Dynamic and Assembly Characteristics of Deep-Cavity Basket Acting as a Host for Inclusion Complexation of Mitoxantrone in Biotic and Abiotic Systems.

We describe the preparation, dynamic, assembly characteristics of vase-shaped basket 13- along with its ability to form an inclusion complex with anticancer drug mitoxantrone in abiotic and biotic systems. This novel cavitand has a deep nonpolar pocket consisting of three naphthalimide sides fused to a bicyclic platform at the bottom while carrying polar glycines at the top. The results of 1 H Nuclear Magnetic Resonance (NMR), 1 H NMR Chemical Exchange Saturation Transfer (CEST), Calorimetry, Hybrid Replica Exchange Molecular Dynamics (REMD), and Microcrystal Electron Diffraction (MicroED) measurements are in line with 1 forming dimer [12 ]6- , to be in equilibrium with monomers 1(R) 3- (relaxed) and 1(S) 3- (squeezed). Through simultaneous line-shape analysis of 1 H NMR data, kinetic and thermodynamic parameters characterizing these equilibria were quantified. Basket 1(R) 3- includes anticancer drug mitoxantrone (MTO2+ ) in its pocket to give stable binary complex [MTO⊂1]- (Kd =2.1 µM) that can be precipitated in vitro with UV light or pH as stimuli. Both in vitro and in vivo studies showed that the basket is nontoxic, while at a higher proportion with respect to MTO it reduced its cytotoxicity in vitro. With well-characterized internal dynamics and dimerization, the ability to include mitoxantrone, and biocompatibility, the stage is set to develop sequestering agents from deep-cavity baskets.

On the Selectivity in the Synthesis of 3-Fluoropiperidines Using BF3-Activated Hypervalent Iodine Reagents.

Fluorinated piperidines find wide applications, most notably in the development of novel therapies and agrochemicals. Cyclization of alkenyl N-tosylamides promoted by BF3-activated aryliodine(III) carboxylates is an attractive strategy to construct 3-fluoropiperidines, but it suffers from selectivity issues arising from competitive oxoaminations and the inability to easily modulate the reactions diastereoselectivity. Herein, we report an itemized optimization of the reaction conditions carried out on both cyclic and acyclic substrates and outline the origins of substrate- and reagent-based stereo-, regio-, and chemoselectivity. Extensive mechanistic studies encompassing multinuclear NMR spectroscopy, deuterium labeling, rearrangements on stereodefined substrates, and careful structural analyses (NMR and X-ray) of the reaction products are performed. This revealed the processes and interactions crucial for achieving controlled preparation of 3-fluoropiperidines using I(III) chemistry and has provided an advanced understanding of the reaction mechanism. In brief, we propose that BF3-coordinated I(III) reagents attack C═C to produce the corresponding iodiranium(III) ion, which then undergoes diastereodetermining 5-exo-cyclization. Transiently formed pyrrolidines with an exocyclic σ-alkyl-I(III) moiety can further undergo aziridinium ion formation or reductive ligand coupling processes, which dictate not only the final product's ring size but also the chemoselectivity. Importantly, the selectivity of the reaction depends on the nature of the ligand bound to I(III) and the presence of electrolytes such as TBABF4. Reported findings will facilitate the usage of ArI(III)-dicarboxylates in the reliable construction of fluorinated azaheterocycles.

Sustained-release losartan from peptide nanofibers promotes chondrogenesis.

Introduction: The central pathologic feature of osteoarthritis (OA) is the progressive loss of articular cartilage, which has a limited regenerative capacity. The TGF-ß1 inhibitor, losartan, can improve cartilage repair by promoting hyaline rather that fibrous cartilage tissue regeneration. However, there are concerns about side effects associated with oral administration and short retention within the joint following intra-articular injections. To facilitate local and sustained intra-articular losartan delivery we have designed an injectable peptide amphiphile (PA) nanofiber that binds losartan. The aims of this study are to characterize the release kinetics of losartan from two different PA nanofiber compositions followed by testing pro-regenerative bioactivity on chondrocytes. Methods: We tested the impact of electrostatic interactions on nanostructure morphology and release kinetics of the negatively charged losartan molecule from either a positively or negatively charged PA nanofiber. Subsequently, cytotoxicity and bioactivity were evaluated in vitro in both normal and an IL-1ß-induced OA chondrocyte model using ATDC5. Results: Both nanofiber systems promoted cell proliferation but that the positively-charged nanofibers also significantly increased glycosaminoglycans production. Furthermore, gene expression analysis suggested that losartan-encapsulated nanofibers had significant anti-inflammatory, anti-degenerative, and cartilage regenerative effects by significantly blocking TGF-ß1 in this in vitro system. Discussion: The results of this study demonstrated that positively charged losartan sustained-release nanofibers may be a novel and useful treatment for cartilage regeneration and OA by blocking TGF-ß1.

Rapid Access to Chiral and Tripodal Cavitands from ß-Pinene.

We report Pd-catalyzed cyclotrimerization of (+)-α-bromoenone, obtained from monoterpene ß-pinene, into an enantiopure cyclotrimer. This C3 symmetric compound has three bicyclo[3.1.1]heptane rings fused to its central benzene with each ring carrying a carbonyl group. The cyclotrimer undergoes diastereoselective threefold alkynylation with the lithium salts of five terminal alkynes (41-63 %, de=4-83 %). The addition enabled a rapid synthesis of a small library of novel chiral cavitands that, in shape, resemble a tripod stand. These molecular tripods include a tris-bicycloannelated benzene head attached to three alkyne legs twisted in one direction to form a nonpolar cavity with polar groups as feet. Tripods with methylpyridinium and methylisoquinolinium legs, respectively, form inclusion complexes with anti-inflammatory and chiral drugs (R)/(S)-ibuprofen and (R)/(S)-naproxen. The mode of binding shows drug molecules docked in the cavity of the host through ion-ion, cation-π, and C-H-π contacts that, in addition of desolvation, give rise to complexes having millimolar to micromolar stability in water. Our findings open the door to creating a myriad of enantiopure tripods with tunable functions that, in the future, might give novel chemosensors, catalysts or sequestering agents.

Closed Aromatic Tubes-Capsularenes.

In this study, we describe a synthetic method for incorporating arenes into closed tubes that we name capsularenes. First, we prepared vase-shaped molecular baskets 4-7. The baskets comprise a benzene base fused to three bicycle[2.2.1]heptane rings that extend into phthalimide (4), naphthalimide (6), and anthraceneimide sides (7), each carrying a dimethoxyethane acetal group. In the presence of catalytic trifluoroacetic acid (TFA), the acetals at top of 4, 6 and 7 change into aliphatic aldehydes followed by their intramolecular cyclization into 1,3,5-trioxane (1 H NMR spectroscopy). Such ring closure is nearly a quantitative process that furnishes differently sized capsularenes 1 (0.7×0.9 nm), 8 (0.7×1.1 nm;) and 9 (0.7×1.4 nm;) characterized by X-Ray crystallography, microcrystal electron diffraction, UV/Vis, fluorescence, cyclic voltammetry, and thermogravimetry. With exceptional rigidity, unique topology, great thermal stability, and perhaps tuneable optoelectronic characteristics, capsularenes hold promise for the construction of novel organic electronic devices.

A computational study of competing conformational selection and induced fit in an abiotic system.

Host-guest complexations can be described by two competing mechanisms, conformational selection (CS) and induced fit (IF). In this work, we used a combination of nudged elastic band (NEB), adaptive steered molecular dynamics (ASMD), and density functional theory (DFT, with a correction for dispersion) to study the dynamics of the pathways (IF/CS) by which two conformers of basket B(+) and B(-) interconvert and trap CX4 guests (X = Cl and Br). While the results from NEB/DFT studies disclosed host-guest noncovalent contacts reducing the basket's conformational dynamics, ASMD methodology suggested an associative mechanism for the guest complexation. With theory in excellent agreement with experiments, NEB and ASMD emerge as the methods of choice for studying dynamics of supramolecular systems.

Enantioselective Construction of Modular and Asymmetric Baskets.

The precise positioning of functional groups about the inner space of abiotic hosts is a challenging task and of interest for developing more effective receptors and catalysts akin to those found in nature. To address it, we herein report a synthetic methodology for preparing basket-like cavitands comprised of three different aromatics as side arms with orthogonal esters at the rim for further functionalization. First, enantioenriched A (borochloronorbornene), B (iodobromonorbornene), and C (boronorbornene) building blocks were obtained by stereoselective syntheses. Second, consecutive A-to-B and then AB-to-C Suzuki-Miyaura (SM) couplings were optimized to give enantioenriched ABC cavitand as the principal product. The robust synthetic protocol allowed us to prepare (a) an enantioenriched basket with three benzene sides and each holding either tBu, Et, or Me esters, (b) both enantiomers of a so-called "spiral staircase" basket with benzene, naphthalene, and anthracene groups surrounding the inner space, and (c) a photo-responsive basket bearing one anthracene and two benzene arms.

From Selection to Instruction and Back: Competing Conformational Selection and Induced Fit Pathways in Abiotic Hosts.

Two limiting cases of molecular recognition, induced fit (IF) and conformational selection (CS), play a central role in allosteric regulation of natural systems. The IF paradigm states that a substrate "instructs" the host to change its shape after complexation, while CS asserts that a guest "selects" the optimal fit from an ensemble of preexisting host conformations. With no studies that quantitatively address the interplay of two limiting pathways in abiotic systems, we herein and for the first time describe the way by which twisted capsule M-1, encompassing two conformers M-1(+) and M-1(-), trap CX4 (X=Cl, Br) to give CX4 ⊂M-1(+) and CX4 ⊂M-1(-), with all four states being in thermal equilibrium. With the assistance of 2D EXSY, we found that CBr4 would, at its lower concentrations, bind M-1 via a M-1(+)→M-1(-)→CBr4 ⊂M-1(-) pathway corresponding to conformational selection. For M-1 complexing CCl4 though, data from 2D EXSY measurements and 1D NMR line-shape analysis suggested that lower CCl4 concentrations would favor CS while the IF pathway prevailed at higher proportions of the guest. Since CS and IF are not mutually exclusive, we reason that our work sets the stage for characterizing the dynamics of a wide range of already existing hosts to broaden our fundamental understanding of their action. The objective is to master the way in which encapsulation takes place for designing novel and allosteric sequestering agents, catalysts and chemosensors akin to those found in nature.

A Hexapodal Capsule for the Recognition of Anions.

We herein describe the preparation, characterization, and recognition characteristics of novel hexapodal capsule 1 composed of two benzenes joined by six hydrogen bonding (HB) groups to encircle space. This barrel-shaped host was obtained by reversible imine condensation of hexakis-aldehyde 2 and hexakis-amine 3 in the presence of oxyanions or halides acting as templates. Fascinatingly, capsule 1 includes 18 HB donating (Csp2-H and N-H) and 12 HB accepting groups (C═O and C═N) surrounding a binding pocket (78 Å3). In this regard, the complexation of fluoride, chloride, carbonate, sulfate, and hydrogen phosphate was probed by NMR spectroscopy (DMSO) and X-ray diffraction analysis to disclose the adaptive nature of 1 undergoing an adjustment of its conformation to complement each anionic guest. Furthermore, the rate by which encapsulated chloride was substituted by sulfate or hydrogen phosphate was slow (>7 days) while the stability of [SO4⊂1]2- was greatest in the series with Ka > 107 M-1 in highly competitive DMSO. With facile access to 1, the stage is set to probe this modular, polyvalent, and novel host to further improve the extraction of tetrahedral oxyanions from waste and the environment or control their chemistry in living systems.

A highly diastereoselective synthesis of deep molecular baskets.

We describe a preparative method for directing Mizoroki-Heck cyclotrimerization of enantioenriched bromonorbornenes into molecular baskets having increasingly deeper and extendable aromatic cavities. Such diastereoselective cyclotrimerizations of the bromo-olefinic substrates resulted from prevalent ß migratory insertions without the formation of palladacycle intermediate(s). The facile access to multigram quantity of a new series of basket-like hosts clears the way for creating novel supramolecular materials for storage, sequestration and catalysis.

A Molecular Capsule with Revolving Doors Partitioning Its Inner Space.

Covalent capsule 1 was designed to include two molecular baskets linked with three mobile pyridines tucked into its inner space. On the basis of both theory (DFT) and experiments (NMR and X-ray crystallography), we found that the pyridine "doors" split the chamber (380 Å3 ) of 1 so that two equally sizeable compartments (190 Å3 ) became joined through a conformationally flexible aromatic barrier. The compartments of such unique host could be populated with CCl4 (88 Å3 ; PC=46 %), CBr4 (106 Å3 ; 56 %) or their combination CCl4 /CBr4 (PC=51 %), with thermodynamic stabilities ΔG° tracking the values of packing coefficients (PC). Halogen (C-X⋅⋅⋅π) and hydrogen bonding (C-H⋅⋅⋅X) contacts held the haloalkane guests in the cavities of 1. The consecutive complexations were found to occur in a negative allosteric manner, which we propose to result from the induced-fit mode of complexation. Newly designed 1 opens a way for probing the effects of inner conformational dynamics on noncovalent interactions, reactivity and intramolecular translation in confined spaces of hollow molecules.

Photoinduced interruption of interannular cooperativity for delivery of cationic guests in water.

The synergy among twelve carboxylates from two hexavalent baskets 16- assisted the encapsulation of one divalent diammonium guest 32+-62+ and the formation of ternary [3-6⊂12]10-. The reduction of basket's multivalency, by photoinduced α-decarboxylation of 16- to give 23-, intercepted the interannular cooperativity operating in the stabilization of capsulpex [3-6⊂12]10- to dramatically diminish the binding affinity towards diammonium guests. As a result, the cationic guests were released into bulk water with 23- assembling into nanoparticles. With numerous drugs carrying positive sites, the finding reported here could now be examined for their light-promoted spatiotemporal delivery.

Twist-Turn-Twist Motif Chaperoned Inside Molecular Baskets.

Two molecular baskets 16-, each with three (S)-glutamic acids at its rim, were found (NMR, ITC) to complex diammonium alkanes 22+-52+ (+H3N(CH2)nNH3+, n = 7-10) giving ternary [2-5⊂12]10- assemblies (K = 107-109 M-2). From the magnetic perturbation of proton nuclei (1H NMR, NICS), we deduced that each guest assumed a U shape within the binary complex, [2-5⊂1]4-. Two ammonium groups were bound in the "anionic nest" at the top of 16-, while the hydrocarbon chain resided in its nonpolar cavity. From detailed ITC analyses, we showed that the binary complex [2-5⊂1]4- forms first and then another 16- capped [2-5⊂1]4- to give [2-5⊂12]10-, via interactions of the carboxylates at the rims with ammoniums on the guest. Long-range NOEs revealed that U-shaped 22+ and 52+ coiled into highly strained twist-turn-twist formations, for the first time observed within an abiotic host while curiously resembling helix-turn-helix motif found in DNA binding proteins.

Multivalent C-H⋠⋠⋠Cl/Br-C Interactions Directing the Resolution of Dynamic and Twisted Capsules.

In this work, we report a mechanism by which stereoisomeric and twisted capsules P/M-1 direct their dynamic chirality in the presence of haloalkane guests. The capsule comprises a static, but twisted, cage that is linked to a dynamic tris(2-pyridylmethyl)amine (TPA) lid at its top. From the results of experimental (NMR spectroscopy and X-ray crystallography) and computational (DFT) studies, the TPA lid was shown to assume clockwise (+) and counterclockwise (-) folds with diastereomeric (but racemic) capsules M-1(+) and M-1(-) interconverting at a rapid rate (ΔG≠ 189K =9.1 kcal mol-1 ). The relative stability of the capsules was found to be a function of guest(s) residing in their interior (243/262 Å3 ) with small CH2 Cl2 (61 Å3 ) yielding roughly equal population of diastereomeric inclusion complexes. Larger guests, such as CCl4 (89 Å3 ) and CBr4 (108 Å3 ), however, formed M-1(-)⊂CX4 at the expense of M-1(+)⊂CX4 in circa 3:1 ratio. To account for the observation, theory (DFT:M06-2X/6-31+G*) and experiments (1 H NMR spectroscopy) were used to deduce that CX4 guests become localized inside the twisted cage of the capsule by forming a C-X⋅⋅⋅π halogen bond [Nc =d/(rH +rX )=0.91-0.92] with the benzene "floor" while encountering electrostatic repulsions with closer naphthalimide boundaries. At last, the TPA lid used its central methylene hydrogens to establish, within the M-1(-)⊂CX4 , three stabilizing C-H⋅⋅⋅X-C interactions with the guest. The same C-H⋅⋅⋅X-C interactions, however, became weaker (or possibly vanished) after the conformational reorganization of the lid and the formation of less stable M-1(+)⊂CX4 complex. On individual basis, C-H⋅⋅⋅X-C intermolecular contacts are weak and hardly detectable in the solution phase. In the case of capsule P/M-1, however, these contacts were multivalent and altogether strong enough to direct the host's dynamic chirality.

Stackable molecular chairs.

We describe the design, preparation and assembly of chair-shaped molecules, comprising a tris-bicyclo[2.2.1]heptane hub that extends into three phthalimides carrying amino acids or short peptides. In water, molecular chairs stack in an antiparallel manner to give hexavalent dendritic structures, in spite of holding equal charges. The here described strategy for increasing the multivalency of peptides is easy to implement in practice, therefore holding potential for creating novel soft materials and therapeutics.

Stereo- and Regioselective Synthesis of Molecular Baskets.

We describe a stereoselective method for obtaining multigram quantities of molecular basket 1 syn in overall 11% yield, using inexpensive cyclopentadiene and diethyl fumarate as starting materials. First, an asymmetric synthesis of enantioenriched bromo(trimethylstannyl)alkene (-)-8 was accomplished by the stereoselective bromination of dibromonorbornene (+)-4 guided by anchimeric assistance and subsequent syn- exo-elimination of tetrabromonorbornane (-)-5a as the key steps. Subsequent Cu(I)-catalyzed cyclotrimerization of (-)-8 was optimized to give 1 syn/ anti in 85% yield and 1:1 ratio of diastereomers. Importantly, the results of our mechanistic experiments were in line with the cyclotrimerization occurring in a chain-type fashion with racemization of a Cu(I) homochiral dimeric intermediate, reducing the stereoselectivity of the transformation. Enabled by more facile access to molecular baskets of type 1 syn, a range of recognition studies can now be completed for producing novel supramolecular catalysts, organophosphorus scavengers, and nanostructured materials.

Photo-induced formation of organic nanoparticles possessing enhanced affinities for complexing nerve agent mimics.

Molecular baskets 16--36-, functionalized with α-amino acids at their rim, undergo photo-induced α-decarboxylations to give amphiphilic 43--63- assembling into nanoparticles. Nanoparticulate 43--63- possess greater affinities for complexing OPs (akin to sarin and cyclosarin) than monomeric 16--36-. With the ability of nanoparticles to function in urine, our study sets the stage for creating novel nanocarriers capable of spatiotemporal sequestration of nerve agents or pesticides in competitive chemical environments.

Light-Triggered Transformation of Molecular Baskets into Organic Nanoparticles.

Discovering novel and functional photoresponsive materials is of interest for improving controlled release of molecules and scavenging toxic compounds for cleaning our environment or designing chemosensors. In this study, we report on the photoinduced decarboxylation of basket 16- , containing three glutamic acids at its rim. This concave compound is, in an aqueous environment (30 mm phosphate buffer at pH 7.0), monomeric (1 H NMR DOSY, DLS) with glutamic acid residues randomly oriented about its rim (1 H NMR and MM-OPLS3). The irradiation (300 nm) of 16- leads to the exclusive removal of its α-carboxylates to give amphiphilic 23- possessing γ-carboxylates. The photochemical transformation is a consecutive reaction with mono- and bis-decarboxylated products observed with 1 H NMR spectroscopy and ESI mass spectrometry. Amphiphilic 23- is a preorganized molecule (MM-OPLS3) that, in water, aggregates into organic nanoparticles (ca. 50-200 nm in diameter; DLS, TEM and cryo-TEM) having a critical aggregation concentration of 12 µm (UV/Vis). As the transition of monomeric 16- into nanoparticulate 23- is triggered with light, we reasoned that stimuli-responsive formation of the soft material lends itself to nanotechnology applications such as controlled release or scavenging of targeted compounds.

A Stimuli-Responsive Molecular Capsule with Switchable Dynamics, Chirality, and Encapsulation Characteristics.

In this study, we report the preparation, conformational dynamics, and recognition characteristics of novel molecular capsule 1 comprising a bowl-shaped framework conjugated to a tris(2-pyridylmethyl)amine (TPA) lid. With the assistance of experiment (1H NMR spectroscopy) and theory (MM and DFT) we found that C3 symmetric 1 is poorly preorganized with three pyridines at the rim adopting a propeller-like orientation and undergoing P-to- M (or vice versa) stereoisomerization (Δ G⧧ < 8 kcal/mol, VT 1H NMR). Capsule 1 binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 with Ka < 7 M-1. Protonation of 1 with HCl, however, gives [1·H]-Cl, with the solid-state structure showing the TPA lid being "flattened" and the +N-H---Cl hydrogen-bonded group residing outside. Importantly, the P-to- M stereoisomerization would for [1·H]-Cl occur with Δ G⧧ = 11 kcal/mol (VT 1H NMR). Less dynamic and more preorganized [1·H]-Cl binds CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4 guests with a greater affinity ( Ka = 100-400 M-1) than 1. The results of our studies suggest that the complexation of increasingly larger guests takes place in an induced-fit fashion, with [1·H]-Cl (a) elongating along its vertical axis and concurrently potentially (b) twisting pyridines from P into M (and vice versa) orientation. The addition of Et3N to [1·H]-Cl⊂CH2Cl2 causes deprotonation of the capsule and the release of CH2Cl2 with the process being fully reversed after the addition of HCl. Allosteric capsule 1 with unique structural and dynamic characteristics is expected to, in the future, assist the construction of complex molecular machines and smart functional materials.

Cycloaddition Reactions of Azomethine Ylides and 1,3-Dienes on the C2v-Symmetrical Pentakisadduct of C60.

The reactivity of the C2v-symmetric pentakisadduct of C60 with azomethine ylides and conjugated dienes was studied experimentally and computationally. This derivative possesses four [6,6] double bonds, each with unique electrophilicity. The Diels-Alder reaction studied is a regiospecific, kinetically and thermodynamically guided [4 + 2] process producing [5:1]-hexaadducts with an octahedral addition pattern. The kinetically controlled Prato reaction gives a mixture of regioisomeric [5:1]-hexaadducts. The synthesis of geometrically well-defined supramolecular architectures may benefit from these new types of highly functionalized [5:1]-hexaadducts.