facile procedure, high yield, and reusability of the used catalyst. Herein, we present a comparative study to synthesize new Schiff bases containing indole moieties using piperidine as an organic base catalyst and Au@TiO2 as a heterogeneous catalyst. In both methods, the products were isolated in high yields and fully characterized using different spectral analysis techniques. The catalyst was reusable four times, and the activity was slightly decreased. The presence of Au increases the number of acidic sites of TiO2, resulting in C=O polarization. Yields of the prepared Schiff bases in the presence of Au@TiO2 and piperidine were comparable. However, Au@TiO2 is an easily separable and recyclable catalyst, which would facilitate the synthesis of organic compounds without applying any hazardous materials. Furthermore, the luminescence behavior of the synthesized Schiff bases exhibited spectral shape dependence on the substituent group. Interestingly, the compounds also displayed deep-blue fluorescence with Commission Internationale de l'Éclairage (CIE) coordinates of y < 0.1. Thus, these materials may contribute to decreasing the energy consumption of the emitting devices.

Synthesis and Antiproliferative Evaluation of 3-Chloroazetidin-2-ones with Antimitotic Activity: Heterocyclic Bridged Analogues of Combretastatin A-4

Antimitotic drugs that target tubulin are among the most widely used chemotherapeutic agents; however, the development of multidrug resistance has limited their clinical activity. We report the synthesis and biological properties of a series of novel 3-chloro-β-lactams and 3,3-dichloro-β-lactams (2-azetidinones) that are structurally related to the tubulin polymerisation inhibitor and vascular targeting agent, Combretastatin A-4. These compounds were evaluated as potential tubulin polymerisation inhibitors and for their antiproliferative effects in breast cancer cells. A number of the compounds showed potent activity in MCF-7 breast cancer cells, e.g., compound 10n (3-chloro-4-(3-hydroxy-4-methoxy-phenyl)-1-(3,4,5-trimethoxyphenyl)azetidin-2-one) and compound 11n (3,3-dichloro-4-(3-hydroxy-4-methoxyphenyl)-1-(3,4,5-trimethoxyphenyl)-azetidin-2-one), with IC50 values of 17 and 31 nM, respectively, and displayed comparable cellular effects to those of Combretastatin A-4. Compound 10n demonstrated minimal cytotoxicity against non-tumorigenic HEK-293T cells and inhibited the in vitro polymerisation of tubulin with significant G2/M phase cell cycle arrest. Immunofluorescence staining of MCF-7 cells confirmed that β-lactam 10n caused a mitotic catastrophe by targeting tubulin. In addition, compound 10n promoted apoptosis by regulating the expression of pro-apoptotic protein BAX and anti-apoptotic proteins Bcl-2 and Mcl-1. Molecular docking was used to explore the potential molecular interactions between novel 3-chloro-β-lactams and the amino acid residues of the colchicine binding active site cavity of β-tubulin. Collectively, these results suggest that 3-chloro-2-azetidinones, such as compound 10n, could be promising lead compounds for further clinical anti-cancer drug development.

Super-Resolution Fluorescence Imaging of Arabidopsis thaliana Transfer Cell Wall Ingrowths using Pseudo-Schiff Labelling Adapted for the Use of Different Dyes

To understand plant growth and development, it is often necessary to investigate the organization of plant cells and plant cell walls. Plant cell walls are often fluorescently labeled for confocal imaging with the dye propidium iodide using a pseudo-Schiff reaction. This reaction binds free amine groups on dye molecules to aldehyde groups on cellulose that result from oxidation with periodic acid. We tested a range of fluorescent dyes carrying free amine groups for their ability to act as pseudo-Schiff reagents. Using the low-pH solution historically used for the Schiff reaction, these alternative dyes failed to label cell walls of Arabidopsis cotyledon vascular tissue as strongly as propidium iodide but replacing the acidic solution with water greatly improved fluorescence labeling. Under these conditions, rhodamine-123 provided improved staining of plant cell walls compared to propidium iodide. We also developed protocols for pseudo-Schiff labeling with ATTO 647N-amine, a dye compatible for super-resolution Stimulated Emission Depletion (STED) imaging. ATTO 647N-amine was used for super-resolution imaging of cell wall ingrowths that occur in phloem parenchyma transfer cells of Arabidopsis, structures whose small size is only slightly larger than the resolution limit of conventional confocal microscopy. Application of surface-rendering software demonstrated the increase in plasma membrane surface area as a consequence of wall ingrowth deposition and suggests that STED-based approaches will be useful for more detailed morphological analysis of wall ingrowth formation. These improvements in pseudo-Schiff labeling for conventional confocal microscopy and STED imaging will be broadly applicable for high-resolution imaging of plant cell walls.

Functionalization of 3-chloroformylcoumarin to coumarin Schiff bases using reusable catalyst: an approach to molecular docking and biological studies

Recently, heterogeneous catalysts have been explored eximiously in the synthesis of heterocyclic compounds. Therefore, here we used solid-supported heterogeneous silica sulfuric acid as a catalyst for the synthesis of Schiff's base of 3-chloroformylcoumarin in view of simplified procedure, reusability and acceptable efficiency, which are required in organic synthesis. An efficient and facile methodology is preferred for synthesis of a class of chromeno-3-substituted derivatives (1a-1l) with good yields. The molecular docking results showed excellent binding interactions with the Mycobacterium tuberculosis InhA-D148G mutant (PDB: 4DQU). The same biomolecules were screened for their in vitro anti-tubercular activity against the M.tb H37Rv strain and antimicrobial studies. Physico-chemistry, toxicity prediction with IC50 value and bioactivity score were also calculated for title compounds. Most active compounds were further tested for cytotoxicity studies and exhibited low-level cytotoxicity against Vero cells. The suggested conjugates are promising lead compounds for the subsequent investigation in search of new anti-tubercular agents. All the conjugates were obtained within the range and followed the Lipinski rule of 5, indicating more 'drug-like' nature.

Metal Complexation Properties of Schiff Bases Containing 1,3,5-Triazine Derived from 2-Hydroxy-1-Naphthaldehyde in Solution. A Simple Spectrofluorimetric Method to Determine Mercury (II)

Four new Schiff base ligands carrying naphthalene groups were prepared from the reaction of 2,4-diamino-6-methyl-1,3,5-triazine and 2,4-diamino-6-undecyl-1,3,5-triazine with 2-hydroxy-1-naphthaldehyde. The influence of a series of metal ions including Cu²⁺, Co²⁺, Hg²⁺, Al³⁺, Cr³⁺, Fe³⁺, Pb²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Mn²⁺, Ag⁺, Ba²⁺, Ca²⁺ and Mg²⁺ on the spectroscopic properties of the ligands was investigated by means of absorption and emission spectrometry. The results of spectrophotometric and spectrofluorimetric titrations disclosed the complexation stoichiometry and complex stability constant of the ligands with metal ions. A simple spectrofluorimetric method was developed using the Schiff base derived from 2,4-diamino-6-undecyl-1,3,5-triazine to determine Hg²⁺ ion. No cleanup or enrichment of the tap water sample was required. A modified standard addition method was used to eliminate matrix effect. The standard addition graph was linear between 0.2 and 2.6 mg/L in determination of Hg²⁺. Detection and quantification limits were 0.08 and 0.23 mg/L, respectively. The simple and cost-effective method can be applied to water samples.