Structure-property relationship in functionalized azobenzene photoswitches and their supramolecular behavior

Herein, we report the design, synthesis, and supramolecular behavior of 30 structurally diverse photoresponsive azobenzene molecular systems. To establish structure-property relationships, azobenzenes appended with N-picolinyl and/or N-benzyl groups tethered directly through carboxamides or via triazolylmethyl carboxamide linkages were explored. We have evaluated the photoswitching characteristics and thermal stability of the Z isomers through systematic studies. All the targets were also screened for their aggregation behavior and supramolecular aspects. Among all the derivatives, a few carboxamide-based systems formed microcrystals upon aggregation, showing light responsiveness. In contrast, the derivatives tethered via triazolylmethyl carboxamide linkage exhibited hydrogel formation with excellent water-absorbing capacity. All supramolecular aspects of the morphology of the microcrystal and hydrogel states and their stimuli-responsiveness have been studied using spectroscopy and various microscopic techniques.

Emerging Aspects of Triazole in Organic Synthesis: Exploring its Potential as a Gelator

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC) - commonly known as the "click reaction" - serves as the most effective and highly reliable tool for facile construction of simple to complex designs at the molecular level. It relates to the formation of carbon heteroatomic systems by joining or clicking small molecular pieces together with the help of various organic reactions such as cycloaddition, conjugate addition, ring-opening, etc. Such dynamic strategy results in the generation of triazole and its derivatives from azides and alkynes with three nitrogen atoms in the five-membered aromatic azole ring that often forms gel-assembled structures having gelating properties. These scaffolds have led to prominent applications in designing advanced soft materials, 3D printing, ion sensing, drug delivery, photonics, separation, and purification. In this review, we mainly emphasize the different mechanistic aspects of triazole formation, which includes the synthesis of sugar-based and non-sugar-based triazoles, and their gel applications reported in the literature for the past ten years, as well as the upcoming scope in different branches of applied sciences.

Growing Impact of Intramolecular Click Chemistry in Organic Synthesis

Click Chemistry, a modular, rapid, and one of the most reliable tool for the regioselective 1,2,3-triazole forming [3+2] reaction of organic azide and terimal alkyne is widely explored in various emerging domains of research ranging from chemical biology to catalysis and medicinal chemistry to material science. This regioselective reaction from a diverse range of azido-alkyne scaffolds has been well performed in both intermolecular as well as intramolecular fashions. In comparison to the intermolecular metal (Cu/Ru/Ni) variant of 'Click Chemistry', the intramolecular click tool is little addressed. The intramolecular click chemistry is exemplified as a mordern tool of cyclization which involves metal-catalyzed (CuAAC/RuAAC) cyclization, organo-catalyzed cyclization, and thermal-induced topochemical reaction. Thus, we report herein the recent approaches on intramolecular azide-alkyne cycloaddition 'Click Chemistry' with their wide-spread emerging applications in the developement of a diverse range of molecules including fused-heterocycles, well-defined peptidomemics, and macrocyclic architectures of various notable features.

Synthesis and Linker-Controlled Self-Assembly of Dendritic Amphiphiles with Branched Fluorinated Tails

Amphiphiles containing fluorinated segments tend to aggregate in the aqueous solution into structure of lower curvature than their hydrocarbon analogs due to their larger diameter. A benefit of supramolecular structures incorporating fluorine moieties is their high electron density, which can be viewed in cryo-TEM with better contrast than their hydrogenated forms. A modular approach has been developed for the synthesis of a new family of nonionic branched amphiphiles consisting of oligoglycerol units (G2) as the hydrophilic part and a branched fluorinated (F27) hydrophobic part. The design of this hydrophobic moiety allows to achieve a higher fluorine density than the previously used straight-chain perfluoroalkanes. Two different chemical approaches, amide, and triazole, are used to link the hydrophilic and hydrophobic segments. In addition, the aggregation behavior is investigated by dynamic light scattering (DLS) and cryo-TEM. The measurements prove the formation of multivesicular (MVVs) and multilamellar (MLVs) vesicles as well as smaller unilamellar vesicles. Further, the cell viability test proves the low cell toxicity of these nanoarchitectures for potential biomedical applications.

Designer peptides as versatile building blocks for functional materials

Peptides and pseudopeptides show distinct self-assembled nanostructures such as fibers, nanotubes, vesicles, micelles, toroids, helices and rods. The formation of such molecular communities through the collective behavior of molecules is not fully understood at a molecular level. All these self-assembled nanostructured materials have a wide range of applications such as drug delivery, gene delivery, biosensing, bioimaging, catalysis, tissue engineering, nano-electronics and sensing. Self-assembly is one of the most efficient and a simple strategy to generate complex functional materials. Owing to its significance, the last few decades witnessed a remarkable advancement in the field of self-assembling peptides with a plethora of new designer synthetic systems being discovered. These systems range from amphiphilic, cyclic, linear and polymeric peptides. This article presents only selected examples of such self-assembling peptides and pseudopeptides.

Structure-activity relationship and molecular modelling studies of quinazolinedione derivatives MMV665916 as potential antimalarial agent

A series of new quinazolinedione derivatives have been readily synthesized and evaluated for their in vitro antiplasmodial growth inhibition activity. Most of the compounds inhibited P. falciparum FcB1 strain in the low to medium micromolar concentration. The 2-ethoxy 8ag', 2-trifluoromethoxy 8ai' and 4-fluoro-2-methoxy 8ak' showed the best inhibitory activity with EC₅₀ values around 5 µM and were non-toxic to the primary human fibroblast cell line AB943. However, these compounds were less potent than the original hit MMV665916, which showed remarkable growth inhibition with EC₅₀ value of 0.4 µM and presented the highest selectivity index (SI > 250). In addition, a novel approach for determining the docking poses of these quinazolinedione derivatives with their potential protein target, the P. falciparum farnesyltransferase PfFT, was investigated.

Evaluation of Amide Bioisosteres Leading to 1,2,3-Triazole Containing Compounds as GPR88 Agonists: Design, Synthesis, and Structure-Activity Relationship Studies

The orphan receptor GPR88 has been implicated in a number of striatal-associated disorders, yet its endogenous ligand has not been discovered. We have previously reported that the amine functionality in the 2-AMPP-derived GPR88 agonists can be replaced with an amide (e.g., 4) without losing activity. Later, we have found that the amide can be replaced with a bioisosteric 1,3,4-oxadiazole with improved potency. Here, we report a further study of amide bioisosteric replacement with a variety of azoles containing three heteroatoms, followed by a focused structure-activity relationship study, leading to the discovery of a series of novel 1,4-disubstituted 1H-1,2,3-triazoles as GPR88 agonists. Collectively, our medicinal chemistry efforts have resulted in a potent, efficacious, and brain-penetrant GPR88 agonist 53 (cAMP EC50 = 14 nM), which is a suitable probe to study GPR88 functions in the brain.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Silver Mediated Banert Cascade with Carbon Nucleophiles

The Banert cascade of propargylic azides can be promoted by simple silver salts, and the triazafulvene intermediate can be intercepted by carbon nucleophiles. Various indoles (>25 examples, up to 92% yield) and electron-rich heterocycles were effective. The Mayr nucleophilicity parameter (N) was found to correlate to the reaction efficiency, which enabled the formation of C_sp³-C_sp² and C_sp³-C_sp³ bonds under otherwise identical conditions from structurally dissimilar nucleophiles.

Enantioselective Nickel-Catalyzed Alkyne-Azide Cycloaddition by Dynamic Kinetic Resolution

The triazole heterocycle has been widely adopted as an isostere for the amide bond. Many native amides are α-chiral, being derived from amino acids. This makes α-N-chiral triazoles attractive building blocks. This report describes the first enantioselective triazole synthesis that proceeds via nickel-catalyzed alkyne-azide cycloaddition (NiAAC). This dynamic kinetic resolution is enabled by a spontaneous [3,3]-sigmatropic rearrangement of the allylic azide. The 1,4,5-trisubstituted triazole products, derived from internal alkynes, are complementary to those commonly obtained by the related CuAAC reaction. Initial mechanistic experiments indicate that the NiAAC reaction proceeds through a monometallic Ni complex, which is distinct from the CuAAC manifold.

Circularly polarized luminescent systems fabricated by Tröger's base derivatives through two different strategies

The Tröger's base derivative rac-TBPP was synthesized and separated into two enantiomers R 2N -TBPP and S 2N -TBPP by chiral column chromatography. These compounds show a strong circularly polarized luminescence with g lum values of +0.0021, and -0.0025, respectively. The second way to fabricate the rac-TBPP-based CPL-active material is to co-gel the fluorescent rac-TBPP with a chiral ᴅ-glutamic acid gelator DGG by co-assembly strategy. At the molar ratio of rac-TBPP/DGG = 1:80, the g lum value of the co-gel was about three times higher than the g lum values of R 2N -TBPP and S 2N -TBPP enantiomers. Interestingly, the CPL handedness of the rac-TBPP/DGG co-gel could be adjusted effectively by changing their stoichiometric ratios.

Amide Bond Bioisosteres: Strategies, Synthesis, and Successes

The amide functional group plays a key role in the composition of biomolecules, including many clinically approved drugs. Bioisosterism is widely employed in the rational modification of lead compounds, being used to increase potency, enhance selectivity, improve pharmacokinetic properties, eliminate toxicity, and acquire novel chemical space to secure intellectual property. The introduction of a bioisostere leads to structural changes in molecular size, shape, electronic distribution, polarity, pK_a, dipole or polarizability, which can be either favorable or detrimental to biological activity. This approach has opened up new avenues in drug design and development resulting in more efficient drug candidates introduced onto the market as well as in the clinical pipeline. Herein, we review the strategic decisions in selecting an amide bioisostere (the why), synthetic routes to each (the how), and success stories of each bioisostere (the implementation) to provide a comprehensive overview of this important toolbox for medicinal chemists.

Ultrashort Peptide Self-Assembly: Front-Runners to Transport Drug and Gene Cargos

The translational therapies to promote interaction between cell and signal come with stringent eligibility criteria. The chemically defined, hierarchically organized, and simpler yet blessed with robust intermolecular association, the peptides, are privileged to make the cut-off for sensing the cell-signal for biologics delivery and tissue engineering. The signature service and insoluble network formation of the peptide self-assemblies as hydrogels have drawn a spell of research activity among the scientists all around the globe in the past decades. The therapeutic peptide market players are anticipating promising growth opportunities due to the ample technological advancements in this field. The presence of the other organic moieties, enzyme substrates and well-established protecting groups like Fmoc and Boc etc., bring the best of both worlds. Since the large sequences of peptides severely limit the purification and their isolation, this article reviews the account of last 5 years' efforts on novel approaches for formulation and development of single molecule amino acids, ultra-short peptide self-assemblies (di- and tri- peptides only) and their derivatives as drug/gene carriers and tissue-engineering systems.

Sulfonamide as amide isostere for fine-tuning the gelation properties of physical gels

(S)-2-Stearamidopentanedioic acid (C18-Glu) is a known LMW gelator that forms supramolecular gels in a variety of solvents. In this work, we have carried out the isosteric substitution of the amide group by a sulfonamide moiety yielding the new isosteric gelator (S)-2-(octadecylsulfonamido)pentanedioic acid (Sulfo-Glu). The gelation ability and the key properties of the corresponding gels were compared in terms of gelation concentration, gel-to-sol transition temperature, mechanical properties, morphology, and gelation kinetics in several organic solvents and water. This comparison was also extended to (S)-2-(4-hexadecyl-1H-1,2,3-triazol-4-yl)pentanedioic acid (Click-Glu), which also constitutes an isostere of C18-Glu. The stabilizing interactions were explored through computational calculations. In general, Sulfo-Glu enabled the formation of non-toxic gels at lower concentrations, faster, and with higher thermal-mechanical stabilities than those obtained with the other isosteres in most solvents. Furthermore, the amide-sulfonamide isosteric substitution also influenced the morphology of the gel networks as well as the release rate of an embedded antibiotic (vancomycin) leading to antibacterial activity in vitro against Staphylococcus aureus.

Diaminomaleonitrile-functionalized gelators in F−/CN− sensing, phase-selective gelation, oil spill recovery and dye removal from water

Diamiomaleonitrile-based gelators 1 and 2 were designed and synthesized. Toluene and 1,2-dichlorobenzene gels of 1 and 2, respectively sense F⁻ and CN⁻ anions. Both 1 and 2 show phase-selective gelation (PSG) with distinguishable feature. Toluene gel of 1 acts as injectable material and is applied in environmental remediation.

Photoredox-Mediated Remote C(sp3)-H Heteroarylation of N-Alkyl Sulfonamides

A Minisci-type δ-selective C(sp³)-H heteroarylation of sulfonyl-protected primary aliphatic amines with N-heteroarenes under photoredox-catalyzed conditions was developed. The reaction typically uses a slight excess of amine reactant. The use of benziodoxole acetate (BI-OAc) oxidant and hexafluoroisopropanol solvent is critical to achieve high yield. Besides methylene C-H bonds, heteroarylation reactions of δ methyl C-H bonds also worked under more forced conditions. The reactions show a broad scope for both amine and N-heteroarene substrates, offering a straightforward method for synthesis of complex δ-heteroarylalkylmines from simple precursors.

Theoretical and experimental studies of an oseltamivir-triazole-based thermoresponsive organogel

Low-molecular weight organic gelators have been of significant interest in recent years because of their interesting properties and potential applications in sensing technology, biomedicine and drug delivery. Herein, the synthesis, characterization and gelation properties of new oseltamivir conjugates are reported. The oseltamivir-triazole conjugate 1 was synthesized via a click-reaction in a 75% yield. The key features of this conjugate include the presence of amide, flexible ester linkages and a triazole scaffold linking a hydrophobic alkyl chain. The conjugate 1, possessing a long alkyl chain, showed gelation properties in various apolar organic solvents. This gelation behavior was not observed in the case of the deesterified conjugate 2; this indicated the necessity of the alkyl chain for gelation. The gelator 1 showed thermoreversible gelation properties in a range of linear alkane solvents (from n-pentane to n-dodecane). A scanning electron microscopic study suggests that the gelator 1 exists as cross-linked structures, which are self-aggregated in the range of submicrometers, as supported by extensive 1H-NMR studies. The rheological parameters supported the occurrence of a soft gelation process, and the gel formed in n-decane was found to be stiffer than that formed in n-hexane. Computational studies suggested that the gelation behavior was indeed due to micelle formation and dependent on the lipophilicity of solvents.

Expanding the limits of amide-triazole isosteric substitution in bisamide-based physical gels

Gelation of organic solvents using N,N'-((1S,2S)-cyclohexane-1,2-diyl)didodecanamide (C12-Cyc) is driven by its self-assembly via antiparallel hydrogen bonds and van der Waals intermolecular interactions. In this work we carried out a dual isosteric substitution of the two amide groups with 1,2,3-triazole rings affording the corresponding isosteric gelator (click-C12-Cyc). A detailed comparative study in terms of the gelation ability and gel properties demonstrated that the 1,2,3-triazoles can take over all of the functions derived from the amide groups offering a versatile strategy for tuning the properties of the corresponding gels. This is not an obvious outcome because the directional amide groups in C12-Cyc constitute the source of the hydrogen bonds to build the 3D self-assembled network. Furthermore, theoretical calculations revealed that click-C12-Cyc can adopt a wide variety of interacting patterns, whose relative stability depends on the polarity of the environment, this is in good agreement with the experimental data obtained regarding its gelation ability. Other important features of click-C12-Cyc for potential practical applications are its non-cytotoxic character and its phase-selective gelation of water-oil mixtures.

5-(1H-1,2,3-Triazol-5-yl)isophthalic Acid: A Versatile Ligand for the Synthesis of New Supramolecular Metallogels

The gelation ability of 5-(1H-1,2,3-triazol-5-yl)isophthalic acid (click-TIA) in the presence of different metal acetates has been studied in different solvents and ligand/metal ratios. This manuscript is focused on the metallogel obtained from the combination of click-TIA and copper(II) acetate, which has been used as a model system in terms of characterization and gelation studies. Sonication treatment of the initial mixture of compounds and the nature of the counter anion were found to be critical factors for the supramolecular assembly of the metal/click-TIA complexes and, hence, for the formation of stable and homogeneous metallogels. The gel materials have been characterized with a variety of techniques including infrared, rheology, UV-vis spectroscopy, powder X-ray diffraction, and scanning electron microscopy.

4-Hydroxybenzaldehyde derived Schiff base gelators: case of the sustainability or rupturing of imine bonds towards the selective sensing of Ag+and Hg2+ionsviasol–gel methodology

The cholesterol appended hydroxybenzaldehyde derived Schiff bases1–4have been designed and synthesized. They are suitable for the naked-eye detection of metal ions such as Hg²⁺and Ag⁺using sol–gel methodology involving either rupturing or maintaining the imine bonds.

Triazole-amide isosteric pyridine-based supramolecular gelators in metal ion and biothiol sensing with excellent performance in adsorption of heavy metal ions and picric acid from water

Pyridine-based gelators 1–4 of triazole-amide isosteric relationship have been considered in metal ion sensing, heavy metal and picric acid adsorption from water. The change from triazole to isosteric amide has marked effect on gelling properties of the gelators.

Low molecular weight supramolecular dehydroepiandrosterone-based gelators: synthesis and molecular modeling study

Three novel isomeric low molecular weight dehydroepiandrosterone-based gelators are synthesized. Their ability to self-assemble is studied in several solvents both experimentally and theoretically by molecular modeling.

Use of a bis-1,2,3-triazole gelator for the preparation of supramolecular metallogels and stabilization of gold nanoparticles

In this work, a series of functional metallogels have been prepared using a bis-1,2,3-triazole gelator prepared using the isosteric substitution method.

Stoichiometry-controlled inversion of circularly polarized luminescence in co-assembly of chiral gelators with an achiral tetraphenylethylene derivative

A supramolecular circularly polarized luminescence (CPL) system was constructed based on the co-gelation of an achiral tetraphenylethylene derivative and chiral organic gelators of glutamic acid in chloroform. And the handedness of CPL can be inverted by stoichiometric ratio.

Synthesis and supramolecular self-assembly of glutamic acid-based squaramides

We describe the preparation and characterization of two new unsymmetrical squaramide-based organogelators. The synthesis of the compounds was carried out by subsequent amine condensations starting from dimethyl squarate. The design of the gelators involved a squaramide core connected on one side to a long aliphatic chain and on the other side to a glutamic acid residue. The gelator bearing the free carboxylic groups showed a lower gelation capacity than its precursor diester derivative. Some selected gels were further studied by infrared spectroscopy, rheology and electron microscopy. Critical gelation concentrations and gel-to-sol transition temperatures were also determined for each case. In addition, the superior squaramide diester gelator was compared with an analogue triazole-based gelator in terms of critical gelation concentration, gelation kinetics and thermal phase transition.

Release of small bioactive molecules from physical gels

Pharmaceutical drugs with low water solubility have always received great attention within the scientific community. The reduced bioavailability and the need of frequent administrations have motivated the investigation of new drug delivery systems. Within this context, drug carriers that release their payload in a sustained way and hence reduce the administration rate are highly demanded. One interesting strategy to meet these requirements is the entrapment of the drugs into gels. So far, the most investigated materials for such drug-loaded gels are derived from polymers and based on covalent linkages. However, over the last decade the use of physical (or supramolecular) gels derived from low molecular weight compounds has experienced strong growth in this field, mainly due to important properties such as injectability, stimuli responsiveness and ease of synthesis. This review summarizes the use of supramolecular gels for the encapsulation and controlled release of small therapeutic molecules.

Pyridine/pyridinium symmetrical bisamides as functional materials: aggregation, selective sensing and drug release

We report the design, synthesis and gelation behavior of some 3-amino pyridine/pyridinium-based bisamides. As an application, gels are useful in the visual detection of cations, anions, biomolecules and in drug release.

Diaminomalenonitrile-decorated cholesterol-based supramolecular gelator: aggregation, multiple analyte (hydrazine, Hg2+ and Cu2+) detection and dye adsorption

A low molecular weight gelator (LMWG) containing a diaminomalenonitrile functional group 1 forms supramolecular gels from DMF–H₂O and 1,2-dichlorobenzene. The DMF/H₂O gel is multi-analyte responsive (Hg²⁺, Cu²⁺ and hydrazine) with practical applications in dye adsorption from water.

Gelatinizing oil in water and its removal via bacteria inhabiting the gels

When crude oil samples were shaken (200 rpm) in seawater samples from the Arabian Gulf at 30 °C, usually oil-gels were produced spontaneously leaving the water quite clear. The gelators could probably be based on cholesteryl derivatives. Microscopic examination of the established gels revealed nanofibrellar structures similar to those described by earlier workers for artificially synthesized gelators. Communities of bacteria including prosthetic and stalked members as well as oil-degrading bacteria were recorded in such gels. Chemical analysis revealed that 88.5% of the oil entrapped by gelation was biodegraded after 40 days at 30 °C. Individual bacterial species isolated from the oil-gels biodegraded in batch cultures between 17.8 and 33.3% of the oil added at time zero in 12 days at 30 °C. Gelation is a promising approach, not only for clean, physical removal of oil spilled in aquatic habitats, as so far suggested, but also in its effective microbiological biodegradation, as the current study revealed.

Scaleable two-component gelator from phthalic acid derivatives and primary alkyl amines: acid-base interaction in the cooperative assembly

A series of phthalic acid derivatives (P) with a carbon-chain tail was designed and synthesized as single-component gelators. A combination of the single-component gelator P and a non-gelling additive n-alkylamine A through acid-base interaction brought about a series of novel phase-selective two-component gelators PA. The gelation capabilities of P and PA, and the structural, morphological, thermo-dynamic and rheological properties of the corresponding gels were investigated. A molecular dynamics simulation showed that the H-bonding network in PA formed between the NH of A and the carbonyl oxygen of P altered the assembly process of gelator P. Crude PA could be synthesized through a one-step process without any purification and could selectively gel the oil phase without a typical heating-cooling process. Moreover, such a crude PA and its gelation process could be amplified to the kilogram scale with high efficiency, which offers a practical economically viable solution to marine oil-spill recovery.

Synthesis and characterization of amide linked triazolyl glycolipids as molecular hydrogelators and organogelators

Eighteen new glycolipids were synthesized and characterized. All eighteen are effective molecular gelators for at least one solvent and eleven are hydrogelators at concentrations of 0.15–1.0 wt%. The hydrogels are suitable carriers for sustained release of chloramphenicol.

Cholesterol appended bis-1,2,3-triazoles as simple supramolecular gelators for the naked eye detection of Ag+, Cu2+ and Hg2+ ions

Cholesterol coupled bis-1,2,3-triazoles have been designed and synthesized. Their gelation abilities and cation responsive behaviors are documented.

Tweaking of the supramolecular gelation properties of a dipeptide based ambidextrous organogelator through the cooperative influence of hydrophobicity, steric bulk and conformational flexibility of the side chain residue of a single hydrophobic α-amino acid encrypted on a designed molecular frame

Fine tuning of gelation behavior via singular alteration of hydrophobic C_α-amino acid on the backbone of a dipeptide based ambidextrous organogelator.