Michel Holler

Glycosidase Inhibition with Fullerene Iminosugar Balls: A Dramatic Multivalent Effect†

The electronic and structural properties of fullerene derivatives make them very attractive candidates for the construction of nanostructures that are potentially useful for applications in materials science and biological chemistry. In particular, the C60 hexakis adducts with a Th-symmetrical octahedral addition pattern initially developed by Hirsch and co-workers are unique organic molecules with an appealing compact spherical scaffold for the construction of multifunctional nanomaterials. However, the synthesis of functionalized fullerene hexakis adducts from malonates and C60 is difficult. 4] This major problem limits the applications of such systems and has been recently solved by the development of synthetic methodologies based on the postfunctionalization of easily accessible building blocks of fullerene hexakis adducts. 6] It has been shown that fullerene hexakis adducts that bear 12 peripheral carbohydrate moieties can be prepared in excellent yields by grafting unprotected sugar derivatives onto the fullerene core. Although these fullerene sugar balls are obviously perfectly suited for applications in the field of carbohydrate–lectin interactions, the evaluation of carbohydrate-processing enzyme inhibition with such multivalent derivatives is less obvious. Indeed, among the possible strategies to attain specific potent glycosidase inhibition, the concept of multivalent design has been clearly overlooked. Most enzymes actually have a single, deep active site that is usually less accessible than the shallow binding pockets or grooves on the lectin surfaces. Consequently, a limited number of binding mechanisms, including statistical rebinding, are possible, whereas multivalent ligands may interact with multiple receptors by additional mechanistic options (e.g., the chelate effect, receptor clustering). It is likely that these factors may have hampered interest in projects directed towards the design of multivalent glycosidase inhibitors. In addition, the experimental results obtained to date were not particularly encouraging. Dito tetravalent analogues of 1-deoxynojirimycin, which is a well-known glycosidase inhibitor, generally displayed comparable if not decreased inhibition compared with their monomeric counterparts. The best result reported to date was found for a trivalent iminosugar that showed a sixfold affinity enhancement towards Jack bean a-mannosidase. Herein we report the synthesis of a fullerene hexakis adduct decorated with 12 iminosugar residues. The inhibition profile of this fullerene iminosugar ball has been systematically evaluated against various glycosidases, and dramatic multivalent effects have been observed for the first time. In order to explore the potential of multivalency on glycosidase inhibition with a globular polytopic ligand constructed around the fullerene scaffold, an N-alkyl analogue of 1-deoxynojirimycin was selected as the peripheral ligand. This class of compounds is indeed poorly selective and displays modest to good glycosidase inhibition. It was thus anticipated that these compounds could be excellent models for the examination of the influence of multivalency on inhibition selectivity over a large range of glycosidases. In addition, the alkyl chain on the endocyclic nitrogen atom of the iminosugar is an ideal spacer that may allow for easy grafting onto the central C60 core by means of a cycloaddition reaction. [16] The synthesis of the azide building block is based on the optimization of a strategy reported independently by Overkleeft et al. and Vasella and co-workers. As shown in Scheme 1, the d-hydroxy amide 2 was obtained directly from commercially available tetra-O-benzyl d-glucopyranose (1) in 78% yield by oxidative amidation with iodine in 30% aqueous ammonia (30%). The main advantage of this onepot process is that aldehyde oxidation and C N bond formation are performed in a single synthetic step. Oxidation of the hydroxy group at C5 followed by intramolecular [*] Prof. P. Compain, C. Decroocq, Dr. D. Hazelard Laboratoire de Synth se Organique et Mol cules Bioactives Universit de Strasbourg et CNRS (UMR 7509) Ecole Europ enne de Chimie, Polym res et Mat riaux 25 rue Becquerel, 67087 Strasbourg (France) Fax: (+ 33)3-6885-2754 E-mail: philippe.compain@unistra.fr

Fullerene sugar balls

Fullerene hexakis-adducts bearing 12 peripheral carbohydrate moieties have been prepared by grafting sugar derivatives onto the fullerene core through the copper mediated Huisgen 1,3-dipolar cycloaddition of azides and alkynes.

Electrophosphorescent homo- and heteroleptic copper(I) complexes prepared from various bis-phosphine ligands

Homo- and heteroleptic copper(I) complexes obtained from various chelating bis-phosphine ligands and Cu(CH3CN)4BF4 have been used for the preparation of light emitting devices.

Polycationic Pillar(5)arene Derivatives: Interaction with DNA and Biological Applications

Dendritic pillar[5]arene derivatives have been efficiently prepared by grafting dendrons with peripheral Boc-protected amine subunits onto a preconstructed pillar[5]arene scaffold. Upon cleavage of the Boc-protected groups, water-soluble pillar[5]arene derivatives with 20 (13) and 40 (14) peripheral ammonium groups have been obtained. The capability of these compounds to form stable nanoparticles with plasmid DNA has been demonstrated by gel electrophoresis, transmission electron microscopy (TEM), and dynamic light scattering (DLS) investigations. Transfection efficiencies of the self-assembled 13/pCMV-Luc and 14/pCMV-Luc polyplexes have been evaluated in vitro with HeLa cells. The transfection efficiencies found for both compounds are good, and pillar[5]arenes 13 and 14 show very low toxicity if any.

Photoinduced electron transfer in a clicked fullerene–porphyrin conjugate

A stable C60 derivative bearing an azide functional group has been prepared and used as a building block for the preparation of a fullerene–porphyrin conjugate (F–P–F) by reaction with a Zn(II)–porphyrin bearing two terminal alkyne groups under the copper mediated Huisgen 1,3-dipolar cycloaddition conditions. The electrochemical and photophysical properties of the resulting multicomponent system have been investigated in detail. In benzonitrile, F–P–F undergoes photoinduced electron transfer and the resulting charge separated state is relatively long lived (τ = 0.48 µs). In contrast, intramolecular energy transfer has been evidenced in toluene, with the generation of the fullerene triplet level upon selective excitation of the porphyrin moiety. In this solvent, a CT emission band is observed in the near-infrared region (λmax = 940 nm) as a consequence of a conformational equilibrium causing, to a minor extent, the formation of intramolecular porphyrin–fullerene tight pairs. This finding is supported by measurements of singlet oxygen sensitization and quenching of the long-lived fullerene centered triplet state in the oxygen free solution.

Electronic properties of oligophenylenevinylene and oligophenyleneethynylene arrays constructed on the upper rim of a calix[4]arene core

Heck and Sonogashira cross-coupling reactions have been used for the functionalization of the upper rim of a tetraiodinated calix[4]arene. In this way, oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) arrays have been constructed on the calix[4]arene core to produce covalent assemblies of four π-conjugated chromophores. Electrochemical properties have been investigated by cyclic voltammetry in different solvent/electrolyte systems. Both OPV and OPE calixarenes show the simultaneous reduction of three out of four units, followed, at more negative potential, by the reduction of the fourth unit. This behaviour can be rationalized in view of a partial deconjugation in one OPV or OPE unit. On the other hand, these calixarenes show only oxidation processes corresponding to the exchange of one electron, thus demonstrating the lack of electrochemical equivalency of the OPV or OPE units, in contrast to the reduction behaviour. Electronic absorption and emission spectra have been recorded in solvents of different polarity (toluene, dichloromethane, and benzonitrile). The absorption spectra of the OPV calixarene do not match to the sum of the component units as a consequence of the partial deconjugation of the OPV arms in the calixarene structure. On the contrary, fluorescence spectra, quantum yields and excited state lifetimes of the OPV calixarene are nearly identical to those of the corresponding model compound. The trend of the absorption and emission spectra for the OPE calixarene is reversed relative to that of the OPV counterpart. Absorption spectra are well-matched with those of four trimeric OPE fragments but emission spectra exhibit, besides the typical OPE monomeric fluorescence, a broader and longer-lived emission feature on the low-energy spectral side, attributable to excimer-type interactions. Therefore the arrangement of OPV or OPE units within the same calix-4-arene skeleton brings about different effects on the electronic and luminescence properties of the multicomponent system.

Chiral tridentate versus bidentate pyridines as catalysts in the enantioselective alkylation of benzaldehyde with diethylzinc

Comparison of the enantioselectivity obtained with either bidentate or tridentate chiral pyridine derivatives demonstrated that both can be very efficient catalysts for the enantioselective addition of diethylzinc to benzaldehyde. This comparison also provided insights for the design of efficient new catalysts based on pyridine.

Pillar[5]arene-based glycoclusters: synthesis and multivalent binding to pathogenic bacterial lectins

The synthesis of pillar[5]arene-based glycoclusters has been readily achieved by CuAAC conjugations of azido- and alkyne-functionalized precursors. The lectin binding properties of the resulting glycosylated multivalent ligands have been studied by at least two complementary techniques to provide a good understanding. Three lectins were selected from bacterial pathogens based on their potential therapeutic applications as anti-adhesives, namely LecA and LecB from Pseudomonas aeruginosa and BambL from Burkholderia ambifaria. As a general trend, multivalency improved the binding to lectins and a higher affinity can be obtained by increasing to a certain limit the length of the spacer arm between the carbohydrate subunits and the central macrocyclic core.

Synthesis and electronic properties of fullerene derivatives substituted with oligophenylenevinylen–ferrocene conjugates

C60-bridge-Fc arrays (C60-Fc, C60-PV-Fc, C60-2PV-Fc, C60-3PV-Fc), bearing a fulleropyrrolidine and a ferrocene (Fc) unit connected via an oligophenylenevinylene (OPV) bridge have been prepared. Suitable dyads (PV-Fc, 2PV-Fc, 3PV-Fc) to be used as references for the study of the electronic properties of the largest arrays have been also synthesized. The electrochemical properties of all the dyad and triad multicomponent arrays have been studied by cyclic voltammetry, evidencing a rich and complex electrochemical pattern due to the presence of several electroactive moieties. The first reduction is always assigned to the fullerene moiety and the first oxidation is centred on the Fc group, making the triad systems suitable candidates for photoinduced electron transfer via the interposed bridge. Photophysical studies evidence a complete quenching of the fluorescence of organic conjugated moieties in PV-Fc, 2PV-Fc, 3PV-Fc, possibly via energy transfer to the Fc unit. In the more complex C60/Fc arrays the quenching of the C60 moieties is ultrafast in CH2Cl2 solution and most likely attributable to electron transfer via the OPV wire. In toluene, the dynamic process of singlet and triplet fullerene quenching can be traced via time resolved fluorescence and transient absorption spectroscopy and the values of the rate constants are smaller with increasing donor–acceptor distance. Definitive assignment of the intercomponent quenching mechanism between the fullerene and the ferrocene moiety (energy or electron transfer) can hardly be obtained. Several repeated attempts to detect the radical anion fingerprint of fulleropyrrolidine with transient absorption failed, even in bimolecular quenching experiments. This supports the view that distance-dependent C60 → Fc singlet–triplet and triplet–triplet energy transfer may compete successfully with the desired charge separation step.

Synthesis and structure of a novel molecular bowl with an all-carbon and acyclic framework

A novel all-carbon molecular bowl 2 where the central functionality is surrounded by two rigid tetramethyl-m-terphenyl units was designed. Bromide 3 was readily synthesized by copper (I)-catalyzed coupling reaction between 4 and 5. X-ray crystallographic analysis established that 3 has a bowl-shaped structure although it is an acyclic molecule. Butyl sulfide 6 and thiol 8 were synthesized via lithiation of 3.