α-Aminophosphonate inhibitors of metallo-ß-lactamases NDM-1 and VIM-2.

The upswing of antibiotic resistance is an escalating threat to human health. Resistance mediated by bacterial metallo-ß-lactamases is of particular concern as these enzymes degrade ß-lactams, our most frequently prescribed class of antibiotics. Inhibition of metallo-ß-lactamases could allow the continued use of existing ß-lactam antibiotics, such as penicillins, cephalosporins and carbapenems, whose applicability is becoming ever more limited. The design, synthesis, and NDM-1, VIM-2, and GIM-1 inhibitory activities (IC50 4.1-506 µM) of a series of novel non-cytotoxic α-aminophosphonate-based inhibitor candidates are presented herein. We disclose the solution NMR spectroscopic and computational investigation of their NDM-1 and VIM-2 binding sites and binding modes. Whereas the binding modes of the inhibitors are similar, VIM-2 showed a somewhat higher conformational flexibility, and complexed a larger number of inhibitor candidates in more varying binding modes than NDM-1. Phosphonate-type inhibitors may be potential candidates for development into therapeutics to combat metallo-ß-lactamase resistant bacteria.

Thiourea-Based Tritopic Halogen-Bonding Acceptors.

A series of thiourea-based tritopic receptor molecules were synthesized to be used as building blocks for halogen-bonded assemblies. Here, 16 new receptor molecules were synthesized from two different 2,4,6-trialkyl-1,3,5-tris(bromomethyl)benzene starting materials via tris(isothiocyanatomethyl)benzene intermediates. The alkyl substituents in the benzene ring proved to be important for isothiocyanate group formation instead of competing thiocyanate group. The synthesis route allowed us to synthesize the isothiocyanate intermediates and further the receptor molecules without the typically used and highly toxic thiophosgene. The synthesized receptor molecules were used to study their halogen-bond acceptor properties with diiodotetrafluorobenzene donors by single-crystal X-ray diffraction method. We were able to obtain five new crystal structures of halogen-bonded complexes, in which all receptors showed two to four accepted C-I⋅⋅⋅S halogen bonds. The observed halogen bonds were highly directional and showed large variation in C=S⋅⋅⋅I acceptor angles, indicating flexible acceptor properties of sulfur.

Self-assembly of new cobalt complexes based on [Co (SCN)4], synthesis, empirical, antioxidant activity, and quantum theory investigations.

The cobalt (II) complexes have been synthesized from the reaction of the cationic entities (3,4-dimethylaniline (1) and histamine (2)) with metallic salt CoCl2⋅6H2O and thiocyanate ion (SCN-) as a ligand in H2O/ethanolic solution and processing by the evaporation crystal growth method at room temperature to get crystals. The synthesized complex has been fully characterized by single-crystal X-ray diffraction. UV-Visible, FTIR spectroscopy, TGA analysis, and DFT circulations were also performed. The crystal structural analysis reveals that the solid (1) {[Co(SCN)4] (C8H12N)3}·Cl crystallizes in the monoclinic system with the space group P21/n and the solid (2) {[Co(SCN)4](C5H11N3)2}·2Cl crystallizes in the monoclinic space group P21/m. Metal cations are joined into corrugated chains parallel to the b-axis direction in (1) and (2) by four thiocyanate anions. The crystal structures of (1) and (2) were calculated using XRPD data, indicating that they are closely connected to the DRX mono-crystal results. Different interactions pack the system into a ring formed by N-H⋯Cl and N-H⋯S hydrogen bonds. C-H⋯π and the π⋯π stacking of anilinuim ring for (1) and N-H⋯S intermolecular interactions for (1) and (2) increase the crystals' robustness. Hirshfeld surface analysis cum 2D fingerprint plots visualize the main intermolecular interactions with their contributions in the solid-state phase. The molecular geometries of both complexes obtained from the crystal structure were used for quantum chemical calculation. Here, frontier orbital analysis and electrostatic potential illustrate the chemical reactivities of metal-organic complexes. QTAIM and NCI analysis reveal the strength of interactions at the electronic level.

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes.

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Deoxygenative Divergent Synthesis: En Route to Quinic Acid Chirons.

The installation of vicinal mesylate and silyl ether groups in a quinic acid derivative generates a system prone for stereoselective borane-catalyzed hydrosilylation through a siloxonium intermediate. The diversification of the reaction conditions allowed the construction of different defunctionalized fragments foreseen as useful synthetic fragments. The selectivity of the hydrosilylation was rationalized on the basis of deuteration experiments and computational studies.

Prenylated Flavonoids from the Roots of Tephrosia rhodesica.

Five new compounds-rhodimer (1), rhodiflavan A (2), rhodiflavan B (3), rhodiflavan C (4), and rhodacarpin (5)-along with 16 known secondary metabolites, were isolated from the CH2Cl2-CH3OH (1:1) extract of the roots of Tephrosia rhodesica. They were identified by NMR spectroscopic, mass spectrometric, X-ray crystallographic, and ECD spectroscopic analyses. The crude extract and the isolated compounds 2-5, 9, 15, and 21 showed activity (100% at 10 µg and IC50 = 5-15 µM) against the chloroquine-sensitive (3D7) strain of Plasmodium falciparum.

Synthesis of 6,12-Disubstituted Methanodibenzo[b,f][1,5]dioxocins: Pyrrolidine Catalyzed Self-Condensation of 2'-Hydroxyacetophenones.

The preparation of unprecedented 6,12-disubstituted methanodibenzo[b,f][1,5]dioxocins from pyrrolidine catalyzed self-condensation of 2'-hydroxyacetophenones is herein described. This method provides easy access to this highly bridged complex core, resulting in construction of two C-O and two C-C bonds, a methylene bridge and two quaternary centers in a single step. The intricate methanodibenzo[b,f][1,5]dioxocin compounds were obtained in up to moderate yields after optimization of the reaction conditions concerning solvent, reaction times and the use of additives. Several halide substituted methanodibenzo[b,f][1,5]dioxocins could be prepared from correspondent 2'-hydroxyacetophenones.

Chiroptical inversion of a planar chiral redox-switchable rotaxane.

A tetrathiafulvalene (TTF)-containing crown ether macrocycle with C s symmetry was designed to implement planar chirality into a redox-active [2]rotaxane. The directionality of the macrocycle atom sequence together with the non-symmetric axle renders the corresponding [2]rotaxane mechanically planar chiral. Enantiomeric separation of the [2]rotaxane was achieved by chiral HPLC. The electrochemical properties - caused by the reversible oxidation of the TTF - are similar to a non-chiral control. Reversible inversion of the main band in the ECD spectra for the individual enantiomers was observed after oxidation. Experimental evidence, conformational analysis and DFT calculations of the neutral and doubly oxidised species indicate that mainly electronic effects of the oxidation are responsible for the chiroptical switching. This is the first electrochemically switchable rotaxane with a reversible inversion of the main ECD band.

Crystal Structures and Cytotoxicity of ent-Kaurane-Type Diterpenoids from Two Aspilia Species.

A phytochemical investigation of the roots of Aspilia plurisetaled to the isolation of ent-kaurane-type diterpenoids and additional phytochemicals (1⁻23). The structures of the isolated compounds were elucidated based on Nuclear Magnetic Resonance (NMR) spectroscopic and mass spectrometric analyses. The absolute configurations of seven of the ent-kaurane-type diterpenoids (3⁻6, 6b, 7 and 8) were determined by single crystal X-ray diffraction studies. Eleven of the compounds were also isolated from the roots and the aerial parts of Aspilia mossambicensis. The literature NMR assignments for compounds 1 and 5 were revised. In a cytotoxicity assay, 12α-methoxy-ent-kaur-9(11),16-dien-19-oic acid (1) (IC50 = 27.3 ± 1.9 µM) and 9ß-hydroxy-15α-angeloyloxy-ent-kaur-16-en-19-oic acid (3) (IC50 = 24.7 ± 2.8 µM) were the most cytotoxic against the hepatocellular carcinoma (Hep-G2) cell line, while 15α-angeloyloxy-16ß,17-epoxy-ent-kauran-19-oic acid (5) (IC50 = 30.7 ± 1.7 µM) was the most cytotoxic against adenocarcinomic human alveolar basal epithelial (A549) cells.

Carbon's Three-Center, Four-Electron Tetrel Bond, Treated Experimentally.

Tetrel bonding is the noncovalent interaction of group IV elements with electron donors. It is a weak, directional interaction that resembles hydrogen and halogen bonding yet remains barely explored. Herein, we present an experimental investigation of the carbon-centered, three-center, four-electron tetrel bond, [N-C-N]+, formed by capturing a carbenium ion with a bidentate Lewis base. NMR-spectroscopic, titration-calorimetric, and reaction-kinetic evidence for the existence and structure of this species is reported. The studied interaction is by far the strongest tetrel bond reported so far and is discussed in comparison with the analogous halogen bond. The necessity of the involvement of a bidentate Lewis base in its formation is demonstrated by providing spectroscopic and crystallographic evidence that a monodentate Lewis base induces a reaction rather than stabilizing the tetrel bond complex. A vastly decreased Lewis basicity of the bidentate ligand or reduced Lewis acidity of the carbenium ion weakens-or even prohibits-the formation of the tetrel bond complex, whereas synthetic modifications facilitating attractive orbital overlaps promote it. As the geometry of the complex resembles the SN2 transition state, it provides a model system for the investigation of fundamental reaction mechanisms and chemical bonding theories.

Enantiomeric Resolution of Asymmetric-Carbon-Free Binuclear Double-Stranded Cobalt(III) Helicates and Their Application as Catalysts in Asymmetric Reactions.

A series of double-stranded binuclear helicates [Co2(H1)2]4+, [Co2(H2)2]4+, and [Co2(H3)2]4+, derived from monodeprotonated bis-pyridyl hydrazine-based ligands of H21, H22, and H23 with one, two, and three -CH2 spacers, were obtained. These asymmetric-carbon-free racemic helicates were separated into their ΔΔ and ΛΛ enantiomers. The resolved helicates were examined for the first time as enantioselective catalysts in asymmetric benzoylation and nitroaldol reactions.

A conformationally adaptive macrocycle: conformational complexity and host-guest chemistry of zorb[4]arene.

Large amplitude conformational change is one of the features of biomolecular recognition and is also the basis for allosteric effects and signal transduction in functional biological systems. However, synthetic receptors with controllable conformational changes are rare. In this article, we present a thorough study on the host-guest chemistry of a conformationally adaptive macrocycle, namely per-O-ethoxyzorb[4]arene (ZB4). Similar to per-O-ethoxyoxatub[4]arene, ZB4 is capable of accommodating a wide range of organic cations. However, ZB4 does not show large amplitude conformational responses to the electronic substituents on the guests. Instead of a linear free-energy relationship, ZB4 follows a parabolic free-energy relationship. This is explained by invoking the influence of secondary C-H···O hydrogen bonds on the primary cation···π interactions based on the information obtained from four representative crystal structures. In addition, heat capacity changes (ΔCp) and enthalpy-entropy compensation phenomena both indicate that solvent reorganization is also involved during the binding. This research further deepens our understanding on the binding behavior of ZB4 and lays the basis for the construction of stimuli-responsive materials with ZB4 as a major component.

Directional Shuttling of a Stimuli-Responsive Cone-Like Macrocycle on a Single-State Symmetric Dumbbell Axle.

Rotaxane-based molecular shuttles are often operated using low-symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid-responsive cone-like macrocycle on a single-state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone-like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.

An aryl-fused redox-active tetrathiafulvalene with enhanced mixed-valence and radical-cation dimer stabilities.

Molecular recognition of stable organic radicals is a relatively novel, but important structural binding motif in supramolecular chemistry. Here, we report on a redox-switchable veratrole-fused tetrathiafulvalene derivative VTTF which is ideally suited for this purpose and for the incorporation into stimuli-responsive systems. As revealed by electrochemistry, UV/Vis measurements, X-ray analysis, and electrocrystallisation, VTTF can be reversibly oxidised to the corresponding radical-cation or dication which shows optoelectronic and structural propterties similar to tetrathiafulvalene and tetrakis(methylthio)tetrathiafulvalene. However, theoretical calculations, variable temperature EPR, and NIR spectroscopy indicate that the dispersion-driven binding in the mixed-valence dimer (VTTF2)˙+ (KMV = 69 M-1 in CH2Cl2) and the radical-cation dimer (VTTF˙+)2 (KRC = 38 M-1 in CH3CN) is significantly enhanced by the additional veratrole π-surface in comparison to pristine tetrathiafulvalene.

Achieving Strong Positive Cooperativity through Activating Weak Non-Covalent Interactions.

Positive cooperativity achieved through activating weak non-covalent interactions is common in biological assemblies but is rarely observed in synthetic complexes. Two new molecular tubes have been synthesized and the syn isomer binds DABCO-based organic cations with high orientational selectivity. Surprisingly, the ternary complex with two hosts and one guest shows a high cooperativity factor (α=580), which is the highest reported for synthetic systems without involving ion-pairing interactions. The X-ray single-crystal structure revealed that the strong positive cooperativity likely originates from eight C-H⋅⋅⋅O hydrogen bonds between the two head-to-head-arranged syn tube molecules. These relatively weak hydrogen bonds were not observed in the free hosts and only emerged in the complex. Furthermore, this complex was used as a basic motif to construct a robust [2+2] cyclic assembly, thus demonstrating its potential in molecular self-assembly.

Chasing Weak Forces: Hierarchically Assembled Helicates as a Probe for the Evaluation of the Energetics of Weak Interactions.

London dispersion forces are the weakest interactions between molecules. Because of this, their influence on chemical processes is often low, but can definitely not be ignored, and even becomes important in cases of molecules with large contact surfaces. Hierarchically assembled dinuclear titanium(IV) helicates represent a rare example in which the direct observation of London dispersion forces is possible in solution even in the presence of strong cohesive solvent effects. Hereby, the dispersion forces do not unlimitedly support the formation of the dimeric complexes. Although they have some favorable enthalpic contribution to the dimerization of the monomeric complex units, large flexible substituents become conformationally restricted by the interactions leading to an entropic disadvantage. The dimeric helicates are entropically destabilized.

Efficient stabilisation of a dihydrogenphosphate tetramer and a dihydrogenpyrophosphate dimer by a cyclic pseudopeptide containing 1,4-disubstituted 1,2,3-triazole moieties.

A cyclic pseudooctapeptide 2 is described containing 1,4-disubstituted 1,2,3-triazole moieties. This compound features eight converging hydrogen bond donors along the ring, namely four amide NH and four triazole CH groups, which enable 2 to engage in interactions with anions. While fully deprotonated sulfate anions exhibit only moderate affinity for 2, protonated anions such as dihydrogenpyrophosphate and dihydrogenphosphate anions are strongly bound. Complexation of the phosphate-derived anions involves sandwiching of a dihydrogenpyrophosphate dimer or a dihydrogenphosphate tetramer between two pseudopeptide rings. X-ray crystallography provided structural information, while 1H NMR spectroscopy, mass spectrometry, and isothermal titration calorimetry demonstrated that these complexes are stable in solution (2.5 vol% water/DMSO) and can even be transferred without decomposition into the gas phase. The observed high thermodynamic stabilities are attributed to the mutual reinforcement of the interactions between the individual complex components, namely, hydrogen-bonding between the anions, multiple hydrogen bonding interactions between the anion aggregates and the triazole CH and NH hydrogen bond donors of 2, and potential dispersive interactions between the closely arranged pseudopeptide rings. Pseudopeptide 2 thus represents a promising lead for the construction of phosphate receptors, whose binding selectivity makes use of the unique ability of certain anions to assemble into higher aggregates.

Squaramide-Catalyzed Asymmetric aza-Friedel-Crafts/N,O-Acetalization Domino Reactions Between 2-Naphthols and Pyrazolinone Ketimines.

N-Boc ketimines derived from pyrazolin-5-ones were explored to develop an unprecedented domino aza-Friedel-Crafts/N,O-acetalization reaction with 2-naphthols. The novel method requires a catalyst loading of only 0.5 mol % of a bifunctional squaramide catalyst, is scalable to gram amounts, and provides a new series of furanonaphthopyrazolidinone derivatives bearing two vicinal tetra-substituted stereogenic centers in excellent yields (95-98 %) and stereoselectivity (>99:1 d.r. and 97-98 % ee). A different reactivity was observed in the case of 1-naphthols and other electron-rich phenols, which led to the aza-Friedel-Crafts adducts in 70-98 % yield and 47-98 % ee.

Asymmetric Synthesis of Functionalized Tricyclic Chromanes via an Organocatalytic Triple Domino Reaction.

A highly stereoselective triple domino reaction for the synthesis of functionalized tricyclic chromane scaffolds has been developed. A secondary amine-catalyzed domino Michael/Michael/aldol condensation reaction between aliphatic aldehydes, nitro-chromenes, and α,ß-unsaturated aldehydes leads to the formation of synthetically important tricyclic chromanes bearing four contiguous stereogenic centers including a tetrasubstituted carbon in good yields (20-66%) and excellent stereoselectivities (>20:1 dr and >99% ee).

X-Ray crystallographic and computational study on uranyl-salophen complexes bearing nitro groups.

In the solid state, salophen-UO2 complexes bearing one, two, or three NO2 groups lack the pronounced ligand curvature that represents a structural hallmark for this class of compounds. A detailed structural study based on single-crystal X-ray crystallography and computational methods, comprising molecular dynamics, gas-phase Hartree Fock, and DFT calculations, was carried out to investigate the coordination properties of the uranyl cation.