Utilizing redox-mediated Bergman cyclization toward the development of dual-action metalloenediyne therapeutics

Pronounced electronic modulation of geometrically-regulated metalloenediyne cyclization

Using a diverse array of thermally robust phosphine enediyne ligands (dxpeb, X = Ph, Ph-pOCH3, Ph-pCF3, Ph-m 2CH3, Ph-m 2CF3, iPr, Cy, and t Bu) a novel suite of cisplatin-like Pt(ii) metalloenediynes (3, Pt(dxpeb)Cl2) has been synthesized and represents unique electronic perturbations on thermal Bergman cyclization kinetics. Complexes 3e (Ph-m 2CF3) and 3f (iPr) are the first of this structure type to be crystallographically characterized with inter alkyne termini distances (3e: 3.13 Å; 3f: 3.10 Å) at the lower end of the widely accepted critical distance range within which enediynes should demonstrate spontaneous ambient temperature cyclization. Despite different electronic profiles, these metalloenediynes adopt a rigid, uniform structure suggesting complexes of the form Pt(dxpeb)Cl2 have orthogonalized geometric and electronic contributions to thermal Bergman cyclization. Kinetic activation parameters determined using 31P NMR spectroscopy highlight the dramatic reactivity and thermal tunability of these complexes. At room temperature, the half-life (t 1/2) of cyclization spans a range of ∼35 hours and for the aryl phosphine derivatives, cycloaromatization rates are 10-30 times faster for complexes with electron donating substituents (3b: Ph-pOCH3; 3d: Ph-m 2CH3) compared to those with electron withdrawing substituents (3c: Ph-pCF3; 3e: Ph-m 2CF3). Computational interrogation of the aryl phosphine metalloenediynes 3a-3e reveals that the origin of this precise electronic control derives from electronic withdrawing group-mediated alkyne carbon polarization that amplifies coulombic repulsion increasing the cyclization barrier height. Additionally, mixing between the in-plane π-orbitals and the phosphine aryl ring system is pronounced for complexes with electron donating substituents which stabilizes the developing C-C bond and lowers the activation barrier. This π-orbital mixing is negligible however, for complexes with electron withdrawing substituents due to an energetic mismatch of the orbital systems. Overall, this work demonstrates that for geometrically rigid frameworks, even remote enediyne functionalization can have pronounced effects on activation barrier.

Identification of a potent palladium-aryldiphosphine catalytic system for high-performance carbonylation of alkenes

The development of stable and efficient ligands is of vital significance to enhance the catalytic performance of carbonylation reactions of alkenes. Herein, an aryldiphosphine ligand (L11) bearing the [Ph2P(ortho-C6H4)]2CH2 skeleton is reported for palladium-catalyzed regioselective carbonylation of alkenes. Compared with the industrially successful Pd/1,2-bis(di-tert-butylphosphinomethyl)benzene catalyst, catalytic efficiency catalyzed by Pd/L11 on methoxycarbonylation of ethylene is obtained, exhibiting better catalytic performance (TON: >2,390,000; TOF: 100,000 h-1; selectivity: >99%) and stronger oxygen-resistance stability. Moreover, a substrate compatibility (122 examples) including chiral and bioactive alkenes or alcohols is achieved with up to 99% yield and 99% regioselectivity. Experimental and computational investigations show that the appropriate bite angle of aryldiphosphine ligand and the favorable interaction of 1,4-dioxane with Pd/L11 synergistically contribute to high activity and selectivity while the electron deficient phosphines originated from electron delocalization endow L11 with excellent oxygen-resistance stability.

Characterization and initial demonstration of in vivo efficacy of a novel heat-activated metalloenediyne anti-cancer agent

Background:

Enediynes are anti-cancer agents that are highly cytotoxic due to their propensity for low thermal activation of radical generation. The diradical intermediate produced from Bergman cyclization of the enediyne moiety may induce DNA damage and cell lethality. The cytotoxicity of enediynes and difficulties in controlling their thermal cyclization has limited their clinical use. We recently showed that enediyne toxicity at 37 °C can be mitigated by metallation, but cytotoxic effects of ‘metalloenediynes’ on cultured tumor cells are potentiated by hyperthermia. Reduction of cytotoxicity at normothermia suggests metalloenediynes will have a large therapeutic margin, with cell death occurring primarily in the heated tumor. Based on our previous in vitro findings, FeSO₄-PyED, an Fe co-factor complex of (Z)-N,N^׳-bis[1-pyridin-2-yl-meth-(E)-ylidene]oct-4-ene-2,6-diyne-1,8-diamine, was prioritized for further in vitro and in vivo testing in normal human melanocytes and melanoma cells.

Methods:

Clonogenic survival, apopotosis and DNA binding assays were used to determine mechanisms of enhancement of FeSO₄-PyED cytotoxicity by hyperthermia. A murine human melanoma xenograft model was used to assess in vivo efficacy of FeSO₄-PyED at 37 or 42.5 °C.

Results:

FeSO₄-PyED is a DNA-binding compound. Enhancement of FeSO₄-PyED cytotoxicity by hyperthermia in melanoma cells was due to Bergman cyclization, diradical formation, and increased apoptosis. Thermal enhancement, however, was not observed in melanocytes. FeSO₄-PyED inhibited tumor growth when melanomas were heated during drug treatment, without inducing normal tissue damage.

Conclusion:

By leveraging the unique thermal activation properties of metalloenediynes, we propose that localized moderate hyperthermia can be used to confine the cytotoxicity of these compounds to tumors, while sparing normal tissue.

Organophosphines in PtP4 derivatives; structural aspects

In this review, we classify and analyze the structural data of more than 80 monomeric platinum coordination complexes with an inner coordination sphere of PtP4 in which only organophosphines are involved. On the basis of the coordination mode of respective organophosphines, these complexes can be divided into six subgroups: Pt(PL)4, Pt(PL)2(PL′)2, Pt(η2-P2L)(PL)2, Pt(η2-P2L)2, Pt(η2-P2L)(η2-P2L′)2, and Pt(η4-P4L). The chelating ligands forming wide varieties of metallocycles: (P=P), (PNP), (PCP), (PC2P), (PP2P), (PC2NP), (PNCNP), (PNPNP), (POHOP), (POBOP), (PCNCP), (PC3P), (PC4P), and (PC2OC2P). The effects of both steric and electronic factors reflect on the values of P-Pt-P chelate angles. The total mean values of Pt-P elongate in the order: 2.289 Å (tetradentate)<2.306 Å (monodentate)<2.320 Å (bidentate). The same order shows the respective covalent bond weaknesses.

The Outliers: Metal-Mediated Radical Reagents for Biological Substrate Degradation

The predictable and controllable interaction of small organic or peptidic molecules with biological substrates is the primary reason most pharmaceuticals are narrowly decorated carbon frameworks. The inhibition or activation binding models are measurable and without side reactions that can cause pathological angst. Yet many diseases, especially those involving rapid proliferation of cells (i.e., cancer) or aggregation of peptides (e.g., heart disease, Alzheimer's disease) have not yet been cured by inhibition therapeutics. Additionally, interventional medicine is often required to alleviate such maladies by physical removal first, followed by molecular-level therapy as a second stage. Thus, there appears to be a niche for more aggressive therapeutics that may employ harsher chemical processes to realize clinical efficacy, albeit without causing catastrophic side effects. Molecules that may be considered for this challenge are not typically biomimetic, nor do they fit the traditional pharmaceutical paradigm. They may have unusual modes of action or undesired reactivity that can be lethal if not controlled. These are the outliers; potential pharmacophores that biology does not know how to manage or adapt to. This is why they may be an intriguing class of agents that needs continuous development. In this Account, we connect the under-developed enediyne family of compounds and our metalloenediyne derivatives to existing radical-based therapeutics such as bleomycin and doxorubicin to illustrate that controlled diradical reactivity, although an outlier mechanism, has a place in the therapeutic portfolio. This is self-evident in that of the 11 natural product enediynes known, 2 have clinical impact, a strong ratio. We expand on the chemical diversity of potential enediyne constructs and focus on the accessible trigger mechanisms to activate diradical formation as a method to control toxicity. Moreover, we further illustrate how electromagnetic fields can be employed to activate both molecular and larger nanomaterial constructs that carry highly concentrated payloads of reactive reagent. Finally, we describe how controlled diradical reactivity can reach beyond traditional therapeutic targets such as DNA, to peptide aggregates found in blood clots, neural fibrils, and membrane scaffolds. It is our belief that cleverly constructed frameworks with well-designed and controlled activation/reaction schemes can lead to novel therapeutics that can challenge evolving viral and bacterial invaders. From this evangelical perspective, our hope is that the conceptual framework, if not the specific designs in this Account, stimulate the readership to develop out-of-the-box therapeutic designs that may combat resistant disease targets.

Anion Control of Lanthanoenediyne Cyclization

A suite of lanthanoenediyne complexes of the form Ln(macrocycle)X₃ (Ln = La³⁺, Ce³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Lu³⁺; X = NO₃^-, Cl^-, OTf^-) was prepared by utilizing an enediyne-containing [2 + 2] hexaaza-macrocycle (2). The solid-state Bergman cyclization temperatures, measured via DSC, decrease with the denticity of X (bidentate NO₃^-, T = 267-292 °C; monodentate Cl^-, T = 238-262 °C; noncoordinating OTf^-, T = 170-183 °C). ¹³C NMR characterization shows that the chemical shifts of the acetylenic carbon atoms also rely on the anion identity. The alkyne carbon closest to the metal binding site, C_A, exhibits a Δδ > 3 ppm downfield shift, while the more distal alkyne carbon, C_B, displays a concomitant Δδ ≤ 2.5 ppm upfield shift, reflecting a depolarization of the alkyne on metal inclusion. For all metals studied, the degree of perturbation follows the trend 2 < NO₃^- < Cl^- < OTf^-. This belies a greater degree of electronic rearrangement in the coordinated macrocycle as the denticity of X and its accompanying shielding of the metal's Lewis acidity decrease. Computationally modeled structures of LnX₃ show a systematic increase in the lanthanide-2 coordination number (CN_La-mc = 2 (NO₃^-), 4 (Cl^-), 5 (H₂O, model for OTf^-)) and a decrease in the mean Ln-N bond length (La-N_average = 2.91 Å (NO₃^-), 2.78 Å (Cl^-), 2.68 Å (H₂O)), further suggesting that a decrease in the anion coordination number correlates with an increase in the metal-macrocycle interaction. Taken together, these data illustrate a Bergman cyclization landscape that is influenced by the bonding of metal to an enediyne ligand but whose reaction barrier is ultimately dominated by the coordinating ability of the accompanying anion.

Enediyne-Comprising Amino Aldehydes in the Passerini Reaction

Multicomponent reactions represent a highly efficient approach to a broad spectrum of structurally diverse compounds starting from simple and affordable compounds. A focused library of tweezers-like compounds is prepared by employing the multicomponent Passerini reaction comprising enediyne-derived amino aldehydes. The reaction proceeds under mild conditions yielding Passerini products in good to excellent yields. Postcondensation modifications of Passerini products are demonstrated through a simple deprotection/coupling approach comprising amino functionality, furnishing enediyne cores with highly decorated arms.

The role of ligand covalency in the selective activation of metalloenediynes for Bergman cyclization

One of the key concerns with the development of radical-generating reactive therapeutics is the ability to control the activation event within a biological environment. To that end, a series of quinoline-metal-loenediynes of the form M(QuiED)·2Cl (M = Cu(II), Fe(II), Mg(II), or Zn(II)) and their independently synthesized cyclized analogs have been prepared in an effort to elucidate Bergman cyclization (BC) reactivity differences in solution. HRMS(ESI) establishes a solution stoichiometry of 1:1 metal to ligand with coordination of one chloride counter ion to the metal center. EPR spectroscopy of Cu(QuiED)·2Cl and Cu (QuiBD)·2Cl denotes an axially-elongated tetragonal octahedron (g║ > g⊥ > 2.0023) with a dx2-y2 ground state, while the electronic absorption spectrum reveals a pπ Cl→Cu(II) LMCT feature at 19,000 cm -1, indicating a solution structure with three nitrogens and a chloride in the equatorial plane with the remaining quinoline nitrogen and solvent in the axial positions. Investigations into the BC activity reveal formation of the cyclized product from the Cu(II) and Fe(II) complexes after 12 h at 45 °C in solution, while no product is observed for the Mg(II) or Zn(II) complexes under identical conditions. The basis of this reactivity difference has been found to be a steric effect leading to metal-ligand bond elongation and thus, a retardation of solution reactivity. These results demonstrate how careful consideration of ligand and complex structure may allow for a degree of control and selective activation of these reactive agents.

Chelation-induced diradical formation as an approach to modulation of the amyloid-β aggregation pathway

Current approaches toward modulation of metal-induced Aβ aggregation pathways involve the development of small molecules that bind metal ions, such as Cu(ii) and Zn(ii), and interact with Aβ. For this effort, we present the enediyne-containing ligand (Z)-N,N'-bis[1-pyridin-2-yl-meth(E)-ylidene]oct-4-ene-2,6-diyne-1,8-diamine (PyED), which upon chelation of Cu(ii) and Zn(ii) undergoes Bergman-cyclization to yield diradical formation. The ability of this chelation-triggered diradical to modulate Aβ aggregation is evaluated relative to the non-radical generating control pyridine-2-ylmethyl-(2-{[(pyridine-2-ylmethylene)-amino]-methyl}-benzyl)-amine (PyBD). Variable-pH, ligand UV-vis titrations reveal pK_a = 3.81(2) for PyBD, indicating it exists mainly in the neutral form at experimental pH. Lipinski's rule parameters and evaluation of blood-brain barrier (BBB) penetration potential by the PAMPA-BBB assay suggest that PyED may be CNS+ and penetrate the BBB. Both PyED and PyBD bind Zn(ii) and Cu(ii) as illustrated by bathochromic shifts of their UV-vis features. Speciation diagrams indicate that Cu(ii)-PyBD is the major species at pH 6.6 with a nanomolar K_d, suggesting the ligand may be capable of interacting with Cu(ii)-Aβ species. In the presence of Aβ_40/42 under hyperthermic conditions (43 °C), the radical-generating PyED demonstrates markedly enhanced activity (2-24 h) toward the modulation of Aβ species as determined by gel electrophoresis. Correspondingly, transmission electron microscopy images of these samples show distinct morphological changes to the fibril structure that are most prominent for Cu(ii)-Aβ cases. The loss of CO₂ from the metal binding region of Aβ in MALDI-TOF mass spectra further suggests that metal-ligand-Aβ interaction with subsequent radical formation may play a role in the aggregation pathway modulation.