Efficient electronic communication of two ruthenium centers through a rigid ditopic N-heterocyclic carbene linker.

A ditopic benzobis(carbene) ligand precursor was prepared that contained a chelating pyridyl moiety to ensure co-planarity of the carbene ligand and the coordination plane of a bound octahedral metal center. Bimetallic ruthenium complexes comprising this ditopic ligand [L4Ru-C,N-bbi-C,N-RuL4] were obtained by a transmetalation methodology (C,N-bbi-C,N=benzobis(N-pyridyl-N'-methyl-imidazolylidene). The two metal centers are electronically decoupled when the ruthenium is in a pseudotetrahedral geometry imparted by a cymene spectator ligand (L4=[(cym)Cl]). Ligand exchange of the Cl(-)/cymene ligands for two bipyridine or four MeCN ligands induced a change of the coordination geometry to octahedral. As a consequence, the ruthenium centers, separated through space by more than 10 Å, become electronically coupled, which is evidenced by two distinctly different metal-centered oxidation processes that are separated by 134 mV (L4=[(bpy)2]; bpy=2,2'-bipyridine) and 244 mV (L4=[(MeCN)4]), respectively. Hush analysis of the intervalence charge-transfer bands in the mixed-valent species indicates substantial valence delocalization in both complexes (delocalization parameter Γ=0.41 and 0.37 in the bpy and MeCN complexes, respectively). Spectroelectrochemical measurements further indicated that the mixed-valent Ru(II)/Ru(III) species and the fully oxidized Ru(III)/Ru(III) complexes gradually decompose when bound to MeCN ligands, whereas the bpy spectators significantly enhance the stability. These results demonstrate the efficiency of carbenes and, in particular, of the bbi ligand scaffold for mediating electron transfer and for the fabrication of molecular redox switches. Moreover, the relevance of spectator ligands is emphasized for tailoring the degree of electronic communication through the benzobis(carbene) linker.

The potential of N-heterocyclic carbene complexes as components for electronically active materials.

The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the - often misleading - 'carbene' label, which implies a substantial degree of M = C pi bonding, we have been interested in evaluating the potential of N-heterocyclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal-carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed the degree of electronic coupling between the metal sites to be quantified and the key parameters that govern the intermetallic communication to be identified.

Highly potent and selective substrate analogue factor Xa inhibitors containing D-homophenylalanine analogues as P3 residue: part 2.

A series of highly potent substrate-analogue factor Xa inhibitors containing D-homophenylalanine analogues as the P3 residue has been identified by systematic optimization of a previously described inhibitor structure. An initial lead, benzylsulfonyl-D-hPhe-Gly-4-amidinobenzylamide (3), inhibits fXa with an inhibition constant of 6.0 nM. Most modifications of the P2 amino acid and P4 benzylsulfonyl group did not improve the affinity and selectivity of the compounds as fXa inhibitors. In contrast, further variation at the P3 position led to inhibitors with significantly enhanced potency and selectivity. Inhibitor 27, benzylsulfonyl-D-homo-2-pyridylalanyl(N-oxide)-Gly-4-amidinobenzylamide, inhibits fXa with a K(i) value of 0.32 nM. The inhibitor has strong anticoagulant activity in plasma and doubles the activated partial thromboplastin time and prothrombin time at concentrations of 280 nM and 170 nM, respectively. Compound 27 inhibits the prothrombinase complex with an IC(50) value of 5 nM and is approximately 50 times more potent than the reference inhibitor DX-9065a in this assay.

Effects of a polyelectrolyte additive on the selective dialysis membrane permeability for low-molecular-weight proteins.

BACKGROUND: Improving the sieving characteristics of dialysis membranes enhances the clearance of low-molecular-weight (LMW) proteins and may have an impact on outcome in patients receiving haemodialysis. To approach this goal, a novel polyelectrolyte additive process was applied to a polyethersulphone (PES) membrane. METHODS: Polyelectrolyte-modified PES was characterized in vitro by measuring complement activation and sieving coefficients of cytochrome c and serum albumin. In a prospective, randomized, cross-over study, instantaneous plasma water clearances and reduction rates of LMW proteins [beta(2)-microglobulin (b2m), cystatin c, myoglobin, retinol binding protein] were determined in eight patients receiving dialysis treatment with PES in comparison with polysulphone (PSU). Biocompatibility was assessed by determination of transient leucopenia, plasma levels of complement C5a, thrombin-antithrombin III (TAT), myeloperoxidase (MPO) and elastase (ELT). RESULTS: PES showed a steeper sieving profile and lower complement activation in vitro compared with PSU. Instantaneous clearance (69 +/- 8 vs. 58 +/- 3 ml/min; P < 0.001) and reduction rate (72.3 +/- 1 5% vs 66.2 +/- 6.1%; P < 0.001) of b2m were significantly higher with PES as compared with PSU. With higher molecular weight of proteins, differences in the solute removal between PES and PSU further increased, whereas albumin loss remained low (PES, 0.53 +/- 0.17 vs PSU, <0.22 g/dialysis). MPO, ELT and TAT did not differ between the two membranes. In contrast, leucopenia was less pronounced and C5a generation was significantly lower during dialysis with PES. CONCLUSIONS: Polyelectrolyte modification of PES results in a higher LMW protein removal and in optimized biocompatibility. Whether these findings translate into better outcome of patients receiving haemodialysis requires further studies.

Synthesis of the first rNHC (remote N-heterocyclic carbene) complexes with no heteroatom in the carbene carbon-containing ring.

Two cationic carbene complexes with no heteroatom in the ring containing the carbene carbon, trans-bromo(2-methyl-2,6-dihydroisoquinolin-6-ylidene)bis(triphenylphosphine)palladium(II) triflate (3) and trans-chloro(1,2-dimethyl-1,7-dihydroquinolin-7-ylidene)bis(triphenylphosphine)palladium(II) triflate (4), were synthesized by oxidative substitution of Pd(PPh3)4 with N-methylated 6-bromoisoquinolinium and 7-chloro-2-methylquinolinium cations, respectively. Compound 3 was also prepared by methylation of neutral trans-bromo(2-methylisoquinolin-6-yl)bis(triphenylphosphine)palladium(II) (5). All complexes were unambiguously characterized by NMR and X-ray crystallographic studies.

Secondary amides of sulfonylated 3-amidinophenylalanine. New potent and selective inhibitors of matriptase.

Matriptase is an epithelium-derived type II transmembrane serine protease and has been implicated in the activation of substrates such as pro-HGF/SF and pro-uPA, which are likely involved in tumor progression and metastasis. Through screening, we have identified bis-basic secondary amides of sulfonylated 3-amidinophenylalanine as matriptase inhibitors. X-ray analyses of analogues 8 and 31 in complex with matriptase revealed that these inhibitors occupy, in addition to part of the previously described S4-binding site, the cleft formed by the molecular surface and the unique 60 loop of matriptase. Therefore, optimization of the inhibitors included the incorporation of appropriate sulfonyl substituents that could improve binding of these inhibitors into both characteristic matriptase subsites. The most potent derivatives inhibit matriptase highly selective with K(i) values below 5 nM. Molecular modeling revealed that their improved affinity results from interaction with the S4 site of matriptase. Analogues 8 and 59 were studied in an orthotopic xenograft mouse model of prostate cancer. Compared to control, both inhibitors reduced tumor growth, as well as tumor dissemination.

New substrate analogue inhibitors of factor Xa containing 4-amidinobenzylamide as P1 residue: part 1.

The trypsin-like serine protease factor Xa (fXa) is located at the convergence point of the intrinsic and extrinsic coagulation cascade, and therefore has emerged as an attractive target for the design of novel anticoagulants. During the development of substrate-analogue urokinase inhibitors we have found that the protection of the P3-dSer side chain leads to a scaffold of potent fXa inhibitors with the general structure R1-SO2-dSer(R2)-Gly-4-amidinobenzylamide. The first lead (3) with an N-terminal benzylsulfonyl group and dSer(tBu) as P3 residue inhibits human fXa with a Ki of 14 nM. A variety of derivatives with modified P4, P3, and P2 residues have been investigated in terms of inhibition of fXa and related proteases and for their anticoagulant potency and elimination behaviour. Most inhibitors were rapidly cleared from the circulation of rats. However, compound 48 (Ki= 3.5 nM), one of the most potent and selective inhibitors containing a dArg as P3 residue was relatively slowly eliminated (t1/2 approximately 1 h). Inhibitor 48 doubled clotting times in human plasma at 0.32 microM (aPTT) and 0.28 microM (PT), and is approximately 10-fold more potent than the reference fXa inhibitor DX-9065a in the inhibition of the prothrombinase complex. The structures of two inhibitors in complex with human fXa were solved by X-ray crystallography.

Conformationally constrained beta-amino acid derivatives by intramolecular [2 + 2]-photocycloaddition of a tetronic acid amide and subsequent lactone ring opening.

[reaction: see text] The N-Boc-protected N-3-alkenyltetronic acid amides 9 and 12 were prepared from tetronic acid bromide (7) and the corresponding amines 6 and 10 by nucleophilic substitution and subsequent acylation in 71% and 39% overall yield. They underwent an intramolecular [2 + 2]-photocycloaddition upon direct irradiation (lambda = 254 nm) to yield diastereoselectively the strained lactones 15 (76%) and 16 (91%) with a 2-azabicyclo[3.2.0]heptane core. In attempts to defunctionalize the 1-hydroxymethyl substituent of the 2-azabicyclo[3.2.0]heptane skeleton, lactone 15 was converted into mesylate 18 (74% overall yield). Intermolecular substitution reactions on this mesylate, however, proceeded sluggishly or failed completely. Lactone 15 could be opened reductively (Dibal-H) or by substitution with benzylamine to the N-Boc-protected 2-azabicyclo[3.2.0]heptanes 21 (71%) and 22 (81%). Conformationally constrained beta-amino acid derivatives were obtained by quantitative N-Boc deprotection of photocycloaddition product 15, followed by N-functionalization and subsequent lactone ring opening. The N-functionalization was conducted by acylation (to 24-26), alkylation (to 27), tosylation (to 28), and isocyanate addition (to 30). The reactions proceeded in yields of 70-84%. Lactone ring opening reactions were conducted with amines to establish the suitability of this process for library synthesis. As an example, the tripeptide 38 was obtained from photocycloaddition product 15 in an overall yield of 51%.

High facial diastereoselectivity in intra- and intermolecular reactions of chiral benzylic cations.

Chiral carbenium ions can be attacked by arene nucleophiles with high facial diastereoselectivity (dr >/= 94/6). Benzylic cations, such as 2, were generated under acidic conditions and reacted with arenes in intra- and intermolecular Friedel-Crafts alkylation reaction. The depicted reaction 1 --> 3 represents one example for the unprecedented, highly diastereoselective intermolecular Friedel-Crafts alkylation reactions which were observed in this study.

Bromination of (phosphine)gold(I) bromide complexes: stoichiometry and structure of products.

The course of the oxidative addition of elemental bromine to complexes of the type (L)AuBr is strongly influenced by the nature of the tertiary phosphine ligand L. Standard square planar gold(III) complexes (L)AuBr3 are obtained not only with L = PMe3 but also with P((I)Pro)3 for which the oxidative addition fails in the corresponding iodine system. Excess bromine is integrated into crystals of the products with the stoichiometry [(Me3P)AuBr3].(Br2) and {[(iPro)3P]AuBr3}.(Br2). Of the series of iodine analogues, an intercalate [(Me3P)AuI3]2.(I2) has been structurally characterized. [((t)Bu)3P]AuBr undergoes ligand redistribution upon treatment with bromine to give a complex reaction mixture, from which {[(tBu)3P]2Au}+(Br3)-.(Br2) could be crystallized. It contains polymeric anions [(Br5)-]n as zig-zag chains. [(o-Tol)3P]AuBr is readily brominated to give [(o-Tol)3P]AuBr3. Contrary to the situation in the gold(I) complex with its linear PAuBr unit, the square planar structure of the PAuBr3 unit causes steric hindering of the rotation of the tolyl groups about the P-C bonds as demonstrated by solution NMR studies. (The corresponding reaction with iodine is known to give only polyiodides with the oxidation state of the gold atom unchanged.) The even more severe congestion in [(Mes)3P]AuBr prevents oxidative addition not only of iodine but also of bromine. With the latter, P-Au cleavage occurs instead affording [(Mes)3PBr]+[AuBr4]-.

A cyclic hexamer of silver trifluoroacetate supported by four triphenylphosphine sulfide template molecules.

Crystallization of silver trifluoroacetate from chloroform solutions containing triphenylphosphine sulfide affords a trigonal and a monoclinic form of a 6:4 complex {[CF3C(O)OAg]6(Ph3PS)4} of C2 symmetry with different amounts of chloroform in the crystals. With the Ph3PS components as template molecules, the CF3C(O)OAg units are assembled to form a 6-membered metallacycle codetermined by metallophilic bonding and enclosed by a 24-membered ring [AgOCO]6. A complex of the type [LAgOC(O)CF3]2, with L representing the isocyanide ligand pTolSO2CH2NC, has been shown to have a conventional bicyclic structure with three-coordinate silver atoms engaged in transannular metallophilic interactions.

Design of novel and selective inhibitors of urokinase-type plasminogen activator with improved pharmacokinetic properties for use as antimetastatic agents.

The serine protease urokinase-type plasminogen activator (uPA) interacts with a specific receptor (uPAR) on the surface of various cell types, including tumor cells, and plays a crucial role in pericellular proteolysis. High levels of uPA and uPAR often correlate with poor prognosis of cancer patients. Therefore, the specific inhibition of uPA with small molecule active-site inhibitors is one strategy to decrease the invasive and metastatic activity of tumor cells. We have developed a series of highly potent and selective uPA inhibitors with a C-terminal 4-amidinobenzylamide residue. Optimization was directed toward reducing the fast elimination from circulation that was observed with initial analogues. The x-ray structures of three inhibitor/uPA complexes have been solved and were used to improve the inhibition efficacy. One of the most potent and selective derivatives, benzylsulfonyl-D-Ser-Ser-4-amidinobenzylamide (inhibitor 26), inhibits uPA with a Ki of 20 nm. This inhibitor was used in a fibrosarcoma model in nude mice using lacZ-tagged human HT1080 cells, to prevent experimental lung metastasis formation. Compared with control (100%), an inhibitor dose of 2 x 1.5 mg/kg/day reduced the number of experimental metastases to 4.6 +/- 1%. Under these conditions inhibitor 26 also significantly prolonged survival. All mice from the control group died within 43 days after tumor cell inoculation, whereas 50% of mice from the inhibitor-treated group survived more than 117 days. This study demonstrates that the specific inhibition of uPA by these inhibitors may be a useful strategy for the treatment of cancer to prevent metastasis.

Saturation transfer difference NMR and computational modeling of a sialoadhesin-sialyl lactose complex.

The siglecs are a family of I-type lectins binding to sialic acids on the cell surface. Sialoadhesin (siglec-1) is expressed at much higher levels in inflammatory macrophages and specifically binds to alpha-2,3-sialylated N-acetyl lactosamine residues of glycan chains. The terminal disaccharide alpha-D-Neu5Ac-(2-->3)-beta-D-Gal is thought to be the main epitope recognized by sialoadhesin. To understand the basis of this biological recognition reaction we combined NMR experiments with a molecular modeling study. We employed saturation transfer difference (STD) NMR experiments to characterize the binding epitope of alpha-2,3-sialylated lactose, alpha-D-Neu5Ac-(2-->3)-beta-D-Gal-(1-->4)-D-Glc 1 to sialoadhesin at atomic resolution. The experimental results were compared to a computational docking model and to X-ray data of a complex of sialyl lactose and sialoadhesin. The data reveal that sialoadhesin mainly recognizes the N-acetyl neuraminic acid and a small part of the galactose moiety of 1. The crystal structure of a complex of sialoadhesin with sialyl lactose 1 was used as a basis for a modeling study using the FlexiDock algorithm. The model generated was very similar to the original crystal structure. Therefore, the X-ray data were used to predict theoretical STD values utilizing the CORCEMA-STD protocol. The good agreement between experimental and theoretical STD values indicates that a combined modeling/STD NMR approach yields a reliable structural model for the complex of sialoadhesin with alpha-D-Neu5Ac-(2-->3)-beta-D-Gal-(1-->4)-D-Glc 1 in aqueous solution.