Monica Civera

Rhodium-Catalyzed Asymmetric Hydrogenation of Olefins with PhthalaPhos, a New Class of Chiral Supramolecular Ligands

A library of 19 binol-derived chiral monophosphites that contain a phthalic acid diamide group (PhthalaPhos) has been designed and synthesized in four steps. These new ligands were screened in the rhodium-catalyzed enantioselective hydrogenation of prochiral dehydroamino esters and enamides. Several members of the library showed excellent enantioselectivity with methyl 2-acetamido acrylate (6 ligands gave >97% ee), methyl (Z)-2-acetamido cinnamate (6 ligands gave >94% ee), and N-(1-phenylvinyl)acetamide (9 ligands gave >95% ee), whilst only a few representatives afforded high enantioselectivities for challenging and industrially relevant substrates N-(3,4-dihydronaphthalen-1-yl)-acetamide (96% ee in one case) and methyl (E)-2-(acetamidomethyl)-3-phenylacrylate (99% ee in one case). In most cases, the new ligands were more active and more stereoselective than their structurally related monodentate phosphites (which are devoid of functional groups that are capable of hydrogen-bonding interactions). Control experiments and kinetic studies were carried out that allowed us to demonstrate that hydrogen-bonding interactions involving the diamide group of the PhthalaPhos ligands strongly contribute to their outstanding catalytic properties. Computational studies carried out on a rhodium precatalyst and on a conceivable intermediate in the hydrogenation catalytic cycle shed some light on the role played by hydrogen bonding, which is likely to act in a substrate-orientation effect.

PhthalaPhos: Chiral Supramolecular Ligands for Enantioselective Rhodium‐Catalyzed Hydrogenation Reactions

Chemists have largely taken inspiration from Nature in the development of new approaches to synthetic challenges. Combinatorial chemistry stems from the concept of evolution, whereby random mutation of a chemical structure gives rise to libraries of compounds, from which an optimal lead can be found with high probability. On the other hand, Nature makes wide use of noncovalent interactions to build its complex supramolecular architectures and to achieve efficient and selective transformations. In recent years, combinatorial and supramolecular approaches to the development of new ligands for asymmetric catalysis has gained momentum. 2d] The term “supramolecular ligand” encompasses all ligands possessing, besides the atom(s) coordinating to the catalytic metal atom, an additional functionality capable of noncovalent interactions (mainly hydrogen or coordinative bonds) which can play the following roles: 1) self-assembly of two monodentate ligands to form a so-called supramolecular bidentate ligand; 2) binding the substrate(s) in proximity to the catalytic metal center in analogy to metalloenzymes. Among the different kinds of noncovalent interactions that have been used so far for developing supramolecular ligands, hydrogen bonds are arguably the most practical and efficient for several reasons: 1) functional groups capable of hydrogen bonding (e.g., amides, ureas, guanidines) are stable and relatively easy to introduce; 2) hydrogen bonds are created dynamically and reversibly in the reaction medium (where catalysis is to take place), are capable of self-repair when broken, and often coexist with other interactions in a “noninvasive” manner. As a result of our continued interest in developing supramolecular ligands, we report herein the design and synthesis of a novel class of chiral monodentate phosphite ligands, named PhthalaPhos, which contain a phthalic acid primary diamide moiety (Scheme 1). The phthalamidic group

Molecular dynamics simulation of aqueous solutions of trimethylamine-N-oxide and tert-butyl alcohol

In this work we have investigated hydration properties of aqueous solutions up to a solute molar fraction X2 = 0.125 of two isosteric molecules – the bioprotectant trimethylamine-N-oxide (TMAO) and the denaturant tert-butyl alcohol (TBA) – using molecular dynamics simulation at 298 K. Statistical analyses of the trajectories show in particular that as the solute concentration increases the number of the water molecules in the first hydration shell decreases uniformly for TMAO, while for TBA it decreases more rapidly in a concentration range where experiments indicate that TBA starts to self-aggregate. No appreciable solute segregation occurs for TMAO even in the most concentrated solution, where on the average each water molecule is shared by two solutes. This result parallels what has been recently found for glycine betaine, an organic osmolyte closely related to TMAO.

Cyclic RGD‐Containing Functionalized Azabicycloalkane Peptides as Potent Integrin Antagonists for Tumor Targeting

Vitronectin receptors αvβ3 and αvβ5 have emerged as potential therapeutic targets for the treatment of osteoporosis, restenosis, ocular disease, tumor‐induced angiogenesis, metastasis, and sickle‐cell anemia. Among a collection of compounds, a new potent integrin antagonist was synthesized, and its binding toward the αvβ3 and αvβ5 receptors was evaluated. This molecule is a suitable candidate as a vector for therapeutics and diagnostics.

Synthesis of Gd and 68Ga Complexes in Conjugation with a Conformationally Optimized RGD Sequence as Potential MRI and PET Tumor‐Imaging Probes

We report the synthesis of novel chelates of Gd and 68Ga with DPTA, DOTA, HP‐DOA3, as well as with AAZTA, a novel chelating agent developed by our research group. These chelating agents were appropriately conjugated, prior to metal complexation, with DB58, an RGD peptidomimetic, conformationally constrained on an azabicycloalkane scaffold and endowed with high affinity for integrin ανβ3. Because ανβ3 is involved in neo‐angiogenesis in solid tumors and is also directly expressed in cancer cells (e.g. glioblastomas, melanomas) and ovarian, breast, and prostate cancers, these constructs could prove useful as molecular imaging probes in cancer diagnosis by MRI or PET techniques. Molecular modeling, integrin binding assays, and relaxivity assessments allowed the selection of compounds suitable for multiple expression on dendrimeric or nanoparticulate structures. These results also led us to an exploratory investigation of 68Ga complexation for the promising 68Ga‐PET technique; the AAZTA complex 15(68Ga) exhibited uptake in a xenograft model of glioblastoma, suggesting potentially useful developments with new probes with improved affinity.

Antiangiogenic Effect of Dual/Selective α5β1/αvβ3 Integrin Antagonists Designed on Partially Modified Retro-Inverso Cyclotetrapeptide Mimetics

Recent evidence highlighted the role of alpha(5)beta(1) integrin in angiogenesis and in regulating alpha(v)beta(3) integrin function. As a consequence, selective alpha(5)beta(1) integrin inhibitors or dual alpha(5)beta(1)/alpha(v)beta(3) integrin inhibitors are considered promising candidates for the development of cancer therapeutic agents. In this paper, we describe the synthesis and pharmacological characterization of a minilibrary of cyclotetrapeptide mimetics containing a PMRI Arg-Gly-Asp sequence. In particular, c[(R)-betaPhepsi(NHCO)Asppsi(NHCO)Gly-Arg] (3) displayed a good activity in inhibiting the alpha(v)beta(3) integrin-mediated cell adhesion of fibronectin or vitronectin, as well as the adhesion of fibronectin to the alpha(5)beta(1) integrin. Interestingly, the diastereomeric compound c[(S)-betaPhepsi(NHCO)Asppsi(NHCO)Gly-Arg] (2) maintained a good efficacy in inhibiting alpha(5)beta(1) integrin while gaining a certain selectivity over alpha(v)beta(3) integrin. These two integrin antagonists significantly inhibited bFGF-induced human endothelial cell tube formation at submicromolar concentrations. Conformational analysis and Molecular Docking calculations suggest that the different alpha(5)beta(1) versus alpha(v)beta(3) selectivity of 2 and 3 can be rationalized on the basis of the alternative display of the aromatic side chain adjacent to Asp.

A Library Approach to the Development of BenzaPhos: Highly Efficient Chiral Supramolecular Ligands for Asymmetric Hydrogenation

A library of chiral supramolecular ligands, named BenzaPhos, of straightforward preparation (two steps from commercially or readily available starting materials) and modular structure, was designed and synthesized. The ligands were screened in the search for new rhodium catalysts for the enantioselective hydrogenation of several benchmark and industrially relevant substrates. Once a series of hits were identified, structural modifications were introduced on three of the best ligands and a small second-generation library was created. Members of the latter library showed outstanding levels of activity and enantioselectivity in the hydrogenation of challenging olefins, such as enamide S4 and β-dehydroamino ester S5 (>99% ee: best value ever reported in both cases). A series of control experiments were undertaken to clarify the role of hydrogen bonding in determining the catalytic properties of the new ligands. The results of these experiments, together with those of computational studies carried out on four dihydride complexes involved in the catalytic hydrogenation of substrate S4, strongly suggest that a substrate orientation takes place in the catalytic cycle by formation of a hydrogen bond between the ligand amide oxygen atom and the substrate amide NH atom.

STD and trNOESY NMR study of receptor-ligand interactions in living cancer cells.

Integrins are transmembrane receptors that link the cytoskeleton to the extracellular matrix (ECM), mediating cell–cell and cell–matrix adhesion and providing the traction for cell mobility and invasion. They are involved in tumour cell proliferation, migration and survival. Integrin avb3 and avb5 expression is correlated with disease progression in various tumor types (melanoma, prostate, glioblastoma). They bind to the Arg-Gly-Asp (RGD) motif as their primary recognition sequence. In recent years, both preclinical and clinical studies have demonstrated the effectiveness of various integrin antagonists in blocking tumor progression. The cyclic peptide c(ArgGly-Asp-d-Phe-NMe-Val) (cilengitide), developed by Kessler and co-workers, is an avb3 and avb5 antagonist that displayed activity in patients with glioblastoma during phase III clinical trials. The most detailed information so far available for understanding of the structural basis of the interaction is provided by the X-ray structure of cilengitide bound to the avb3 headpiece: the arginine of the RGD motif associates with the av subunit and the aspartate coordinates the bivalent metal ion on the b3 subunit. [5] Because the RGD motif occurs in many extracellular matrix ligands, the recognition specificity would be expected to be modified by other residues and to depend on the conformation of the RGD sequence. Detailed comparison with different ligands for the same integrin binding site might shed light on the essential elements that determine their interactions, specificities and affinities, and might also allow the rational design of new antagonists. The properties of integrins embedded into cell membranes can differ from those of purified receptors, so we have carried out a NMR study (by saturation transfer difference and trNOESY techniques) of the interactions between ECV304 cells (bladder cancer cells in which integrin avb3 is highly expressed ) and the two cyclic RGD mimics 1 and 2 (Scheme 1), with different configurational and structural features. We show that the interactions between these small ligands and membrane-bound proteins can be observed by NMR directly in H2O buffer suspensions of living cells. The data allowed us to identify (by STD) the portions of the ligands in closest contact with the protein and to define (by trNOESY) the preferred conformations of the bound ligands. These NMR techniques focus on the NMR signals of the ligands, exploiting NOE effects between protein and ligands in the protein– ligand complexes. The ligands, the cyclic RGD pentapeptide mimics 1 and 2 (Scheme 1), previously described by workers from our laboratory, are each characterized by a benzylic group connected to carbon 3 of the lactam unit, but differ in the configurations of the C-3 and C-7 lactam stereocentres. As already reported, the conformational preferences of these compounds in the free state are modulated by the configurations of the bicyclic lactam, and so are their affinities for avb3 integrin. These activities were estimated by competition experiments with [I]echistatin for binding to purified avb3 receptors. [7] The resulting IC50 values for 1 and 2 were 6.4 and 154 nm, respectively. Adhesion assay tests were also performed for 1 with ECV304 bladder cancer cells, a cell line derived from solid tumours that highly expresses avb3 integrin. The IC50 values were estimated in competition experiments versus vitronectin (12.5 0.2 mm) and versus fibronectin (7.5 1.9 mm). The micromolar activities obtained in the adhesion assay on ECV cells represent an ideal range for testing interactions between the ligands and the integral membrane protein of intact cells by means of trNOE and STD experiments. NMR experiments : STD and trNOESY experiments were performed with cell suspensions in nondeuterated buffer. The average number of avb3 receptors per ECV304 cell was estimated by fluorescence analysis (FACS) to be 4 10. Approximately 10 cells were used for ligands 1 or 2 (0.7 mmol) in a Scheme 1. Integrin ligands and corresponding avb3 binding affinities.

Dimeric Smac mimetics/IAP inhibitors as in vivo-active pro-apoptotic agents. Part II: Structural and biological characterization.

Novel pro-apoptotic, homodimeric and heterodimeric Smac mimetics/IAPs inhibitors connected through head-head (8), tail-tail (9) or head-tail linkers (10), were biologically and structurally characterized. In vitro characterization (binding to BIR3 and linker-BIR2-BIR3 domains from XIAP and cIAP1, cytotoxicity assays) identified early leads from each dimer family. Computational models and structural studies (crystallography, NMR, gel filtration) partially rationalized the observed properties for each dimer class. Tail-tail dimer 9a was shown to be active in a breast and in an ovary tumor model, highlighting the potential of dimeric Smac mimetics/IAP inhibitors based on the N-AVPI-like 4-substituted 1-aza-2-oxobicyclo[5.3.0]decane scaffold as potential antineoplastic agents.

Development of isoxazoline-containing peptidomimetics as dual αvβ3 and α5β1 integrin ligands.

Isoxazoline‐containing peptidomimetics, designed to be effective αvβ3 and α5β1 integrin ligands, were synthesized through an original procedure involving N,O‐bis(trimethylsilyl)hydroxyamine conjugate addition to alkylidene acetoacetates, followed by intramolecular hemiketalization. To mimic the RGD recognition sequence, basic and acidic terminal appendages were introduced, and the final products were tested in cell adhesion inhibition assays. All the synthesized compounds proved to be excellent ligands for both integrin receptors, and a strong influence on intracellular signaling and phosphorylation pathways was demonstrated by evaluation of fibronectin‐induced phosphorylation of ERK. The molecular basis of the observed inhibitory activity was suggested on the results of docking experiments.