Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions.

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners established that this step occurs without redox involvement of the pyridine(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C(sp3)-C(sp3) reductive coupling. A series of labeling experiments with pyridine(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C(sp3)-C(sp3) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure-activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as the promotion of chemical recycling of cycloaddition polymers.

Piperazine squaric acid diamides, a novel class of allosteric P2X7 receptor antagonists.

The P2X7 receptor (P2X7R) stands out among the purinergic receptors due to its strong involvement in the regulation of tumor growth and metastasis formation as well as in innate immune responses and afferent signal transmission. Numerous studies have pointed out the beneficial effects of P2X7R antagonism for the treatment of a variety of cancer types, inflammatory diseases, and chronic pain. Herein we describe the development of novel P2X7R antagonists, incorporating piperazine squaric diamides as a central element. Besides improving the antagonists' potency from pIC50 values of 5.7-7.6, ADME properties (logD7.4 value, plasma protein binding, in vitro metabolic stability) of the generated compounds were investigated and optimized to provide novel P2X7R antagonists with drug-like properties. Furthermore, docking studies revealed the antagonists binding to the allosteric binding pocket in two distinct binding poses, depending on the substitution of the central piperazine moiety.

Squaraine dyes as serum albumins probes: Synthesis, photophysical experiments and molecular docking studies.

Three barbiturate squaraine dyes derived from indolenine or benzothiazole, with different barbituric acid derivatives were prepared, characterized and photophysically evaluated by standard spectroscopic methods. As expectable for squaraines, these dyes showed narrow and intense absorption and emission bands in the Vis/NIR region. The interaction of synthesized dyes with bovine and human serum albumins (BSA and HSA) was also evaluated in phosphate buffer (PB). The results revealed that upon the addition of BSA or HSA the complex dye-protein emit more fluorescence, and the emission intensity is directly proportional to the concentration of protein used (0-3.5 µM). The titration tests allowed to calculate the binding constants, in an order of magnitude of 104-106 M, as well as the limits of detection and quantification in the nanomolar tens range. All dyes showed a good response to the interaction with both proteins, but the most pronounced envisioning their use as protein labeling was observed for the squaraine dye derived from the indolenine with a 1,3-dimethylbarbituric acid moiety. The molecular docking studies revealed the existence of a binding between the compounds and four sites on the HSA molecule, where one of these four locations is a new binding site with which this series of dye interacts.

A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes.

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.

New Catalytic Radical Process Involving 1,4-Hydrogen Atom Abstraction: Asymmetric Construction of Cyclobutanones.

While alkyl radicals have been well demonstrated to undergo both 1,5- and 1,6-hydrogen atom abstraction (HAA) reactions, 1,4-HAA is typically a challenging process both entropically and enthalpically. Consequently, chemical transformations based on 1,4-HAA have been scarcely developed. Guided by the general mechanistic principles of metalloradical catalysis (MRC), 1,4-HAA has been successfully incorporated as a key step, followed by 4-exo-tet radical substitution (RS), for the development of a new catalytic radical process that enables asymmetric 1,4-C-H alkylation of diazoketones for stereoselective construction of cyclobutanone structures. The key to success is the optimization of the Co(II)-based metalloradical catalyst through judicious modulation of D2-symmetric chiral amidoporphyrin ligand to adopt proper steric, electronic, and chiral environments that can utilize a network of noncovalent attractive interactions for effective activation of the substrate and subsequent radical intermediates. Supported by an optimal chiral ligand, the Co(II)-based metalloradical system, which operates under mild conditions, is capable of 1,4-C-H alkylation of α-aryldiazoketones with varied electronic and steric properties to construct chiral α,ß-disubstituted cyclobutanones in good to high yields with high diastereoselectivities and enantioselectivities, generating dinitrogen as the only byproduct. Combined computational and experimental studies have shed light on the mechanistic details of the new catalytic radical process, including the revelation of facile 1,4-HAA and 4-exo-tet-RS steps. The resulting enantioenriched α,ß-disubstituted cyclobutanones, as showcased with several enantiospecific transformations to other types of cyclic structures, may find useful applications in stereoselective organic synthesis.

Olefin-Supported Cationic Copper Catalysts for Photochemical Synthesis of Structurally Complex Cyclobutanes.

The sole method available for the photocycloaddition of unconjugated aliphatic alkenes is the Cu-catalyzed Salomon-Kochi reaction. The [Cu(OTf)]2 ⋅benzene catalyst that has been standard in this reaction for many decades, however, is air-sensitive, prone to photodecomposition, and poorly reactive towards sterically bulky alkene substrates. Using bench-stable precursors, an improved catalyst system with superior reactivity and photostability has been designed, and it offers significantly expanded substrate scope. The utility of this new catalyst for the preparation of sterically crowded cyclobutane structures is highlighted through the preparation of the cores of the natural products sulcatine G and perforatol.

Synthesis and biological effect of lysosome-targeting fluorescent anion transporters with enhanced anionophoric activity.

Two lysosome-targeting fluorescent anion transporters derived from coumarins, trifluoromethylated arylsquaramides and morpholines were synthesized, and their specificity and efficiency to target and alkalize lysosomes were investigated. They are able to target lysosomes specifically. Compared with the previous analogue without trifluoromethyl substituents, these two conjugates, in particular the one having a 3,5-bis(trifluoromethyl) substituent, exhibit significantly higher ability to facilitate the transport of chloride anions, alkalize lysosomes and reduce the activity of lysosomal Cathepsin B enzyme. The present finding suggests that improving the anionophoric activity of lysosome-targeting fluorescent anion transporters is favorable to the efficiency to alkalize lysosomes and deactivate lysosomal Cathepsin B enzyme.

Enantioselective Type II Cycloaddition of Alkynes via C-C Activation of Cyclobutanones: Rapid and Asymmetric Construction of [3.3.1] Bridged Bicycles.

Synthesis of bridged scaffolds via Type II cyclization constitutes substantial challenges due to the intrinsic ring strain accumulated in reaction transition states. Catalytic enantioselective Type II-cyclization methods are even rarer. Here, we describe a detailed study of developing a Rh(I)-catalyzed enantioselective intramolecular Type II cyclization of alkynes via C-C activation of cyclobutanones. This method offers a rapid approach to access a wide range of functionalized [3.3.1]-bridged bicycles along with an exocyclic olefin and an all-carbon quaternary stereocenter. Excellent enantioselectivity has been achieved using a combination of cationic rhodium(I) and DTBM-segphos. Attributed to the redox neutral and strong acid/base-free reaction conditions, high chemoselectivity has also been observed. For the oxygen-tethered substrates, the reaction can proceed at room temperature. In addition, partial kinetic resolution has been achieved for substrates with existing stereocenters, forging interesting chiral tricyclic scaffolds. The methylalkyne-derived substrates gave unexpected dimeric structures in good yield with excellent enantioselectivity and complete diastereoselectivity. Furthermore, the bridged bicyclic products can be diversely functionalized through simple transformations. Finally, mechanistic studies reveal a surprising reaction pathway that involves forming a metal-stabilized anti-Bredt olefin intermediate.

Design and Synthesis of Near-Infrared Mechanically Interlocked Molecules for Specific Targeting of Mitochondria.

The entrapment of squaraine (SQ) within a molecular container to form rotaxane has been shown to improve the dye stability and the fluorescence proficiency inside the mitochondria. The macrocycle provides shelter and protects the near-infrared (NIR) SQ chromophore from nucleophilic attacks made by the exposed thiol of Cys-containing mitochondrial proteins and mitochondrial glutathione. Herein a microwave-assisted template-directed clipping reaction on low-loading 2-chlorotrityl chloride resin is used to develop an NIR unsymmetrical squaraine rotaxane in high quantum yield.

Squaraine Dyes: Molecular Design for Different Applications and Remaining Challenges.

Squaraine dyes are a class of organic dyes with strong and narrow absorption bands in the near-infrared. Despite high molar absorptivities and fluorescence quantum yields, these dyes have been less explored than other dye scaffolds due to their susceptibility to nucleophilic attack. Recent strategies in probe design including encapsulation, conjugation to biomolecules, and new synthetic modifications have seen squaraine dyes emerging into the forefront of biomedical imaging and other applications. Herein, we provide a concise overview of (1) the synthesis of symmetrical and unsymmetrical squaraine dyes, (2) the relationship between structure and photophysical properties of squaraine dyes, and (3) current applications of squaraine dyes in the literature. Given the recent successes at overcoming the limitations of squaraine dyes, they show high potential in biological imaging, in photodynamic and photothermal therapies, and as molecular sensors.

Lessons in Strain and Stability: Enantioselective Synthesis of (+)-[5]-Ladderanoic Acid.

The synthesis of structurally complex and highly strained natural products provides unique challenges and unexpected opportunities for the development of new reactions and strategies. Herein, the synthesis of (+)-[5]-ladderanoic acid is reported. En route to the target, unusual and unexpected strain release driven transformations were uncovered. This occurrence required a drastic revision of the synthetic design that ultimately led to the development of a novel stepwise cyclobutane assembly by an allylboration/Zweifel olefination sequence.

Kinetic Resolution via Rh-Catalyzed C-C Activation of Cyclobutanones at Room Temperature.

Herein we describe the development of a highly selective kinetic resolution of cyclobutanones via a Rh-catalyzed "cut-and-sew" reaction with selectivity factor up to 785. This reaction takes place at room temperature with excellent efficiency. Various trans-5,6-fused bicycles and C2-substituted cyclobutanones were obtained with excellent ee's that can be further used as chiral building blocks. DFT calculations reveal the crucial roles of the DTBM-segphos ligand in stabilizing the rate- and enantioselectivity-determining C-C oxidative addition transition state via favorable ligand-substrate dispersion interactions.

Interactions between an amphipathic di-histidine peptide and a metal affinity chromatographic resin derived from a bis(tacn)butane chelating ligand.

The interactive behavior of an amphipathic peptide with the Cu2+ , Ni2+ , and Zn2+ complexes of 1,4-bis(triazacyclonon-1-yl)butane), bis(tacn)but , immobilized onto Sepharose CL-4B, has been investigated. The effects of incubation time, as well as the incubation buffer pH and ionic strength, have been examined. The binding data have been interrogated using Langmuir, Langmuir-Freundlich, bi-Langmuir, and Temkin isothermal models and Scatchard plots. These results confirm that this amphipathic peptide binds with relatively high capacities to the immobilized Cu2+ - and Ni2+ -1,4-bis(triazacyclonon-1-yl)butane)-Sepharose CL-4B sorbents via at least two discrete sites. However, the corresponding immobilized Zn2+ -sorbent had low binding capacity. Moreover, the magnitude of the binding capacities of these sorbents was dependent on the pH and ionic strength of the incubation buffer. These results are relevant to the isolation of E. coli expressed recombinant proteins that incorporate this and related amphipathic peptide tags, containing two or more histidine residues, located at the N- or C-terminus of the recombinant protein, and the co-purification of low abundance host cell proteins of diverse structure, by immobilized metal ion affinity chromatographic methods.

Synthesis and DNase I inhibitory properties of new benzocyclobutane-2,5-diones.

Aim: Eight new benzocyclobutane-2,5-diones (1a-1h) were synthesized, and their inhibitory properties against bovine pancreatic DNase I were examined in vitro. Methods & results: Compounds 1a-1h were synthesized using photocycloaddition of duroquinone with various phenyl-substituted ethylenes in the presence of 18W compact fluorescent lamp (visible light). Two compounds, 1,3,4,6-tetramethyl-7-phenylbicyclo[4.2.0]oct-3-ene-2,5-dione (1a) and 1,3,4,6-tetramethyl-7-p-tolylbicyclo[4.2.0]oct-3-ene-2,5-dione (1b) inhibited DNase I in a noncompetitive manner with IC50 values below 150 µM and showed to be more potent DNase I inhibitors than crystal violet, used as a positive control. In order to analyze potential binding sites for the studied compounds with DNase I, molecular docking study was performed. Conclusion: The studied benzocyclobutane-2,5-diones offer a good starting point for a design of new DNase I inhibitors.

Catalytic Enantioselective Synthesis of Cyclobutenes from Alkynes and Alkenyl Derivatives.

Discovery of enantioselective catalytic reactions for the preparation of chiral compounds from readily available precursors, using scalable and environmentally benign chemistry, can greatly impact their design, synthesis, and eventually manufacture on scale. Functionalized cyclobutanes and cyclobutenes are important structural motifs seen in many bioactive natural products and pharmaceutically relevant small molecules. They are also useful precursors for other classes of organic compounds such as other cycloalkane derivatives, heterocyclic compounds, stereodefined 1,3-dienes, and ligands for catalytic asymmetric synthesis. The simplest approach to make cyclobutenes is through an enantioselective [2 + 2]-cycloaddition between an alkyne and an alkenyl derivative, a reaction which has a long history. Yet known reactions of this class that give acceptable enantioselectivities are of very narrow scope and are strictly limited to activated alkynes and highly reactive alkenes. Here, we disclose a broadly applicable enantioselective [2 + 2]-cycloaddition between wide variety of alkynes and alkenyl derivatives, two of the most abundant classes of organic precursors. The key cycloaddition reaction employs catalysts derived from readily synthesized ligands and an earth-abundant metal, cobalt. Over 50 different cyclobutenes with enantioselectivities in the range of 86-97% ee are documented. With the diverse functional groups present in these compounds, further diastereoselective transformations are easily envisaged for synthesis of highly functionalized cyclobutanes and cyclobutenes. Some of the novel observations made during these studies including a key role of a cationic Co(I)-intermediate, ligand and counterion effects on the reactions, can be expected to have broad implications in homogeneous catalysis beyond the highly valuable synthetic intermediates that are accessible by this route.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands.

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from ß-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Synthesis of Cyclobutane Analogue 4: Preparation of Purine and Pyrimidine Carbocyclic Nucleoside Derivatives.

The coupling of 2-bromo-3-benzoyloxycyclobutanone with purine under basic conditions produces two regioisomers consisting of the N-7 and N-9 alkylated products in equal amounts in their racemic forms. The distribution of the isomers is consistent with the charge delocalization between the N-7 and N-9 positions of the purinyl anion. The structural assignments and relative stereochemistry of each regioisomer were based on 1 and 2D NMR techniques. The relative stereochemistry of the C-2 and C-3 substituents in each regioisomer was the trans orientation consistent with steric factors in the coupling step. The N-9 regioisomer was reduced with sodium borohydride to give the all trans cyclobutanol as the major product in a stereoselective manner. The alcohol was debenzoylated with sodium methoxide in a transesterification step to give the nucleoside analogue. The regioisomeric pyrimidine nucleosides were prepared by Vorbrüggen coupling of the 3-hydroxymethylcyclobutanone triflate with either thymine or uracil followed by stereoselective hydride addition. Regiospecificity of the coupling at the N-1 position was observed and stereoselective reduction to the trans-disubstituted cyclobutanol structure assignments was based on NMR data.

New barium, strontium and strontium-doped barium squarates: synthesis, crystal structures and DNA/BSA binding, antioxidant and in vitro cytotoxicity studies.

A new set of differently hydrated barium and strontium squarates, namely poly[[triaqua(µ-1,2-dioxocyclobut-3-ene-1,2-diolato)barium] monohydrate], {[Ba(C4O4)(H2O)3]·H2O}n (1), poly[[diaqua(µ-1,2-dioxocyclobut-3-ene-1,2-diolato)strontium] monohydrate], {[Sr(C4O4)(H2O)2]·H2O}n (2), and poly[[triaqua(µ-1,2-dioxocyclobut-3-ene-1,2-diolato)barium/strontium(0.85/0.15)] monohydrate], {[Ba0.85Sr0.15(C4O4)(H2O)3]·H2O}n (3), is reported. The study of their crystal structures indicates that all the complexes crystallize in the triclinic space group P-1. Complexes 1 and 3 have a rare combination of squarate units coordinated through monodentate O atoms to two different metal atoms and through two bidentate O atoms to three different metal atoms. Furthermore, they have three coordinated water molecules to give a coordination number of nine. The squarate ligands in complex 2 exhibit two different coordination modes: (i) monodentate O atoms coordinated to four different Sr atoms and (ii) two monodentate O atoms coordinated to two different metal atoms with the other two O atoms bidentate to four different Sr atoms. All the compounds decompose to give the respective carbonates when heated to 800 °C, as evidenced by thermogravimetry/differential thermal analysis (TG-DTA), which are clusters of nanoparticles. Complexes 1 and 3 show additional endothermic peaks at 811 and 820 °C, respectively, indicating the phase transition of BaCO3 from an orthorhombic (α-Pmcn) to a trigonal phase (ß-R3m). All three complexes have significant DNA-binding constants, ranging from 2.45 × 104 to 9.41 × 104 M-1 against EB-CT (ethidium bromide-calf thymus) DNA and protein binding constants ranging from 1.1 × 105 to 8.6 × 105 with bovine serum albumin. The in vitro cytotoxicity of the complexes is indicated by the IC50 values, which range from 128.8 to 261.3 µg ml-1. Complex 3 shows better BSA binding, antioxidant activity against the DPPH radical and cytotoxicity than complexes 1 and 2.

Enantioselective [2+2] Cycloadditions of Cinnamate Esters: Generalizing Lewis Acid Catalysis of Triplet Energy Transfer.

We report the enantioselective [2+2] cycloaddition of simple cinnamate esters, the products of which are useful synthons for the controlled assembly of cyclobutane natural products. This method utilizes a cocatalytic system in which a chiral Lewis acid accelerates the transfer of triplet energy from an excited-state Ir(III) photocatalyst to the cinnamate ester. Computational evidence indicates that the principal role of the Lewis acid cocatalyst is to lower the absolute energies of the substrate frontier molecular orbitals, leading to greater electronic coupling between the sensitizer and substrate and increasing the rate of the energy transfer event. These results suggest Lewis acids can have multiple beneficial effects on triplet sensitization reactions, impacting both the thermodynamic driving force and kinetics of Dexter energy transfer.

Synthesis, Photochemical and In Vitro Cytotoxic Evaluation of New Iodinated Aminosquaraines as Potential Sensitizers for Photodynamic Therapy.

In this work, several benzothiazole-based aminosquaraine dyes, displaying strong absorption within the so-called phototherapeutic window (650⁻800 nm), were synthesized. The ability, of all the new dyes, to generate singlet oxygen was assessed by determining the correspondent phosphorescence emission and through the comparison with a standard. The quantum yields of singlet oxygen generation were determined and exhibited to be strongly dependent on the nature of the amino substituents introduced in the squaric ring. The photodynamic activity of the synthesized dyes was tested against four human tumor cell lines: breast (MCF-7), lung (NCI-H460), cervical (HeLa) and hepatocellular (HepG2) carcinomas; and a non-tumor porcine liver primary cell culture (PLP2). All the compounds synthesized were found to be able to inhibit tumor cells growth upon irradiation more than in the dark, in most of the cases, very significantly. Considering the photodynamic activity exhibited and the low toxicity displayed for the non-tumor cells, some of the synthetized dyes can be regarded as potential candidates as photosensitizers for PDT.