PHOTOCYCLOADDITION REACTION OF 5,7-DIMETHOXYCOUMARIN TO THYMIDINE

Abstract— The photocycloaddition reaction of 5,7-dimethoxycoumarin to thymidine on direct irradiation (λ > 300 nm) is studied as a model for photosensitization reaction of furocoumarins. The major photoadducts were isolated by silica gel column and gel permeation chromatography. Each component of the photoadducts was further separated by reverse phase, paired-ion high performance liquid chromatography. The structure of these photoproducts isolated is consistent with 1:1 C₄-cycloadducts in accordance with characteristics of their UV, IR, NMR and mass spectra and elemental analysis data. The stereochemistry of each isomer was studied by Fourier transform NMR, UV and IR spectra. The fraction C has the anti head-to-tail configuration and the fraction D has the configuration of anti head-to-head. The fractions A and B probably have the syn configuration.     

Confined supramolecular nanostructures of mesogen-bearing amphiphiles

Stable surface nanostructures with different morphology have been successfully constructed by modifying the chemical structure of synthetic amphiphiles; by introducing mesogenic groups into bolaform amphiphiles, stable spaghetti-like or stripe-like nanostructures can be obtained; it is believed that such a kind of surface structure could be used for templating synthesis and assembly.     

Microwave spectrum of aminoborane,BH2NH2

The unstable species aminoborane, BH₂NH₂, has been identified as a reaction product of ammonia with diborane by microwave spectroscopy. The rotational constants determined are A = 138212 ± 4 MHz, B = 27487.83 ± 0.10 MHz and C = 22878.44 ± 0.11 MHz for ¹¹BH₂NH₂ and A = 138199 ± 6 MHz, B = 28420.36 ± 0.11 MHz and C = 23520.78 ± 0.12 MHz for ¹⁰BH₂NH₂. The dipole moment is 1.844 ± 0.015 D.     

Intercalating lithium into the lattice of silver nanoparticles boosts catalytic hydrogenation of carbon–oxygen bonds

Coinage metal nanoparticles with high dispersion can serve as highly efficient heterogeneous catalysts. However, owing to their low melting point, poor thermal stability remains a major obstacle towards their application under reaction conditions. It is a common practice to use porous inorganic templates such as mesoporous silica SBA-15 to disperse Ag nanoparticles (NPs) against aggregation but their stability is far from satisfactory. Here, we show that the catalytic activity for hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG) over Ag NPs dispersed on SBA-15 silica can be further promoted by incorporation of alkali metal ions at small loading, which follows the inverse order of their cationic size: Li⁺ > Na⁺ > K⁺ > Rb⁺. Among these, 5Ag₁–Li_0.05/SBA-15 can double the MG yield compared to pristine 5Ag/SBA-15 under identical conditions with superior thermal stability. Akin to the effect of an ionic surfactant on stabilization of a micro-emulsion, the cationic charge of an alkali metal ion can maintain dispersion and modulate the surface valence of Ag NPs. Interstitial Li in the octahedral holes of the face center packed Ag lattice is for the first time confirmed by X-ray pair distribution function and electron ptychography. It is believed that this interstitial-stabilization of coinage metal nanoparticles could be broadly applicable to multi-metallic nanomaterials for a broad range of C–O bond activating catalytic reactions of esters.

Coinage metal nanoparticles with high dispersion can serve as highly efficient heterogeneous catalysts.     

Synthesis, structure, and transformation of novel osmium azine and ylide complexes

Three novel triosmium complexes with unusual coordination characteristics are reported. Treatment of the hydridotriosmium cluster (mu-H)2Os3(CO)10 with CNNPPh3 in CH2Cl2 gave complexes (mu-H)Os3(CO)(10)(mu2-eta2-C(H)NNPPh3) (1) and (mu-H)Os3(CO)10(mu2-eta1-CHPPh3) (2). Complex 1 represents the first example of the existence of a coordinated phosphinazine ligand. An in-situ 1H NMR study showed that the reaction of (mu-H)2Os3(CO)10 with CNNPPh3 produced complex 1 as the initial product in 100% conversion. The latter is not stable in solution and slowly eliminates nitrogen to form an unusual ylide complex 2 in quantitative yield. The thermolysis of 2 in refluxing toluene afforded (mu-H)3Os3(CO)9(mu3-eta1-CCO2CH2Ph) (3) as a colorless compound. Complexes 1-3 were characterized by spectroscopic methods and single-crystal X-ray diffraction analysis. The interesting feature of structure 3 is the presence of a mu3-alkylidyne ligand where the symmetrically triply bridged CCO2CH2Ph fragment lies perpendicular to and above the triosmium triangle.     

Lead-selective membrane electrodes based on dithiophenediazacrown ether derivatives

Three double-armed diazacrown ethers with two thiophene side groups, 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC), 7,16-dithenoyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTODC), and 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC), have been synthesized and used as novel neutral lead(II) ionophores in ion selective electrode applications. The relationship between the molecular structure of these ionophores and electrochemical properties (linear range, response time, selectivity) of the membrane electrode is discussed. The optimum conditions for the preparation of the electrodes are described. The optimized dithenoyldiazacrown had a detection limit of pPb = 5.7, and Nernstian range with slope 29.2 mV decade⁻¹ from pPb = 5.0 to 2.7. Mercury and silver ions are the major interferences. These electrodes are applied to potentiometric titrations of lead(II) ions and show promise for the determination of lead ions in water samples.     

Synthesis and spectral characterization of some complexes of platinum(II) containing η-methyleugenol

Several complexes of the formula trans-[Pt(Meug)(Am)Cl₂], Meug: methyleugenol (4-allyl-1,2-dimethoxybenzene), a η²-coordinated olefin, and Am: ammine, methylamine, diethylamine, o-toluidine, m-toluidine, p-toluidine, o-anisidine, m-anisidine and p-anisidine have been prepared. UV, IR, Raman, ¹H NMR, ¹³C NMR and 2D NMR spectra of the complexes were recorded and analyzed.     

Efficient conversion of substituted aryl thioureas to 2-aminobenzothiazoles using benzyltrimethylammonium tribromide

The reaction of molecular bromine (Br2) with arylthioureas is known to produce 2-aminobenzothiazoles (Hugerschoff reaction). We show here that benzyltrimethylammonium tribromide (1, PhCH2NMe3Br3), a stable, crystalline organic ammonium tribromide (OATB), can be readily utilized as an alternative electrophilic bromine source. It is easier to control the stoichiometry of addition with an OATB, which minimizes aromatic bromination caused by excess reagent. We have developed a direct procedure from isothiocyanates and amines using tetrabutylammonium thiocyanate (Bu4NSCN) and PhCH2NMe3Br3 to afford functionalized 2-aminobenzothiazoles.     

Construction of diverse peptide structural architectures via chemoselective peptide ligation

Herein, we report the development of a facile synthetic strategy for constructing diverse peptide structural architectures via chemoselective peptide ligation. The key advancement involved is to utilize the benzofuran moiety as the peptide salicylaldehyde ester surrogate, and Dap–Ser/Lys–Ser dipeptide as the hydroxyl amino functionality, which could be successfully introduced at the side chain of peptides enabling peptide ligation. With this method, the side chain-to-side chain cyclic peptide, branched/bridged peptides, tailed cyclic peptides and multi-cyclic peptides have been designed and successfully synthesized with native peptidic linkages at the ligation sites. This strategy has provided an alternative strategic opportunity for synthetic peptide development. It also serves as an inspiration for the structural design of PPI inhibitors with new modalities.

Methods of introducing peptide salicylaldehyde esters and hydroxyl amine functionality into the peptide side chain have been developed. Diverse peptide structural motifs were constructed via ligation with native amide linkages at the ligation sites.     

Syntheses,Characterization and Crystal Structures of Three Structurally Similar Dinuclear Schiff Base Cadmium(II) Complexes with Bridging Azide or Thiocyanate Ligands

Three novel Schiff base cadmium(II) complexes, derived from the end‐on (μ‐1,1‐N₃) azide or end‐to‐end (μ‐1,3‐NCS) thio cyanate bridges and similar tridentate Schiff base ligands, have been synthesized under similar synthetic procedures and their crystal structures determined by X‐ray diffraction methods. They are the dinuclear double end‐on azide‐bridged [Cd₂(L1)₂(N₃)₂(μ‐1,1‐N₃)₂] ( 1 ), the dinuclear double end‐on azide‐bridged [Cd₂(L2)₂(N₃)₂(μ‐1,1‐N₃)₂] ( 2 ), and the dinuclear double end‐to‐end thiocyanate‐bridged [Cd₂(L3)₂(NCS)₂(μ_1,3‐NCS)₂] ( 3 ), where L1, L2 and L3 are three similar tridentate Schiff bases obtained by condensation of 2‐pyridylaldehyde with N,N‐diethylethane‐1,2‐diamine, of 2‐pyridylaldehyde with N‐isopropylethane‐1,2‐diamine, and of 2‐pyridylaldehyde with N,N‐dimethylpropane‐1,3‐diamine, respectively. Each cadmium(II) centre in the complexes is in a distorted octahedral coordination. There is a crystallographic inversion centre in each of the complexes. The similar small ligands used as the secondary ligands in the preparation of the cadmium(II) complexes with similar Schiff bases can result in similar structures.