Selective recognition of fluoride and acetate by a newly designed ruthenium framework: experimental and theoretical investigations

An effective anion sensor, [Ru(II)(bpy)(2)(H(2)L(-))](+) (1(+)), based on a redox and photoactive {Ru(II)(bpy)(2)} moiety and a new ligand (H(3)L = 5-(1H-benzo[d]imidazol-2-yl)-1H-imidazole-4-carboxylic acid), has been developed for selective recognition of fluoride (F(-)) and acetate (OAc(-)) ions. Crystal structures of the free ligand, H(3)L and [1](ClO(4)) reveal the existence of strong intramolecular and intermolecular hydrogen bonding interactions. The structure of [1](ClO(4)) shows that the benzimidazole N-H of H(2)L(-) is hydrogen bonded with the pendant carboxylate oxygen while the imidazole N-H remains free for possible hydrogen bonding interaction with the anions. The potential anion sensing features of 1(+) have been studied by different experimental and theoretical (DFT) investigations using a wide variety of anions, such as F(-), Cl(-), Br(-), I(-), HSO(4)(-), H(2)PO(4)(-), OAc(-) and SCN(-). Cyclic voltammetry and differential pulse voltammetry established that 1(+) is an excellent electrochemical sensor for the selective recognition of F(-) and OAc(-) anions. 1(+) is also found to be a selective colorimetric sensor for F(-) or OAc(-) anions where the MLCT band of the receptor at 498 nm is red shifted to 538 nm in the presence of one equivalent of F(-) or OAc(-) with a distinct change in colour from reddish-orange to pink. The binding constant between 1(+) and F(-) or OAc(-) has been determined to be logK = 7.61 or 7.88, respectively, based on spectrophotometric titration in CH(3)CN. The quenching of the emission band of 1(+) at 716 nm (λ(ex) = 440 nm, Φ = 0.01 at 298 K in CH(3)CN) in the presence of one equivalent of F(-) or OAc(-), as well as two distinct lifetimes of the quenched and unquenched forms of the receptor 1(+), makes it also a suitable fluorescence-based sensor. All the above experiments, in combination with (1)H NMR, suggest the formation of a 1:1 adduct between the receptor (1(+)) and the anion (F(-) or OAc(-)). The formation of 1:1 adduct {[1(+)·F(-)] or [1(+)·OAc(-)]} has been further evidenced by in situ ESI-MS(+) in CH(3)CN. Though the receptor, 1(+), is comprised of two N-H protons associated with the coordinated H(2)L(-) ligand, only the free imidazole N-H proton participates in the hydrogen bonding interactions with the incoming anions, while the intramolecularly hydrogen bonded benzimidazole N-H proton remains intact as evidenced by the crystal structure of the final product (1). The hydrogen bond mediated anion sensing mechanism, over the direct deprotonation pathway, in 1(+) has been further justified by a DFT study and subsequent NBO analysis.     

Dearomatizing cyclization of arylsulfonylalkoxymethyl lithiums: a route to the podophyllotoxin skeleton

[reaction: see text] The phenylsulfonyl group promotes the dearomatizing cyclization of tethered organolithiums onto aromatic rings. With an ether tether, the cyclizations create a new tetrahydrofuran ring, and both cyclization and subsequent electrophilic quenches proceed with high levels of diastereoselectivity. The sulfonyl group can be removed from the cyclized products oxidatively or reductively. The dearomatizing cyclization of a naphthyl sulfone was used in the synthesis of a close structural analogue of podophyllotoxin.     

Transformation of a metal-organic framework from the NbO to PtS net

Two metal-organic frameworks (MOFs), MOF-501 and MOF-502, respectively, formulated as Co(2)(BPTC)(H(2)O)(5).G(x) and Co(2)(BPTC)(H(2)O)(DMF)(2).G(x) (BPTC = 3,3',5,5'-biphenyltetracarboxylate; G = guest molecules), have been synthesized and structurally characterized, and their topologies were found to be based on the NbO (MOF-501) and PtS (MOF-502) nets. Heating MOF-501 in solution results in the more thermodynamically favored MOF-502.     

Divergent Synthesis of 1,2,3,4-Tetrasubstituted Cyclobutenes from a Common Scaffold: Enantioselective Desymmetrization by Dual-Catalyzed Photoredox Cross-Coupling

Four-membered carbocycles are important structural motifs found in several natural products and drugs. Amongst those, cyclobutenes are attractive intermediates because the residual olefin can be manipulated selectively into various saturated and unsaturated analogs. Few methods exist to access chiral tri- and tetra-C-substituted cyclobutenes and they are generally limited in terms of diversification. Herein, a divergent synthetic strategy was developed where a single optically enriched scaffold is diversified into a variety of derivatives with different substitution patterns. To this end, the enantioselective desymmetrization of prochiral 1,2-dibromocyclobutene imides was enabled by a dual Ir/Ni-catalyzed photoredox C(sp²)−C(sp³) cross-coupling with an alkyltrifluoroborate salt to install a convertible carbon fragment in good yields and >90 % enantiomeric excess. Exceptional mono-coupling selectivity is observed and the resulting chiral bromocyclobutene serves as a common scaffold that can be transformed in a divergent manner into several valuable 1,2,3,4-tetra-C-substituted cyclobutane products while maintaining optical purity.     

Conversion of a phenylphosphinidene-capped ruthenium cluster compound with a closo-octahedral Ru4P2 framework to one with a closo-trigonal prismatic skeleton by the addition of carbon monoxide and protons

Treatment of [Ru₄(μ₄-PPh)₂(μ₂-CO)(CO)₁₀] with NaBH₄ leads to the formation of [Ru₄(μ₄-PPh)₂(μ₂-H)(μ₂-CO)₂(CO)₈]⁻ which readily adds protons and carbon monoxide to produce [Ru₄(μ₃-PPh)₂(μ₂-H)₂(CO)₁₂]; X-ray crystallographic studies have revealed that while the Ru₄P₂ framework adopts a closo octahedral geometry in [Ru₄(μ₄-PPh)₂(μ₂-H)(μ₂-CO)₂(CO)₈]⁻, it adopts a closo trigonal prismatic one in [Ru₄(μ₃-PPh)₂(μ₂-H)₂(CO)₁₂]     

Failure to verify a reported synthesis of the aconitine skeleton

Attempts to verify the first three stages of a claimed synthesis of the aconitine skeleton (Samir Chatterjee, $\text{Tetrahedron Lett.}$ 3249 (1979) have given totally different results. It is concluded that the reported products of these three stages were not in fact obtained.     

Anodic coupling reactions: a sequential cyclization route to the arteannuin ring skeleton

A pair of intramolecular anodic olefin coupling reactions has been used to construct the arteannuin ring skeleton. Both coupling reactions took advantage of a furan ring as one of the coupling partners. In the first, it was found that an enol ether derived from an aldehyde was not an effective initiating group for the reaction. Instead, the cyclization benefited strongly from the use of a N,O-ketene acetal initiating group. In the second cyclization, an endocyclic enol ether was coupled to the furan ring. This second electrolysis reaction generated the key tetrasubstituted carbon at the center of the arteannuin ring skeleton.     

Tandem aza-Michael additions under high pressure: a shortcut to the azanorbornyl skeleton

The hyperbaric aza-Michael addition of mono- and diamines on α,β-unsaturated β,β-disubstituted mono- and diesters has been studied. While in the case of monoester, this reaction provides a β-aminoester presenting a quaternary center, a direct and efficient access to diester or lactams featuring an azanorbornyl skeleton was obtained when starting from a diester, following an unprecedented double aza-Michael addition.     

Conversion of homocysteine to methionine by methionine synthase: a density functional study

This communication reports a theoretical study of the conversion of homocysteine to methionine by methionine synthase. The reaction pathway is based on density functional calculations with large basis sets, including thermodynamic, relativistic, and solvent effects. We find that the suggested SN2 mechanism explains well the experimentally observed reaction rate. The results show that the reaction is highly polar, as reflected in the change of charge density along the reaction coordinate. It is enhanced in the protein by two effects: deprotonation of the bound substrate and desolvation of substrate and cofactor in the rate-determining step.     

The Synthesis and Reactions of Yomogi Alcohol. Conversion of the artemisyl skeleton to the santolinyl skeleton by a 1,2-shift of a vinyl group. Synthesis of santolinatriene

The synthesis of yomogi alcohol (2, 5, 5-trimethylhepta-3,6-dien-2-ol, 2 ) is described, and experiments directed towards its allylic rearrangement to artemisia alcohol detervatives have been carried out. Acidic reagents open the ring of yomogi alcohol epoxide ( 16 ) at with participation of the 6,7-double-bond, a shift of the vinyl group results to yield a compound with the santolinyl skeleton. The same reagents are without effect when this double bond reduced. Action of butyllithium of the benzaldehyde acetal ( 41 ) of 2, 5-dimethyl-4-vinyl-2, dihydroxy-hex-5-ene ( 28 ), obtained by acid-catalyzed ring opening of yomogi alcohol epoxide in the presence of benzaldehyde, leads to santolinatriene ( 42 ). This vinyl shift is not observed in the case of O-acetyl yomogi alcohol epoxide ( 46 ), from white a compound believed to be an oxetan 48 (R ? COCH₃) is formed with concomitent shift of the acetate group. Further unusual reactions of the oxetan are described, and some observation about the epoxidation of sterically hindered allyl alcohols and their acetates are made.     

Synthesis of novel heterocycles related to the dynemicin a ring skeleton

Studies directed towards the construction of the CDE ring framework of dynemycin A ( 1 ) are reported. A series of quinoline based dienophiles containing an activating group (e.g. 5 , Scheme 1), reacted with acyclic dienes (e.g. 4 , Scheme 1) in a Diels-Alder fashion under increased pressure to afford a variety of heterocyclic systems related to the CDE skeleton of dynemycin A. Reaction of dienophile 15 with cyclic diene 29 led to the novel hetereocycle 34 whereas attempted decarbonylation of 41 led to a novel rhodium-promoted intramolecular carbonylation of the terminal acetylene furnishing compound 43 .     

Liquid-crystalline coumarin derivatives: contribution to the tailoring of metal-free sensitizers for solar cells

Coumarins have been used in a wide range of applications, such as dye-sensitised solar cells, laser dyes and optical sensors. In order to further explore the properties of these materials, three new coumarin derivatives were obtained with different terminal arylalkyne linkages to the 6-position of the coumarin core. The synthesised materials were characterised by NMR, absorption and emission spectroscopy, and the liquid crystal properties were investigated through differential scanning calorimetry and polarised optical microscopy. In addition, dye-sensitised solar cells were assembled to evaluate the photoelectrochemical properties of the materials. Only the coumarin with a naphthyl group exhibited stable smectic A and nematic mesophases. All the coumarins were photoemissive in the range 420–461 nm. The adsorption of these dyes on TiO₂ was observed by UV–vis spectroscopy; in addition, by incident photon-to-electron conversion efficiency and I–V curves, photocurrent generation was observed.     

Characterization,crystal structure and cytotoxic activity of a rare iridoid glycoside from Lonicera saccata

A new iridoid glycoside, methyl (3R,4R,4aS,7S,7aR)‐3‐hydroxy‐7‐methyl‐5‐oxooctahydrocyclopenta[c]pyran‐4‐carboxylate‐3‐O‐β‐d ‐(1′S,2′R,3′S,4′S,5′R)‐glucopyranoside, named loniceroside A, C₁₇H₂₆O₁₀, ( 1 ), was obtained from the aerial parts of Lonicera saccata. Its structure was established based on an analysis of spectroscopic data, including 1D NMR, 2D NMR and HRESIMS, and the configurations of the chiral C atoms were determined by X‐ray crystallographic analysis. The single‐crystal structure reveals that the cyclopenta[c]pyran scaffold is formed from a five‐membered ring and a chair‐like six‐membered ring connected through two bridgehead chiral C atoms. In the solid state, the glucose group of ( 1 ) plays an important role in constructing an unusual supramolecular motif. The structure analysis revealed adjacent molecules linked together through intermolecular O—H…O hydrogen bonds to generate a banded structure. Furthermore, the banded structures are linked into a three‐dimensional network by interesting hydrogen bonds. Biogenetically, compound ( 1 ) carries a glucopyranosyloxy moiety at the C‐3 position, representing a rare structural feature for naturally occurring iridoid glycosides. The growth inhibitory effects against human cervical carcinoma cells (Hela), human lung adenocarcinoma cells (A549), human acute mononuclear granulocyte leukaemia (THP‐1) and the human liver hepatocellular carcinoma cell line (HepG2) were evaluated by the MTT method.     

Rearrangement of a cyclohexyl radical to a cyclopentylmethyl radical on the avermectin skeleton

The rearrangement of a substituted cyclohexyl radical to a cyclopentylmethyl radical on the skeleton of avermectin B1 was observed experimentally and explored computationally. The Stork-Nishiyama methodology was applied to the macrocycle of interest followed by a Tamao oxidation. The expected 5-6 fused ring product was observed in minor amounts. The major product was a 5-5 fused ring resulting from apparent conversion of the initially formed cyclohexyl radical to a cyclopentylmethyl radical. Preliminary computational results indicate that substituents in the macrocycle induce the rearrangement.     

Simulation of chaperonin effect on protein folding: a shift from nucleation-condensation to framework mechanism

The iterative annealing mechanism (IAM) of chaperonin-assisted protein folding is explored in a framework of a well-established coarse-grained protein modeling tool, which enables the study of protein dynamics in a time-scale well beyond classical all-atom molecular mechanics. The chaperonin mechanism of action is simulated for two paradigm systems of protein folding, B domain of protein A (BdpA) and B1 domain of protein G (GB1), and compared to chaperonin-free simulations presented here for BdpA and recently published for GB1. The prediction of the BdpA transition state ensemble (TSE) is in perfect agreement with experimental findings. It is shown that periodic distortion of the polypeptide chains by hydrophobic chaperonin interactions can promote rapid folding and leads to a decrease in folding temperature. It is also demonstrated how chaperonin action prevents kinetically trapped conformations and modulates the observed folding mechanisms from nucleation-condensation to a more framework-like.     

A direct entry to the 1-methoxyindole skeleton and to the corresponding indoles by a novel rearrangement: general syntheses of substituted 1-methoxyindoles

A short and efficient route to 1-methoxyindoles via a novel rearrangement is disclosed. This route involves only three steps from commercially available nitro compounds. The methodology is also generalized with a variety of examples to afford a series of 2-substituted-1-methoxyindoles possessing an electron-withdrawing group at position 3. In addition, a 1-methoxyindole compound 10 was converted to the corresponding indole 11 under mild conditions thereby constituting a new synthesis of substituted indoles.     

Synthesis of the tetracyclic core skeleton of the lundurines by a gold-catalyzed cyclization

The 1H-azocino[5,4-b]indole skeleton of the lundurines has been prepared by the 8-endo-dig cyclization of an alkynylindole using AuCl₃ or other gold complexes as catalysts.     

Structure and anti-HIV activity of micrandilactones B and C, new nortriterpenoids possessing a unique skeleton from Schisandra micrantha

Two new highly oxygenated nortriterpenoids with a unique norcycloartane skeleton, micrandilactones B and C, were isolated from Schisandra micrantha; micrandilactone C exhibited an EC50 value of 7.71 microg/mL (SI > 25.94) against HIV-1 replication with minimal cytotoxicity, and the potent anti-HIV-1 activity and unique structural features of make it a promising lead for therapeutic development of a new generation of anti-HIV drug.     

Solid-state single-crystal-to-single-crystal transformation from a 2D layer to a 3D framework mediated by lattice iodine release

We describe an exceedingly rare example of solid-state single-crystal-to-single-crystal transformation from a 2D layer to a known 3D framework via lattice iodine release, which involves the formation of a new Cu-O ligand bond and a change in the metal coordination geometry.