N,N'-diarylsquaramides: general, high-yielding synthesis and applications in colorimetric anion sensing

Unlocking Photocatalytic Activity of Acridinium Salts by Anion-Binding Co-Catalysis

The in situ generation of active photoredox organic catalysts upon anion-binding co-catalysis by making use of the ionic nature of common photosensitizers is reported. Hence, the merge of anion-binding and photocatalysis permitted the modulation of the photocatalytic activity of simple acridinium halide salts, building an effective anion-binding - photoredox ion pair complex able to promote a variety of visible light driven transformations, such as anti-Markovnikov addition to olefins, Diels-Alder and the desilylative C-C bond forming reactions. Anion-binding studies, together with steady-state and time-resolved spectroscopy analysis, supported the postulated ion pair formation between the thiourea hydrogen-bond donor organocatalyst and the acridinium salt, which proved essential for unlocking the photocatalytic activity of the photosensitizer.

Squaramide-based receptors in anion supramolecular chemistry: insights into anion binding, sensing, transport and extraction

Over the last 15 years, squaramide-based receptors have attracted the attention of supramolecular chemists working in the field of anion recognition. Herein, we highlight examples of squaramide-based receptors that are able to bind, sense, extract and transport anions.

Anion-Binding Properties of Aliphatic Symmetric Squaramide Receptors

Squaramides (SQs), which are very popular for their H-bonding ability, have attracted great interest due to their wide range of applications such as asymmetric synthesis, pharmacology, and anion transportation. In this study, aliphatic symmetric SQs based on cis/trans-1,2-diaminocyclohexane (DACH) substituted with cyclic tertiary amines, synthesized in four steps under simple reaction conditions, were investigated for the first time for their ability to bind Cl-, Br-, and I- anions. The changes in cis/trans geometric isomers and the cyclic ring (pyrrolidine vs piperidine) were found to have a combined effect on the degree of anion binding. The spectroscopic titrations of the SQs with TBA-Cl, TBA-Br, and TBA-I in the range of 0.2 to 20.0 equiv were monitored by 1H NMR, and the analyses of the magnitude of chemical shift differences in the NH peaks of the SQs in course of titration were performed by DynaFit and BindFit programs for the calculation of their Ka values. All symmetric SQs I-IV were found to selectively bind Cl- anion more strongly than Br- anion to varying degrees depending on the SQ derivatives. Especially, SQ IV, which has a symmetric trans-DACH and a pyrrolidine ring, was found to have the highest Cl- anion-binding ability compared to the other SQs. However, the SQs did not show any change in the chemical shift of the NH proton in 1H NMR upon successive addition of TBA-I, indicating that they do not interact with I- anion. The stoichiometries of the complexation behavior of SQs I-IV toward Cl- and Br- anions were also analyzed by Job plots.

Synthesis and evaluation of druglike parameters via in silico techniques for a series of heterocyclic monosquarate-amide derivatives as potential carboxylic acid bioisosteres

Herein, we present a synthetic compound library comprising of 13 structurally diverse heterocyclic monosquarate-amide derivatives. The compounds featured in this library were designed as potential bioisosteric replacements carboxylic acid moiety's. A good selection of the compounds presented exhibit unique molecular architecture and have shown promising results following in silico evaluation of 'druglike properties' using Swiss ADME. The research presented in this work focuses on the preparation of derivatives of 3,4-dihydroxycyclobut-3-ene-1,2-dione, a known carboxylic acid bioisostere.

Oxocarbon Acids and their Derivatives in Biological and Medicinal Chemistry

The biological and medicinal chemistry of the oxocarbon acids 2,3- dihydroxycycloprop-2-en-1-one (deltic acid), 3,4-dihydroxycyclobut-3-ene-1,2-dione (squaric acid), 4,5-dihydroxy-4-cyclopentene-1,2,3-trione (croconic acid), 5,6-dihydroxycyclohex- 5-ene-1,2,3,4-tetrone (rhodizonic acid) and their derivatives is reviewed and their key chemical properties and reactions are discussed. Applications of these compounds as potential bioisosteres in biological and medicinal chemistry are examined. Reviewed areas include cell imaging, bioconjugation reactions, antiviral, antibacterial, anticancer, enzyme inhibition, and receptor pharmacology.

Carbonylation as a Key Step in New Tandem Reactions - A Route to BODIPYs

Herein, a highly efficient five-step reaction sequence to BODIPYs is presented. The key step is the combination of transition metal-catalyzed in-situ generation of aldehydes and their subsequent organocatalytic activation to yield dipyrromethanes, which are further converted to the corresponding BODIPY. Classic syntheses towards BODIPYs have relied on aldehydes or acid chlorides, which are often not commercially available and rather sensitive to handle. The presented approach starts from readily available and stable alkenes or aryl-bromides, which allows to extend the range of readily available BODIPYs that can be tailored for their specific use. The synthesis of 55 derivatives with overall yields of up to 78 % demonstrates the wide applicability and advantages of the presented method.

From Squaric Acid Amides (SQAs) to Quinoxaline-Based SQAs─Evolution of a Redox-Active Cathode Material for Organic Polymer Batteries

The search for new redox-active organic materials (ROMs) is essential for the development of sustainable energy-storage solutions. In this study, we present a new class of cyclobuta[b]quinoxaline-1,2-diones or squaric acid quinoxalines (SQXs) as highly promising candidates for ROMs featuring exceptional stability and high redox potentials. While simple 1,2- and 1,3-squaric acid amides (SQAs), initially reported by Hünig and coworkers decades ago, turned out to exhibit low stability in their radical cation oxidation states, we demonstrate that embedding the nitrogen atoms into a quinoxaline heterocycle leads to robust two-electron SQX redox systems. A series of SQX compounds, as well as their corresponding radical cations, were prepared and fully characterized, including EPR spectroscopy, UV-vis spectroscopy, and X-ray diffraction. Based on the promising electrochemical properties and high stability of the new ROM, we developed SQX-functionalized polymers and investigated their physical and electrochemical properties for energy-storage applications. These polymers showed remarkable thermal stability well above 200 °C with reversible redox properties and potentials of about 3.6 V vs Li+/Li. By testing the galvanostatic cycling performance in half-cells with lithium-metal counter electrodes, a styrene-based polymer with SQX redox side groups showed stable cycling for single-electron oxidation for more than 100 cycles. These findings render this new class of redox-active polymers as highly promising materials for future energy-storage applications.

Solvent-Dependent Conformational Switching of N-Methyl-N,N'-diarylsquaramide

N,N'-Diarylsquaramide and N,N'-dialkylsquaramide are conformationally stable linkers with extended (trans, trans) and folded (cis, cis) structures, respectively, independently of external conditions. Here, we show that N-monomethylated N,N'-diarylsquaramides generally take a (trans, cis) structure in the crystal but show a solvent-dependent conformational equilibrium in solution. In particular, the stable conformer of N-methyl-N,N'-bis(1-naphthyl)squaramide (1f) changes depending upon the solvent. Thus, aromatic N-monomethylated squaramides could find application as components of environment-responsive molecular switches.

Potentiometric Sensing of Nonsteroidal Painkillers by Acyclic Squaramide Ionophores

We report here a small library of a new type of acyclic squaramide receptors (L1-L5) as selective ionophores for the detection of ketoprofen and naproxen anions (KF- and NS-, respectively) in aqueous media. 1H NMR binding studies show a high affinity of these squaramide receptors toward KF- and NS-, suggesting the formation of H-bonds between the two guests and the receptors through indole and -NH groups. Compounds L1-L5 have been tested as ionophores for the detection of KF- and NS- inside solvent PVC-based polymeric membranes. The optimal membrane compositions were established through the careful variation of the ligand/tridodecylmethylammonium chloride (TDMACl) anion-exchanger ratio. All of the tested acyclic squaramide receptors L1-L5 have high affinity toward KF- and NS- and anti-Hofmeister selectivity, with L4 and L5 showing the highest sensitivity and selectivity to NS-. The utility of the developed sensors for a high precision detection of KF- in pharmaceutical compositions with low relative errors of analysis (RSD, 0.99-1.4%) and recoveries, R%, in the range 95.1-111.8% has been demonstrated. Additionally, the chemometric approach has been involved to effectively discriminate between the structurally very similar KF- and NS-, and the possibility of detecting these analytes at concentrations as low as 0.07 μM with R2 of 0.947 and at 0.15 μM with R2 of 0.919 for NS- and KF-, respectively, was shown.

Fatty Acid-Activated Proton Transport by Bisaryl Anion Transporters Depolarises Mitochondria and Reduces the Viability of MDA-MB-231 Breast Cancer Cells

In respiring mitochondria, the proton gradient across the inner mitochondrial membrane is used to drive ATP production. Mitochondrial uncouplers, which are typically weak acid protonophores, can disrupt this process to induce mitochondrial dysfunction and apoptosis in cancer cells. We have shown that bisaryl urea-based anion transporters can also mediate mitochondrial uncoupling through a novel fatty acid-activated proton transport mechanism, where the bisaryl urea promotes the transbilayer movement of deprotonated fatty acids and proton transport. In this paper, we investigated the impact of replacing the urea group with squaramide, amide and diurea anion binding motifs. Bisaryl squaramides were found to depolarise mitochondria and reduce MDA-MB-231 breast cancer cell viability to similar extents as their urea counterpart. Bisaryl amides and diureas were less active and required higher concentrations to produce these effects. For all scaffolds, the substitution of the bisaryl rings with lipophilic electron-withdrawing groups was required for activity. An investigation of the proton transport mechanism in vesicles showed that active compounds participate in fatty acid-activated proton transport, except for a squaramide analogue, which was sufficiently acidic to act as a classical protonophore and transport protons in the absence of free fatty acids.

Electroactive molecularly imprinted polymer nanoparticles for selective glyphosate determination

Redox-active molecularly imprinted polymer nanoparticles selective for glyphosate, MIP-Gly NPs, were devised, synthesized, and subsequently integrated onto platinum screen-printed electrodes (Pt-SPEs) to fabricate a chemosensor for selective determination of glyphosate (Gly) without the need for redox probe in the test solution. That was because, ferrocenylmethyl methacrylate was added to the polymerization mixtures during the NPs synthesis so that the resulting MIP-Gly NPs contained covalently immobilized ferrocenyl moieties as the reporting redox ingredient, conferring these NPs with electroactive properties. MIP-Gly NPs of four different compositions were evaluated. The herein described approach represents a simple and effective way to endow MIP NPs with electrochemical reporting capabilities with neither the need to functionalize them post-synthesis nor to use electrochemical mediators present in the tested solution during the analyte determinations. MIP-Gly NPs synthesized using allylamine and squaramide-based monomers appeared most selective to Gly. The Pt-SPEs modified with MIP-Gly NPs were characterized with differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). Changes in the DPV peak originating from the oxidation of the ferrocenyl moieties in these MIP-Gly NPs served as the analytical signal. The DPV limit of detection and the linear dynamic concentration range for Gly were 3.7 pM and 25 pM-500 pM, respectively. Moreover, the selectivity of the fabricated chemosensors was sufficiently high to determine Gly successfully in spiked river water samples.

Catalytic aza-Nazarov cyclization reactions to access α-methylene-γ-lactam heterocycles

We have developed a catalytic aza-Nazarov reaction of N-acyliminium salts generated in situ from the reaction of a variety of cyclic and acyclic imines with α,β-unsaturated acyl chlorides to afford substituted α-methylene-γ-lactam heterocycles. The reactions proceed effectively in the presence of catalytic (20 mol %) amounts of AgOTf as an anion exchange agent or hydrogen-bond donors such as squaramides and thioureas as anion-binding organocatalysts. The aza-Nazarov cyclization of 3,4-dihydroisoquinolines with α,β-unsaturated acyl chlorides gives tricyclic lactam products 7 in up to 79% yield with full diastereocontrol (dr = >99:1). The use of acyclic imines in a similar catalytic aza-Nazarov reaction with 20 mol % of AgOTf results in the formation of α-methylene-γ-lactam heterocycles 19 in up to 76% yield and with good to high diastereoselectivities (4.3:1 to 16:1). We have demonstrated the scalability of the reaction with a gram-scale example. The relative stereochemistry of the α-methylene-γ-lactam products 19 has been determined via the single-crystal X-ray analysis of lactam 19l. In order to shed light on the details of the reaction mechanism, we have performed carefully designed mechanistic studies which consist of experiments on the effect of β-silicon stabilization, the alkene geometry of the α,β-unsaturated acyl chloride reactants, and adventitious water on the success of the catalytic aza-Nazarov reaction.

Synthesis and DNase I Inhibitory Properties of New Squaramides

Three new monosquaramides (3a-c) were synthesized, characterized by IR, NMR and X-ray, and evaluated for inhibitory activity against deoxyribonuclease I (DNase I) and xanthine oxidase (XO) in vitro. The target compounds inhibited DNase I with IC₅₀ values below 100 μM, being at the same time more potent DNase I inhibitors than crystal violet, used as a positive control. 3-Ethoxy-4-((1-(pyridin-3-yl)propan-2-yl)amino)cyclobut-3-ene-1,2-dione (3c) stood out as the most potent compound, exhibiting a slightly better IC₅₀ value (48.04 ± 7.98 μM) compared to the other two compounds. In order to analyze potential binding sites for the studied compounds with DNase I, a molecular docking study was performed. Compounds 3a-c are among the most potent small organic DNase I inhibitors tested to date.

pH-Dependent transport of amino acids across lipid bilayers by simple monotopic anion carriers

The transport of amino acids across lipid membranes is vital for the proper functioning of every living cell. In spite of that, examples of synthetic transporters that can facilitate amino acid transport are rare. This is mainly because at physiological conditions amino acids predominantly exist as highly polar zwitterions and proper shielding of their charged termini, which is necessary for fast diffusion across lipophilic membranes, requires complex and synthetically challenging heteroditopic receptors. Here we report the first simple monotopic anion receptor, dithioamide 1, that efficiently transports a variety of natural amino acids across lipid bilayers at physiological pH. Mechanistic studies revealed that the receptor rapidly transports deprotonated amino acids, even though at pH 7.4 these forms account for less than 3% of the total amino acid concentration. We also describe a new fluorescent assay for the selective measurement of the transport of deprotonated amino acids into liposomes. The new assay allowed us to study the pH-dependence of amino acid transport and elucidate the mechanism of transport by 1, as well as to explain its exceptionally high activity. With the newly developed assay we screened also four other representative examples of monotopic anion transporters, of which two showed promising activity. Our results imply that heteroditopic receptors are not necessary for achieving high amino acid transport activities and that many of the previously reported anionophores might be active amino acid transporters. Based on these findings, we propose a new strategy for the development of artificial amino acid transporters with improved properties.

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

Anion-Induced Reversible Actuation of Squaramide-Crosslinked Polymer Gels

Supramolecular anion binding to squaramide crosslinkers in poly(N,N-dimethylacrylamide) gel networks enhances swelling and allows reversible chemically driven actuation. The volume swelling ratio of the gels is shown to depend on both the type of anion and its concentration. ¹H NMR and UV-vis titrations with the squaramide crosslinkers reveal a relationship between anion binding affinity and the concentration-dependent swelling behavior. Gel swelling is shown to be reversible, and by embedding a solid support into rod-shaped gels, soft actuators are fabricated that undergo forward and backward bending motion in response to changing anion concentration. The swelling and bending process, which is accompanied by intense green coloration of the gel, is achieved by using only low amounts of crosslinker. This macroscopic actuation achieved by anion binding to specific molecular entities in the polymer network will open new opportunities in the field of chemically responsive materials.

Hydrogen bond activated glycosylation under mild conditions

Herein, we report a new glycosylation system for the highly efficient and stereoselective formation of glycosidic bonds using glycosyl N-phenyl trifluoroacetimidate (PTFAI) donors and a charged thiourea hydrogen-bond-donor catalyst. The glycosylation protocol features broad substrate scope, controllable stereoselectivity, good to excellent yields and exceptionally mild catalysis conditions. Benefitting from the mild reaction conditions, this new hydrogen bond-mediated glycosylation system in combination with a hydrogen bond-mediated aglycon delivery system provides a reliable method for the synthesis of challenging phenolic glycosides. In addition, a chemoselective glycosylation procedure was developed using different imidate donors (trichloroacetimidates, N-phenyl trifluoroacetimidates, N-4-nitrophenyl trifluoroacetimidates, benzoxazolyl imidates and 6-nitro-benzothiazolyl imidates) and it was applied for a trisaccharide synthesis through a novel one-pot single catalyst strategy.

A mild glycosylation system was developed using glycosyl imidate donors and a charge-enhanced thiourea H-bond donor catalyst. The method can be used for the effective synthesis of O-, C-, S- and N-glycosides and chemoselective one-pot glycosylation.

Synthesis and mechanism of biological action of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators

Lysosomal pH is an important modulator for many cellular processes. An agent that is capable of regulating lysosomal pH may find a wide range of potential applications in the field of biomedicine. In this study, we describe the synthesis of a family of morpholinyl-bearing arylsquaramides as small-molecule lysosomal pH modulators. These compounds are able to efficiently facilitate the transmembrane transport of chloride anions as mobile carriers across vesicular and cellular phospholipid membranes. They are capable of specifically alkalizing liposomes, disrupting the homeostasis of lysosomal pH and inactivivating lysosomal Cathepsin B enzyme. Anion transport is considered as the probable mechanism of action for the high efficiency of these compounds to modulate lysosomal pH. The present findings present a novel means to efficiently regulate lysosomal pH, which is in contrast to the methods shown by conventional lysosomal pH modulators that generally function by either acting as a weak base/acid, or releasing a basic/acidic component in lysosomal environments to change lysosomal pH.

A family of small-molecule arylsquaramides were synthesized as transmembrane anion transporters for modulating lysosomal pH.

Squaramide-Based Self-Associating Amphiphiles for Anion Recognition

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using 1 H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO42- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO42- anion when compared against other biologically relevant anions.

Sensing mechanism of fluorescent sensor to Cu2+ based on inhibiting ultra-fast intramolecular proton transfer process

A novel and efficient chemosensor 1 for detecting Cu²⁺ has recently been developed. However, the photophysical properties of chemosensor 1 and its response mechanism to Cu²⁺ are still unclear. Herein, the density functional theory and the time-dependent density functional theory approaches are implemented to investigate the excited state behavior of chemosensor 1 and its sensing mechanism for Cu²⁺ is revealed. Through constructing the potential energy curve with the dihedral angle of hydroxide radical as a variable, the irreversibility of the adjustment of the hydrogen proton direction is determined. This feature provides a favorable geometric configuration condition for the formation of intramolecular hydrogen bond. Moreover, the reduced density gradient analysis and topological analysis are performed to visualize the hydrogen bond strength, it is found that the hydrogen bond is enhanced in first singlet excited state (S₁) compared with that in ground state (S₀). The chemosensor 1 has only a low potential barrier in the S₁ state, indicating that it could undergo an ultra-fast excited state intramolecular proton transfer (ESIPT) process. Furthermore, the reaction sites of chemosensor 1 and Cu²⁺ is theoretically predicted by the electrostatic potential analysis and the coordination mode of 1 + Cu²⁺-H⁺ is confirmed. Thus, we verify that the deprotonation inhibits the ESIPT behavior and leads to fluorescence quenching to achieve the recognition of chemosensor 1 to Cu²⁺. In addition, the binding energy of Cu²⁺ with chemosensor 1 is greater than that of Mg²⁺ and Zn²⁺, the high selectivity of chemosensor 1 to Cu²⁺ is illustrated. Our investigation clarifies the sensing mechanism of chemosensor 1 to Cu²⁺ based on inhibiting ultra-fast ESIPT process, which provides a theoretical basis for the development of new metal ion sensors.

Diaminomethylenemalononitriles and Diaminomethyleneindanediones as Dual Hydrogen Bond Donors for Anion Recognition

Diaminomethylenemalononitriles (DMMs) and diaminomethyleneindanediones (DMIs) are dual H-bond donors that have previously been used as organocatalysts, but their anion binding ability has not been investigated. We report the synthesis of both alkyl- and aryl-substituted DMMs and DMIs, together with a comparison of their anion binding ability with that of the analogous thioureas. The DMMs display affinity for monovalent anions, with similar anion binding affinities observed to that of the thioureas in acetonitrile, albeit with differing trends for the N,N'-dialkyl versus N,N'-diaryl compounds. In contrast, the DMIs do not bind to monovalent anions under similar conditions as a result of conformational locking through the formation of intramolecular H-bonds. This can be overcome upon addition of sulfate ions, and binding of sulfate is enhanced in a more competitive solvent (DMSO).

Design Platform for Sustainable Catalysis with Radicals: Electrochemical Activation of Cp2 TiCl2 for Catalysis Unveiled

The combination of synthesis, rotating ring-disk electrode (RRDE) and cyclic voltammetry (CV) measurements, and computational investigations with the aid of DFT methods shows how a thiourea, a squaramide, and a bissulfonamide as additives affect the Eq Cr equilibrium of Cp2 TiCl2 . We have, for the first time, provided quantitative data for the Eq Cr equilibrium and have determined the stoichiometry of adduct formation of [Cp2 Ti(III)Cl2 ]- , [Cp2 Ti(III)Cl] and [Cp2 Ti(IV)Cl2 ] and the additives. By studying the structures of the complexes formed by DFT methods, we have established the Gibbs energies and enthalpies of complex formation as well as the adduct structures. The results not only demonstrate the correctness of our use of the Eq Cr equilibrium as predictor for sustainable catalysis. They are also a design platform for the development of novel additives in particular for enantioselective catalysis.

Switching-On Fluorescence by Copper (II) and Basic Anions: A Case Study with a Pyrene-Functionalized Squaramide

The new symmetric acyclic N,N’-bis(1-pyrenyl) squaramide (H₂L) functionalized with the pyrene moiety as a fluorogenic fragment has been designed and its ability to selectively detect specific anions and metals investigated. H₂L selectively binds Cl⁻ both in solution (DMSO 0.5% H₂O and MeCN) and in the solid state, and allows to selectively detect Cu²⁺ in MeCN with the formation of a 2:1 metal-receptor complex, with a green intense emission appreciable by naked eye under the UV lamp. The H₂L copper complex preserves its emission properties in the presence of Cl⁻. The addition of basic anions (OH⁻, CN⁻, and F⁻) up to 10 equivalents caused the deprotonation of the squaramide NHs and a dramatic change of the emission properties of the H₂L copper complex.

Hydrogen Bond Donor Catalyzed Cationic Polymerization of Vinyl Ethers

The synthesis of high-molecular-weight poly(vinyl ethers) under mild conditions is a significant challenge, since cationic polymerization reactions are highly sensitive to chain-transfer and termination events. We identified a novel and highly effective hydrogen bond donor (HBD)-organic acid pair that can facilitate controlled cationic polymerization of vinyl ethers under ambient conditions with excellent monomer compatibility. Poly(vinyl ethers) of molar masses exceeding 50 kg mol-1 can be produced within 1 h without elaborate reagent purification. Modification of the HBD structure allowed tuning of the polymerization rate, while DFT calculations helped elucidate crucial intermolecular interactions between the HBD, organic acid, and polymer chain end.

Influence of the Z/E Isomerism on the Pathway Complexity of a Squaramide-Based Macrocycle

The rising interest on pathway complexity in supramolecular polymerization has prompted the finding of novel monomer designs able to stabilize kinetically trapped species and generate supramolecular polymorphs. In the present work, the exploitation of the Z/E (geometrical) isomerism of squaramide (SQ) units to produce various self-assembled isoforms and complex supramolecular polymerization pathways in methylcyclohexane/CHCl₃ mixtures is reported for the first time. This is achieved by using a new bissquaramidic macrocycle (MSq) that self-assembles into two markedly different thermodynamic aggregates, AggA (discrete cyclic structures) and AggB (fibrillar structures), depending on the solvent composition and concentration. Remarkably, UV-vis, ¹ H NMR, and FT-IR experiments together with quantum-chemical calculations indicate that these two distinct aggregates are formed via two different hydrogen bonding patterns (side-to-side in AggA and head-to-tail in AggB) due to different conformations in the SQ units (Z,E in AggA and Z,Z in AggB). The ability of MSq to supramolecularly polymerize into two distinct aggregates is utilized to induce the kinetic-to-thermodynamic transformation from AggA to AggB, which occurs via an on-pathway mechanism. It is believed that this system provides new insights for the design of potential supramolecular polymorphic materials by using squaramide units.

Construction of Aromatic Multilayered Structures Based on the Conformational Properties of N,N'-Dimethylated Squaramide

Squaramide is a highly rigid four-membered ring system bearing two carbonyl and two amino groups, and its derivatives have found applications in many fields. We synthesized several N,N'-dimethylated oligosquaramides linked via phenyl groups, and investigated their structures in the crystalline state and in solution. Compounds 1, 2 (bissquaramides), and 13 (trissquaramide) exist as folded structures in the crystalline state, in which the aromatic rings are located in a face-to-face position. In their multilayered structures, the benzene rings are more nearly parallel and are closer to each other, compared with those in N,N'-dimethylated oligoureas. Individual molecules of meta-connected compounds 2 and 13 show a helical structure with all-R or all-S axis chirality, but afford only racemic crystals. The NMR spectra indicated that these compounds retain well-ordered folded structures in solution. The unique steric and electronic properties of N,N'-dimethylated squaramide can provide access to novel functional aromatic multilayered and helical foldamers.

Electroactive Anion Receptor with High Affinity for Arsenate

Herein, we present the synthesis and characterization of a macrocyclic polyamide cage that incorporates redox-active 1,4-dithiin units. UV/vis titration experiments with eight anions in acetonitrile revealed high affinity for H₂AsO₄^- (log β₂ = 10.4_-0.4^+0.4) and HCO₃^- (log β₂ = 8.3_-0.4^+0.3) over other common anionic guests, such as Cl^- (log K_1:1 = 3.20_-0.02^+0.03), HSO₄^- (log K_1:1 = 3.57_-0.03^+0.02), and H₂PO₄^- (log K_1:1 = 4.24_-0.04^+0.05), by the selective formation of HG₂ complexes. The recognition of arsenate over phosphate is rare among both proteins and synthetic receptors, and though the origin of selectivity is not known, exploiting the difference in the binding stoichiometry represents an underexplored avenue toward developing receptors that can differentiate between the two anions. Additional analysis by ¹H NMR in 1:3 CD₂Cl₂/MeCN-d₃ found a strong dependence of anion binding stoichiometry with the solvent employed. Finally, titration experiments with cyclic voltammetry provided varying and complex responses for each anion tested, though reaction between the anion and receptors was observed in most cases. These results implicate 1,4-dithiins as interesting recognition moieties in the construction of supramolecular receptors.

Anion Binding Affinity: Acidity versus Conformational Effects

High-level quantum chemical calculations were used to elucidate the gas- and solution-phase conformational equilibria for a series of symmetrically substituted (thio)ureas, (thio)squaramides, and croconamides. Gas-phase calculations predict that the thermodynamic conformer of many of these anion receptors is not the dual-hydrogen-bond-facilitating anti-anti conformer as is commonly assumed. For N,N'-diaryl thiosquaramides and croconamides, the syn-syn conformer is typically the predominant conformer. Solution-phase calculations show that the anti-anti conformer is increasingly stabilized as the polarity of the solvent increases. However, the syn-syn conformer remains the lowest energy conformation for croconamides. These predictions are used to explain the acidity versus chloride binding affinity correlations recently reported for some of these compounds. The chloride binding constants for thioureas and croconamides are significantly lower than expected on the basis of their pK_a values, and this may be due in part to the need for these receptors to reorganize into the anti-anti conformer. Experimental NMR nuclear Overhauser effect (NOE) measurements of an asymmetrically substituted squaramide and its thio analogue are consistent with the syn-syn conformation being predominant at 298 K. The conformational equilibria should therefore be an important consideration for the design and development of future anion receptors and organocatalysts.

Synthesis, Labeling and Preclinical Evaluation of a Squaric Acid Containing PSMA Inhibitor Labeled with 68 Ga: A Comparison with PSMA-11 and PSMA-617

The L-lysine urea-L-glutamate (KuE) represents a key motif in recent diagnostic and therapeutic radiopharmaceuticals targeting the prostate specific membrane antigen (PSMA). Using a squaric acid moiety for coupling of KuE with a radioactive label, the squaric acid as a linker in the PSMA ligand seems to mimic the aromatic structure of the naphthylalanine unit on PSMA-617. In this work, we investigate the influence of squaric acid moiety on the biological activity of the compound carrying a KuE motif and three typical chelates. The derivatives TRAM.SA.KuE, DOTAGA.SA.KuE and NODAGA.SA.KuE were all synthesized in straightforward organic reactions and purified by HPLC afterward. Different amounts of tracer were labeled at different temperatures with ⁶⁸ Ga. PET examinations were performed on NMRInu/nu nude mice with an LNCaP tumor on the right hind leg including ex vivo investigations of the organs. For comparison, ⁶⁸ Ga-derivatives of PSMA-11 and PSMA-617, the derivatives most commonly used in clinics, were investigated in the same animal model.

Simple squaramide receptors for highly efficient anion binding in aqueous media and transmembrane transport

A family of acyclic squaramide receptors (L1–L5) have been synthesised with the aim to evaluate how the presence of additional H-bond donor groups on the squaramide scaffold could affect their affinity towards anions and transport ability.

Self-assembly of amphiphilic aryl-squaramides in water driven by dipolar π–π interactions

Amphiphilic aryl-squaramides self-assemble via unprecedented dipolar π–π interactions forming well-defined supramolecular aggregates and self-consistent hydrogels in water

A squaraine-based dipicolylamine derivative acting as a turn-on mercury(ii) fluorescent probe in water

A squaraine-based ligand, sbdpa, has been synthesized, and the behaviour of its metal complexes was investigated. It was found that it acts as a Hg²⁺ chemosensor in aqueous solution.

Synthesis and characterization of cyclobutenedione-bithiophene π-conjugated polymers: acetal-protecting strategy for Kumada-Tamao-Corriu coupling polymerization between aryl bromide and Grignard reagents

Cyclobutenedione is an aromatic ring that exhibits strong electron-withdrawing properties but is susceptible to undesired reactions with nucleophiles. Herein, Kumada–Tamao–Corriu coupling polymerization of a cyclobutenedione monomer whose carbonyl groups are protected as acetals was achieved. Hydrolysis of the acetals afforded donor–acceptor type π-conjugated polymers consisting of cyclobutenedione as an acceptor unit and bithiophene as a donor unit. The acetal-protected monomer was also subjected to Suzuki–Miyaura coupling polymerization. The absorption and emission spectra of the deprotected polymers shifted to the longer wavelength compared with the acetal-protected polymers.

Donor–acceptor-type π-conjugated polymers consisting of cyclobutenedione as an acceptor and bithiophene as a donor were synthesized by Kumada–Tamao–Corriu coupling polymerization of an acetal-protected monomer.

Receptors for sulfate that function across a wide pH range in mixed aqueous-DMSO media

Water soluble squaramide macrocycles (MSQs) display high sulfate binding affinities in aqueous DMSO mixtures. The introduction of pyridine spacers into the macrocycles resulted in increased sulfate binding affinity in comparison to compounds with benzene spacers. [3]MSQ 6 was found to be a selective ligand for SO42- in highly competitive conditions and over a wide pH range (3.2-14).

Organocatalytic synthesis of enantiopure spiro acenaphthyl-pyrrolizidine/pyrrolidines: justifying the regioselectivity based on a distortion/interaction model

An efficient organocatalytic [3 + 2] reaction with Schreiner's thiourea organocatalyst for the synthesis of a small library of novel enantiopure stable spiroacenaphthyl-pyrrolidines/pyrrolizidines with high regio- and diastereoselectivity (up to 99%) is described for the first time. These chiral compounds were synthesized by a three-component 1,3-dipolar cycloaddition of (E)-1-(2-oxoacenaphthylen-1(2H)-ylidene) pyrrolidin-1-ium-2-ide as a dipolar and (S)-cinnamoyl/crotonoyl oxazolidinone as a dipolarophile. The absolute configuration of cycloadducts was confirmed by X-ray diffraction analysis. The origin of catalyst reactivity and regio- and stereoselectivity was investigated through DFT calculations. DFT calculations showed that the regioselectivity was controlled by the distortion (deformation) of reactants and Schreiner's thiourea acts as a LUMO-lowering catalyst.

Condition Screening for Sustainable Catalysis in Single-Electron Steps by Cyclic Voltammetry: Additives and Solvents

Cyclic voltammetry-based screening method for Cp₂ TiX-catalyzed reactions is extended to the screening of solvents other than tetrahydrofuran for bulk electrolysis of the catalyst and radical arylation. It was found that CH₃ CN can be used as a solvent for both processes without additives. Furthermore, in tetrahydrofuran, squaramide L2 is more efficient than the previously reported supramolecular halide binder, Schreiner's thiourea L1. The results extend the usefulness of the proposed time and resource-efficient screening method for designing catalysis reactions in single-electron steps.

Phosphate and polyphosphate anion recognition by a dinuclear copper(ii) complex of an unsymmetrical squaramide

At pH = 7.4, ATP⁴⁻ is selectively removed from water containing PhPO₄²⁻, Haep⁻, AMP²⁻ and ADP³⁻ by the studied squaramide-based dicopper(ii) complex.

Fluorescent squaramide ligands for cellular imaging and their encapsulation in cubosomes

Two new fluorescent squaramides bearing quinoline (L1) and naphthalene (L2) as fluorogenic fragments were synthesized and investigated as possible cellular imaging probes as free molecules and when loaded in monoolein-based cubosomes.