Nms-Amides: An Amine Protecting Group with Unique Stability and Selectivity.

p-Toluenesulfonyl (Tosyl) and nitrobenzenesulfonyl (Nosyl) are two of the most common sulfonyl protecting groups for amines in contemporary organic synthesis. While p-toluenesulfonamides are known for their high stability/robustness, their use in multistep synthesis is plagued by difficult removal. Nitrobenzenesulfonamides, on the other hand, are easily cleaved but display limited stability to various reaction conditions. In an effort to resolve this predicament, we herein present a new sulfonamide protecting group, which we term Nms. Initially developed through in silico studies, Nms-amides overcome these previous limitations and leave no room for compromise. We have investigated the incorporation, robustness and cleavability of this group and found it to be superior to traditional sulfonamide protecting groups in a broad range of case studies.

Reinstatement of synaptic plasticity in the aging brain through specific dopamine transporter inhibition.

Aging-related neurological deficits negatively impact mental health, productivity, and social interactions leading to a pronounced socioeconomic burden. Since declining brain dopamine signaling during aging is associated with the onset of neurological impairments, we produced a selective dopamine transporter (DAT) inhibitor to restore endogenous dopamine levels and improve cognitive function. We describe the synthesis and pharmacological profile of (S,S)-CE-158, a highly specific DAT inhibitor, which increases dopamine levels in brain regions associated with cognition. We find both a potentiation of neurotransmission and coincident restoration of dendritic spines in the dorsal hippocampus, indicative of reinstatement of dopamine-induced synaptic plasticity in aging rodents. Treatment with (S,S)-CE-158 significantly improved behavioral flexibility in scopolamine-compromised animals and increased the number of spontaneously active prefrontal cortical neurons, both in young and aging rodents. In addition, (S,S)-CE-158 restored learning and memory recall in aging rats comparable to their young performance in a hippocampus-dependent hole board test. In sum, we present a well-tolerated, highly selective DAT inhibitor that normalizes the age-related decline in cognitive function at a synaptic level through increased dopamine signaling.

Synthesis and dopamine receptor binding of dihydrexidine and SKF 38393 catecholamine-based analogues.

Dopamine is an important neurotransmitter that regulates numerous essential functions, including cognition and voluntary movement. As such, it serves as an important scaffold for synthesis of novel analogues as part of drug development effort to obtain drugs for treatment of neurodegenerative diseases, such as Parkinson's disease. To that end, similarity search of the ZINC database based on two known dopamine-1 receptor (D1R) agonists, dihydrexidine (DHX) and SKF 38393, respectively, was used to predict novel chemical entities with potential binding to D1R. Three compounds that showed the highest similarity index were selected for synthesis and bioactivity profiling. All main synthesis products as well as the isolated intermediates, were properly characterized. The physico-chemical analyses were performed using HRESIMS, GC/MS, LC/MS with UV-Vis detection, and FTIR, 1H NMR and 13C NMR spectroscopy. Binding to D1 and D2 receptors and inhibition of dopamine reuptake via dopamine transporter were measured for the synthesized analogues of DHX and SKF 38393.

Reactivity of Diamines in Acyclic Diamino Carbene Gold Complexes.

Acyclic diamino carbenes (ADCs) are interesting alternatives to their more widely studied N-heterocyclic carbene counterparts, particularly due to their greater synthetic accessibility and properties such as increased sigma donation and structural flexibility. ADC gold complexes are typically obtained through the reaction of equimolar amounts of primary/secondary amines on gold-coordinated isocyanide ligands. As such, the reaction of diamine nucleophiles to isocyanide gold complexes was expected to lead to bis-ADC gold compounds with potential applications in catalysis or as novel precursors for gold nanomaterials. However, the reaction of primary diamines with two equivalents of isocyanide gold chlorides resulted in only one of the amine groups reacting with the isocyanide carbon. The resulting ADC gold complexes bearing free amines dimerized via coordination of the amine to the partner gold atom, resulting in cyclic, dimeric gold complexes. In contrast, when secondary diamines were used, both amines reacted with an isocyanide carbon, leading to the expected bis-ADC gold complexes. Density functional theory calculations were performed to elucidate the differences in the reactivities between primary and secondary diamines. It was found that the primary amines were associated with higher reaction barriers than the secondary amines and hence slower reaction rates, with the formation of the second carbenes in the bis-ADC compounds being inhibitingly slow. It was also found that diamines have a unique reactivity due to the second amine serving as an internal proton shuttle.

Defect {(WVIO7)WVI4} and Full {(WVIO7)WVI5} Pentagonal Units as Synthons for the Generation of Nanosized Main Group V Heteropolyoxotungstates.

We report on the synthesis and characterization of three new nanosized main group V heteropolyoxotungstates KxNay[H2(XWVI9O33)(WVI5O12)(X2WVI29O103)]·nH2O {X3W43} (x = 11, y = 16, and n = 115.5 for X = SbIII; x = 20, y = 7, and n = 68 for X = BiIII) and K8Na15[H16(CoII(H2O)2)0.9(CoII(H2O)3)2(WVI3.1O14)(SbIIIWVI9O33)(SbIII2WVI30O106)(H2O)]·53H2O {Co3Sb3W42}. On the basis of the key parameters for the one-pot synthesis strategy of {Bi3W43}, a rational step-by-step approach was developed using the known Krebs-type polyoxotungstate (POT) K12[SbV2WVI22O74(OH)2]·27H2O {Sb2W22} as a nonlacunary precursor leading to the synthesis and characterization of {Sb3W43} and {Co3Sb3W42}. Solid-state characterization of the three new representatives {Bi3W43}, {Sb3W43}, and {Co3Sb3W42} by single-crystal and powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and elemental analysis, along with characterization in solution by UV/vis spectroscopy shows that {Bi3W43}, {Sb3W43}, and {Co3Sb3W42} represent the first main group V heteropolyoxotungstates encapsulating a defect {(WVIO7)WVI4} ({X3W43}, X = BiIII and SbIII) or full {(WVIO7)WVI5} ({Co3Sb3W42}) pentagonal unit. With 43 tungsten metal centers, {X3W43} (X = BiIII and SbIII) are the largest unsubstituted tungstoantimonate- and bismuthate clusters reported to date. By using time-dependent UV/vis spectroscopy, the isostructural representatives {Sb3W43} and {Bi3W43} were subjected to a comprehensive study on their catalytic properties as homogeneous electron-transfer catalysts for the reduction of K3[FeIII(CN)6] as a model substrate revealing up to 5.8 times higher substrate conversions in the first 240 min (35% for {Sb3W43}, 29% for {Bi3W43}) as compared to the uncatalyzed reaction (<6% without catalyst after 240 min) under otherwise identical conditions.

The Smallest Polyoxotungstate Retained by TRIS-Stabilization.

A polycondensation reaction of the orthotungstate anion WO42-, buffered at pH 7.5 in a TRIS-HCl (0.15 M) solution, results in the first example of a discrete polyoxotungstate anion, with just two W ions stabilized with TRIS ligands. It was isolated and characterized as Na2[WVI2O6(C4O3NH10)2]·6H2O by single-crystal and powder X-ray diffraction, FT-IR spectroscopy, thermogravimetrical analysis (TGA), and elemental analysis in solid state and by electro-spray ionization mass spectrometry (ESI-MS), 13C, and 183W NMR, as well as Raman spectroscopy in solution. This synthesis demonstrates the crucial and new role of the added tris-alkoxy ligand in the development of a new hybrid TRIS-isopolytungstate with the lowest known nuclearity (so far) and the terminal oxygens substituted with two nitrogen atoms arising from amines of the TRIS ligands.

Speciation of Transition-Metal-Substituted Keggin-Type Silicotungstates Affected by the Co-crystallization Conditions with Proteinase K.

We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts Na14[(A-α-SiW10O37)2{Co4(OH)2(H2O)2}]·37H2O (Na{SiW10Co2}2) and Na14[(A-α-SiW10O37)2{Ni4(OH)2(H2O)2}]·77.5H2O (Na{SiW10Ni2}2), which were prepared by applying a new synthesis protocol and characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Proteinase K was applied as a model protein and the polyoxotungstate (POT)-protein interactions of Na{SiW10Co2}2 and Na{SiW10Ni2}2 were studied side by side with the literature-known K5Na3[A-α-SiW9O34(OH)3{Co4(OAc)3}]·28.5H2O ({SiW9Co4}) featuring the same number of transition metals. Testing the solution behavior of applied POTs under the crystallization conditions (sodium acetate buffer, pH 5.5) by time-dependent UV/vis spectroscopy and electrospray ionization mass spectrometry speciation studies revealed an initial dissociation of the sandwich POTs to the disubstituted Keggin anions HxNa5-x[SiW10Co2O38]3- and HxNa5-x[SiW10Ni2O38]3- ({SiW10M2}, M = CoII and NiII) followed by partial rearrangement to the monosubstituted compounds (α-{SiW11Co} and α-{SiW11Ni}) after 1 week of aging. The protein crystal structure analysis revealed monosubstituted α-Keggin POTs in two conserved binding positions for all three investigated compounds, with one of these positions featuring a covalent attachment of the POT anion to an aspartate carboxylate. Despite the presence of both mono- and disubstituted anions in a crystallization mixture, proteinase K selectively binds to monosubstituted anions because of their preferred charge density for POT-protein interaction.

Aluminum-Substituted Keggin Germanotungstate [HAl(H2O)GeW11O39]4-: Synthesis, Characterization, and Antibacterial Activity.

We report on the new monosubstituted aluminum Keggin-type germanotungstate (C4H12N)4[HAlGeW11O39(H2O)]·11H2O ([Al(H2O)GeW11]4-), which has been synthesized at room temperature via rearrangement of the dilacunary [γ-GeW10O36]8- polyoxometalate precursor. [Al(H2O)GeW11]4- has been characterized thoroughly both in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by cyclic voltammetry (CV) 183W, 27Al NMR and UV-vis spectroscopy. A study on the antibacterial properties of [Al(H2O)GeW11]4- and the known aluminum(III)-centered Keggin polyoxotungstates (Al-POTs) α-Na5[AlW12O40] (α-[AlW12O40]5-) and Na6[Al(AlOH2)W11O39] ([Al(AlOH2)W11O39]6-) revealed enhanced activity for all three Al-POTs against the Gram-negative bacterium Moraxella catarrhalis (minimum inhibitory concentration (MIC) up to 4 µg mL-1) and the Gram-positive Enterococcus faecalis (MIC up to 128 µg mL-1) compared to the inactive Al(NO3)3 salt (MIC > 256 µg mL-1). CV indicates the redox activity of the Al-POTs as a dominating factor for the observed antibacterial activity with increased tendency to reduction, resulting in increased antibacterial activity of the POT.

Triapine Analogues and Their Copper(II) Complexes: Synthesis, Characterization, Solution Speciation, Redox Activity, Cytotoxicity, and mR2 RNR Inhibition.

Three new thiosemicarbazones (TSCs) HL1-HL3 as triapine analogues bearing a redox-active phenolic moiety at the terminal nitrogen atom were prepared. Reactions of HL1-HL3 with CuCl2·2H2O in anoxic methanol afforded three copper(II) complexes, namely, Cu(HL1)Cl2 (1), [Cu(L2)Cl] (2'), and Cu(HL3)Cl2 (3), in good yields. Solution speciation studies revealed that the metal-free ligands are stable as HL1-HL3 at pH 7.4, while being air-sensitive in the basic pH range. In dimethyl sulfoxide they exist as a mixture of E and Z isomers. A mechanism of the E/Z isomerization with an inversion at the nitrogen atom of the Schiff base imine bond is proposed. The monocationic complexes [Cu(L1-3)]+ are the most abundant species in aqueous solutions at pH 7.4. Electrochemical and spectroelectrochemical studies of 1, 2', and 3 confirmed their redox activity in both the cathodic and the anodic region of potentials. The one-electron reduction was identified as metal-centered by electron paramagnetic resonance spectroelectrochemistry. An electrochemical oxidation pointed out the ligand-centered oxidation, while chemical oxidations of HL1 and HL2 as well as 1 and 2' afforded several two-electron and four-electron oxidation products, which were isolated and comprehensively characterized. Complexes 1 and 2' showed an antiproliferative activity in Colo205 and Colo320 cancer cell lines with half-maximal inhibitory concentration values in the low micromolar concentration range, while 3 with the most closely related ligand to triapine displayed the best selectivity for cancer cells versus normal fibroblast cells (MRC-5). HL1 and 1 in the presence of 1,4-dithiothreitol are as potent inhibitors of mR2 ribonucleotide reductase as triapine.

Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties.

Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions. Herein, we report a new member of the Keggin archetype, the Cr-centered Keggin anion [α-CrW12O40]5- (CrW12), with the unusual tetrahedral coordination of CrIII reported for the first time in POMs conferring unattended magnetic properties. POM chemistry has recently presented excellent examples of single-molecule and single-ion magnets (SMMs and SIMs) as well as molecular spin qubits; however, the majority of POM-based SIMs reported to date contain lanthanoid ions. CrW12, as the first example of a chromium(III) SIM, exhibits slow relaxation of magnetization and quantum tunneling with a single-ion magnetic behavior even above 10 K with an energy barrier for the reversal of the magnetization of 3.0 K. The first 3d-metal SIM based on a nonlacunary Keggin anion is the foundation for a new research area in POM chemistry.

Improving the Stability of Maleimide-Thiol Conjugation for Drug Targeting.

Maleimides are essential compounds for drug conjugation reactions via thiols to antibodies, peptides and other targeting units. However, one main drawback is the occurrence of thiol exchange reactions with, for example, glutathione resulting in loss of the targeting ability. A new strategy to overcome such retro-Michael exchange processes of maleimide-thiol conjugates by stabilization of the thiosuccinimide via a transcyclization reaction is presented. This reaction enables the straightforward synthesis of stable maleimide-thiol adducts essential in drug-conjugation applications.

Synthesis, Modification, and Biological Evaluation of a Library of Novel Water-Soluble Thiopyridone-Based Organometallic Complexes and Their Unexpected (Biological) Behavior.

A series of 16 dinuclear thiopyridone-based organometallics with excellent water solubility, increased stability and remarkable cytotoxicity were synthesized and characterized. The complexes of this work formed dimeric species featuring a double positive charge in polar protic solvents, accounting for their outstanding solubility in aqueous solution. Most of them displayed higher antiproliferative activity than their parental thiomaltol complex, with unexpected cytotoxicity trends depending on the employed metal center, ligand modification, and cell line. Insights into their behavior in biological systems were gathered by means of amino-acid interaction studies, cytotoxicity tests in 3D spheroid models, laser ablation, cellular accumulation measurements, as well as cell cycle experiments.

The First Anticancer Tris(pyrazolyl)borate Molybdenum(IV) Complexes: Tested in Vitro and in Vivo-A Comparison of O,O-, S,O-, and N,N-Chelate Effects.

The synthesis, characterization and biological activity of molybdenum(IV) complexes containing Trofimenko's scorpionato ligand, hydrotris(3-isopropylpyrazolyl)borate (TpiPr ), in addition to varying biologically active as well as other conventional ligands is described. Ligands employed include (O,O-) (S,O-) (N,N-) donors that have been successfully coordinated to the molybdenum center by means of oxygen-atom transfer (OAT) reactions from the known MoVI starting material, TpiPr MoO2 Cl. The synthesized complexes were characterized by standard analytical methods and where possible by X-ray diffraction analysis. The aqueous stability of the compounds was studied by means of UV/Vis spectroscopy and the impact of the attached ligand scaffolds on the oxidation potentials (MoIV to MoV ) was studied by cyclic voltammetry. Utilizing polyvinylpyrrolidone (PVP) as a solubilizing agent, adequate aqueous solubility for biological tests was obtained. Anticancer activity tests and preliminary mode of action studies have been performed in vitro and in vivo.

Binding of a Fatty Acid-Functionalized Anderson-Type Polyoxometalate to Human Serum Albumin.

The Anderson-type hexamolybdoaluminate functionalized with lauric acid (LA), (TBA)3[Al(OH)3Mo6O18{(OCH2)3CNHCOC11H23}]·9H2O (TBA-AlMo6-LA, where TBA = tetrabutylammonium), was prepared via two synthetic routes and characterized by thermogravimetric and elemental analyses, mass spectrometry, IR and 1H NMR spectroscopy, and powder and single-crystal X-ray diffraction. The interaction of TBA-AlMo6-LA with human serum albumin (HSA) was investigated via fluorescence and circular dichroism spectroscopy. The results revealed that TBA-AlMo6-LA binds strongly to HSA (63% quenching at an HSA/TBA-AlMo6-LA ratio of 1:1), exhibiting static quenching. In contrast to TBA-AlMo6-LA, the nonfunctionalized polyoxometalate, Na3(H2O)6[Al(OH)6Mo6O18]·2H2O (AlMo6), showed weak binding toward HSA (22% quenching at a HSA/AlMo6 ratio of 1:25). HSA binding was confirmed by X-ray structure analysis of the HSA-Myr-AlMo6-LA complex (Myr = myristate). These results provide a promising lead for the design of novel polyoxometalate-based hybrids that are able to exploit HSA as a delivery vehicle to improve their pharmacokinetics and bioactivity.

Synthesis, Characterization, and Phosphoesterase Activity of a Series of 4f- and 4d-Sandwich-Type Germanotungstates [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV, m = 6).

We report on a family of five new 4f- and 4d-doped sandwich-type germanotungstates with the general formula [(n-C4H9)4N]l/mH2[(M(H2O)3)(γ-GeW10O35)2]·3(CH3)2CO [M(H2O)3(GeW10)2] (M = CeIII, NdIII, GdIII, ErIII, l = 7; ZrIV, m = 6), which have been synthesized at room temperature in an acetone-water mixture. Among the compound series, [Zr(H2O)3(GeW10)2]8-, which has been obtained in the presence of 30% H2O2, represents the first example of a 4d-substituted germanotungstate incorporating the intact dilacunary [γ-GeIVW10O36]8- building block. All compounds were characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction (XRD), IR spectroscopy, thermogravimetric analysis (TGA), and elemental analysis and in solution by NMR and UV-vis spectroscopy. The phosphoesterase activity of [Ce(H2O)3(GeW10)2]9- and [Zr(H2O)3(GeW10)2]8- toward the model substrates 4-nitrophenyl phosphate (NPP) and O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP) was monitored with 1H- and 31P-NMR spectroscopy revealing an acceleration of the hydrolytic reaction by an order of magnitude (kcorr = 3.44 (±0.30) × 10-4 min-1 for [Ce(H2O)3(GeW10)2]9- and kcorr = 5.36 (±0.05) × 10-4 min-1 for [Zr(H2O)3(GeW10)2]8-) as compared to the uncatalyzed reaction (kuncat = 2.60 (±0.10) × 10-5 min-1). [Ce(H2O)3(GeW10)2]9- demonstrated improved antibacterial activity toward Moraxella catarrhalis (MIC 32 µg/mL), compared to the unsubstituted [GeW10O36]8- POM (MIC 64 µg/mL).

Cation-Directed Synthetic Strategy Using 4f Tungstoantimonates as Nonlacunary Precursors for the Generation of 3d-4f Clusters.

The first synthetic pathway using a series of four nonlacunary 4f-heterometal-substituted polyoxotungstate clusters Na21[(Ln(H2O)(OH)2(CH3COO))3(WO4)(SbW9O33)3]·nH2O (NaLnSbW9; Ln = TbIII, DyIII, HoIII, ErIII, YIII) as precursors for the directed preparation of nine new 3d-4f heterometallic tungstoantimonates K5Na12H3[TM(H2O)Ln3(H2O)5(W3O11)(SbW9O33)3]·nH2O (KTMLnSbW9; TM = CoII, NiII; Ln = TbIII, DyIII, HoIII, ErIII, YIII) has been developed. Systematic studies revealed an increased K content in the aqueous acidic reaction mixture to be the key step in the cation-directed preparation of 3d-4f compounds; among those, the Co-containing members represent the first examples of KCoLnSbW9 (Ln = TbIII, DyIII, HoIII, ErIII, YIII) heterometallic tungstoantimonates exhibiting the SbW9 building block. All 13 compounds have been characterized thoroughly in the solid state by powder and single-crystal X-ray diffraction (XRD), revealing a cyclic trimeric polyoxometalate architecture with three SbW9 units encapsulating a planar triangle of LnIII ions in the case of NaLnSbW9 and a heterometallic core of one TMII and three LnIII for KTMLnSbW9 (TM = CoII, NiII; Ln = TbIII, DyIII, HoIII, ErIII, YIII). The results obtained by XRD are supplemented by complementary characterization methods in the solid state such as IR spectroscopy, thermogravimetric analysis, and elemental analysis as well as in solution by UV-vis spectroscopy. Detailed magnetic studies on the representative compounds KTMDySbW9 (TM = CoII, NiII) and KCoYSbW9 of the series revealed field-induced slow magnetic relaxation.

A Many-Faced Alkaloid: Polymorphism of (-)-Monophyllidin.

The synthesis of the alkaloid (-)-monophyllidin is described. The molecule is a hybrid of xanthoxyline and (S)-proline, accessible in one-step through a Mannich reaction. In the solid-state, defined structural arrangements with different physical properties are formed. Single crystal X-ray diffraction revealed structures of six distinct polymorphs. In the crystalline state, the alkaloid can host small polar molecules (preferably water), while the (S)-proline moiety is present in the zwitterionic state. Combined with the chelate, which is already present in the xanthoxyline substructure, an ideal disposition for multiple hydrogen bond networks evolve. Therefore, highly water-soluble polymorphs of monophyllidin can form. This structural flexibility explains the many faces of the molecule in terms of structure as well as analytical data. Furthermore, speculations about the biological role of the molecule, with regard to the manifold interactions with water, are presented.

Halogen-Imparted Reactivity in Lithium Carbenoid Mediated Homologations of Imine Surrogates: Direct Assembly of bis-Trifluoromethyl-ß-Diketiminates and the Dual Role of LiCH2I.

The selective formal insertion (homologation) of a carbon unit bridging the two trifluoroacetamidoyl chlorides (TFAICs) units is reported. The tactic is levered on a highly chemoselective homologation-metalation-acyl nucleophilic substitution sequence which precisely enables to assemble novel trifluoromethylated ß-diketiminates within a single synthetic operation. Unlike previous homologations conducted with LiCH2Cl furnishing aziridines, herein we exploit the unique capability of iodomethyllithium to act contemporaneously as a C1 source (homologating effect) and metalating agent. The mechanistic rationale grounded on experimental evidences supports the hypothesized proposal and, the structural analysis gathers key aspects of this class of valuable ligands in catalysis.

Direct Single- and Double-Side Triol-Functionalization of the Mixed Type Anderson Polyoxotungstate [Cr(OH)3W6O21]6.

Since the first successful triol-functionalization of the Anderson polyoxometalates, the six protons of their central octahedron X(OH)6 (X-heteroatom, p- or d-element) have been considered as a prerequisite for their functionalization with tripodal alcohols, and therefore, the functionalization of Anderson structures from the unprotonated sides have never been reported. Here, we describe the triol-functionalization of [Cr(OH)3W6O21]6- leading to the single-side grafted anions [Cr(OCH2)3CRW6O21]6- (CrW6-tris-R, R = -C2H5, -NH2, -CH2OH) and the unprecedented double-side functionalized anion [Cr((OCH2)3CC2H5)2W6O18]3- (CrW6-(tris-C2H5)2), despite the lack of protons in the parent anion in the solid state. CrW6-(tris-C2H5)2 demonstrates the first example of double-side functionalized Anderson POT with the partially one-side protonated corresponding parent anion. The new heteropolytungstates were characterized by single-crystal X-ray diffraction, elemental analysis, Fourier-transform infrared spectroscopy, thermal gravimetric analysis, cyclic voltammetry, and electrospray ionization mass spectrometry. Density functional theory calculations were performed to investigate and compare the stability among the different isomers of the parent anion [Cr(OH)3W6O21]6-.

Improved Access to Chiral Tetranaphthoazepinium-Based Organocatalysts Using Aqueous Ammonia as Nitrogen Source.

The class of 3,3'-diaryl substituted tetranaphthobisazepinium bromides has found wide application as highly efficient C2-symmetrical phase-transfer catalysts (PTCs, Maruoka type catalysts). Unfortunately, the synthesis requires a large number of steps and hampers the build-up of catalyst libraries which are often desired for screening experiments. Here, we present a more economic strategy using dinaphthoazepine 7 as the common key intermediate. Only at this stage various aryl substituents are introduced, and only two individual steps are required to access target structures. This protocol was applied to synthesize ten tetranaphthobisazepinium compounds 1a-1j. Their efficiency as PTCs was tested in the asymmetric substitution of tert-butyl 2-((diphenylmethylene)amino)acetate. Enantioselectivities up to 92% have been observed with new catalysts.