Crystal Structure and Spectroscopic Properties of Bis[trans‐dichlorobis(2,2‐dimethylpropane‐1,3‐diamine)chromium(III)] Tetrachlorozincate

The structure of trans‐[Cr(Me₂tn)₂Cl₂]₂ZnCl₄ (Me₂tn = 2,2‐dimethylpropane‐1,3‐diamine) was determined by a single‐crystal X‐ray diffraction study at 173 K. The analysis reveals that there are three crystallographically independent chromium(III) complex cations in the title compound. The chromium(III) atoms are coordinated by four nitrogen atoms of Me₂tn and two chlorine atoms in a trans arrangement, displaying a distorted octahedral geometry. The two six‐membered chelate rings in three complex cations are oriented in an anti chair–chair conformation with respect to each other. The Cr–N and Cr–Cl bond lengths average 2.0862(2) and 2.3112(6) Å, respectively. The ZnCl₄^2– have slightly distorted tetrahedral arrangement with Zn–Cl lengths and the Cl–Zn–Cl angles are influenced by hydrogen bonding. The resolved absorption maxima in the electronic d–d spectrum were fitted with a secular determinant for a quartet energy state of the d³ configuration in a tetragonal field. It is confirmed that the nitrogen atoms of the Me₂tn ligand are strong σ donors, but the chloro ligands have weak σ‐ and π donor properties toward the chromium(III) ion.     

Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr0.25Ti0.75]O3 ceramic

In this work, we report the Rietveld refinement, microstructure, conductivity and impedance properties of Ba[Zr_0.25Ti_0.75]O₃ ceramic synthesized by solid state reaction. This ceramic was characterized by X-ray diffraction, Rietveld refinement, scanning electron microscopy and energy dispersive X-ray spectrometry. Impedance spectroscopy analyses reveals a non-Debye relaxation phenomenon being its relaxation frequency moving toward to positive side with increase of temperature. A significant shift in impedance loss peaks toward higher frequency side indicates conduction in material and favoring the long range motion of mobile charge carriers. The frequency dependent ac conductivity at different temperatures indicates that the conduction process is thermally activated. The variation of dc conductivity exhibited a negative temperature coefficient of resistance behavior. The ac conductivity data are used to evaluate the density of states at Fermi level and activation energy of this ceramic. The dc electrical and thermal conductivities of grain and grain boundary have been discussed.     

An insight into our research on vinyl sulfone-modified pyranosides and furanosides

This review describes the research by the authors on the synthesis of vinyl sulfone-modified carbohydrates and the application of this new class of Michael acceptors in the generation of a wide range of aminosugars, branched-chain sugars, cyclopropanted carbohydrates, densely functionalized cyclopropanes, isonucleosides and pyrroles.     

Efficient plasmid DNA cleavage by copper(II) complexes of 1,4,7-triazacyclononane ligands featuring xylyl-linked guanidinium groups

Three new metal-coordinating ligands, L(1), L(2), and L(3), have been prepared by appending o-, m-, and p-xylylguanidine pendants, respectively, to one of the nitrogen atoms of 1,4,7-triazacyclononane (tacn). The copper(II) complexes of these ligands are able to accelerate cleavage of the P-O bonds within the model phosphodiesters bis(p-nitrophenyl)phosphate (BNPP) and [2-(hydroxypropyl)-p-nitrophenyl]phosphate (HPNPP), as well as supercoiled pBR 322 plasmid DNA. Their reactivity toward BNPP and HPNPP is not significantly different from that of the nonguanidinylated analogues, [Cu(tacn)(OH(2))(2)](2+) and [Cu(1-benzyl-tacn)(OH(2))(2)](2+), but they cleave plasmid DNA at considerably faster rates than either of these two complexes. The complex of L(1), [Cu(L(1)H(+))(OH(2))(2)](3+), is the most active of the series, cleaving the supercoiled plasmid DNA (form I) to the relaxed circular form (form II) with a k(obs) value of (2.7 ± 0.3) × 10(-4) s(-1), which corresponds to a rate enhancement of 22- and 12-fold compared to those of [Cu(tacn)(OH(2))(2)](2+) and [Cu(1-benzyl-tacn)(OH(2))(2)](2+), respectively. Because of the relatively fast rate of plasmid DNA cleavage, an observed rate constant of (1.2 ± 0.5) × 10(-5) s(-1) for cleavage of form II DNA to form III was also able to be determined. The X-ray crystal structures of the copper(II) complexes of L(1) and L(3) show that the distorted square-pyramidal copper(II) coordination sphere is occupied by three nitrogen atoms from the tacn ring and two chloride ions. In both complexes, the protonated guanidinium pendants extend away from the metal and form hydrogen bonds with solvent molecules and counterions present in the crystal lattice. In the complex of L(1), the distance between the guanidinium group and the copper(II) center is similar to that separating the adjacent phosphodiester groups in DNA (ca. 6 ?). The overall geometry of the complex is also such that if the guanidinium group were to form charge-assisted hydrogen-bonding interactions with a phosphodiester group, a metal-bound hydroxide would be well-positioned to affect the nucleophilic attack on the neighboring phosphodiester linkage. The enhanced reactivity of the complex of L(1) at neutral pH appears to also be, in part, due to the relatively low pK(a) of 6.4 for one of the coordinated water molecules.     

A general route to mono- and disubstituted divinyl sulfones: acyclic Michael acceptors for the synthesis of polyfunctionalized cyclic sulfones

Nucleophilic C-S bond formation using easily available β-hydroxysulfonate derivatives allowed direct access to new mono- and disubstituted divinyl sulfones. Our strategy uses thioethanol (HSCH(2)CH(2)OH) and its analogues such as HSCH(2)CH(Y)OH generated in situ. The strategy also allows the synthesis of modified divinyl sulfones attached to chiral appendages like carbohydrates. Bis-heteronucleophilic Michael addition reactions with 1 equiv of a primary amine afforded new generations of S,S-dioxothiomorpholine derivatives known for their therapeutic applications. Further synthetic manipulations of some of these cyclic compounds led to the synthesis of novel bicyclic derivatives.     

A metal-free route towards 1,5-disubstituted 1,2,3-triazolylmethylene linked disaccharides: Synthesis in a biodegradable hydroxyl-ammonium-based aqueous ionic liquid media

Suitably protected carbohydrates were joined together using 1,5-disubstituted 1,2,3-triazolylmethylene (1,5-DTM) linkers. The DTM linker was built by the 1,3-dipolar cycloaddition reactions of a series of sugar azides with vinyl sulfonylmethylene-modified furanose or pyranose under metal free conditions. Three different biodegradable hydroxylammonium based ionic liquids were studied in water as the reaction media. The N,N-dimethyl ethanolammonium formate-water mixture was found to be the best reaction medium because the reaction time was shortened considerably to generate a dozen new 1,5-DTM-linked disaccharides.     

Diastereoselective C-C bond formation at C-5 of vinyl sulfone-modified hex-5-enofuranosyl carbohydrates: diversity-oriented synthesis of branched-chain sugars and beyond

This is the first report on the diastereoselective addition of carbon nucleophiles to vinyl sulfone-modified hex-5-enofuranosides. The stereoelectronic properties of the substituents at the C-3 position and their interactions with the incoming carbon nucleophiles control the diastereoselectivity of addition at the C-5 position, favoring the formation of l-ido derivatives as major products in most of the cases studied. This new concept of stereocontrolled carbon-carbon bond formation in vinyl sulfone-modified carbohydrates is general in nature. The novel chirons generated by this diversity-oriented synthetic method have been implemented in the preparation of a wide range of hexofuranosyl C-5 branched-chain sugars, bicyclic derivatives, chirally pure enals, and densely functionalized carbocycles.     

Dinucleosides with non-natural backbones: a new class of ribonuclease A and angiogenin inhibitors

Ribonuclease?A (RNase A) serves as a convenient model enzyme in the identification and development of inhibitors of proteins that are members of the ribonuclease superfamily. This is principally because the biological activity of these proteins, such as angiogenin, is linked to their catalytic ribonucleolytic activity. In an attempt to inhibit the biological activity of angiogenin, which involves new blood vessel formation, we employed different dinucleosides with varied non-natural backbones. These compounds were synthesized by coupling aminonucleosides with dicarboxylic acids and amino- and carboxynucleosides with an amino acid. These molecules show competitive inhibition with inhibition constant (K(i)) values of (59±3) and (155±5) μM for RNase A. The compounds were also found to inhibit angiogenin in a competitive fashion with corresponding K(i) values in the micromolar range. The presence of an additional polar group attached to the backbone of dinucleosides was found to be responsible for the tight binding with both proteins. The specificity of different ribonucleolytic subsites were found to be altered because of the incorporation of a non-natural backbone in between the two nucleosidic moieties. In spite of the replacement of the phosphate group by non-natural linkers, these molecules were found to selectively interact with the ribonucleolytic site residues of angiogenin, whereas the cell binding site and nuclear translocation site residues remain unperturbed. Docked conformations of the synthesized compounds with RNase A and angiogenin suggest a binding preference for the thymine-adenine pair over the thymine-thymine pair.