Hydrogen bonding in dimers of tritolyl and tritosylurea derivatives of triphenylmethanes

The crystal structure of the homodimer formed by the tritolylurea 3a proves the existence of a belt of six bifurcated hydrogen bonds between both NH and the O=C groups of the adjacent urea residues. For the tritosylurea 3b, four additional three-center hydrogen bonds, also involving the SO2 oxygen, are found in the crystalline state. Molecular dynamics simulations in a chloroform box confirm these patterns of the hydrogen bonds and the resulting elongation of the dimer 3b. 3b in comparison to 3a x 3a. The calculated complexation energies for the three dimeric combinations are nearly identical in agreement with the simultaneous formation of heterodimer 3a x 3b in a mixture of 3a and 3b.     

Rapid reduction of pentavalent antimony by trypanothione: potential relevance to antimonial activation

The dithiol trypanothione can reduce an antiparasitic pentavalent antimony agent to trivalent rapidly; this reduction process is both pH and temperature dependent and trypanothione may therefore play an important role in the activation of the drug.     

Unusual conformations of 1,3‐dialk­oxy­thia­calix[4]arenes in the solid state

The structures of three syn‐1,3‐dialkoxy­thia­calix[4]arenes with unusual conformations in the solid state are reported. The pinched cone conformation of syn‐2²,4²‐dihydroxy‐1²,3²‐bis­(prop‐2‐enyl­oxy)thia­calix[4]arene, C₃₀H₂₄O₄S₄, (3a), is stabilized by two intra­molecular hydrogen bonds, remarkably formed from both OH groups to the same ether O atom. In syn‐2²,4²‐dihydroxy‐1⁵,2⁵,3⁵,4⁵‐tetra­nitro‐1²,3²‐bis­(prop‐2‐enyl­oxy)thia­calix[4]arene acetone disolvate, C₃₀H₂₀N₄O₁₂S₄·2C₃H₆O, (3b1), the mol­ecule is found in the 1,3‐alternate conformation. The crystallographic C2 symmetry is due to a twofold rotation axis running through the centre of the calixarene ring. The hydroxy groups cannot form intra­molecular hydrogen bonds as in (3a) and both are bonded to an acetone solvent mol­ecule. The mol­ecule of the pseudo‐polymorph of (3b1) in which the same compound crystallized without any solvent, viz. (3b2), is located on a crystallographic mirror plane. Only one of the two hydroxy groups forms a hydrogen bond, and this is with a nitro group of a neighbouring mol­ecule as acceptor. Mol­ecular mechanics calculations for syn‐1,3‐diethers suggest a preference of the 1,3‐alternate over the usual cone conformation for thia­calix[4]arene versus calix[4]arene and for para‐nitro versus para‐H derivatives.     

Templated metal catalysis for single nucleotide specific DNA sequence detection

A catalytic DNA-templated reaction of hydrolysis of an ester group in an N-modified peptide nucleic acid, which is activated by a Cu2+ complex-PNA, has been discovered and optimized. Both the ester-containing PNA and the metal complex PNA bind neighboring sites on a template DNA. This brings the reacting groups (the ester and the Cu2+ complex) in proximity to each other and accelerates the hydrolysis of the ester approximately 500 times in comparison with its hydrolysis in the absence of the template. The hydrolysis reaction provides >10(2)-fold kinetic discrimination between DNAs that are different from each other at a single nucleotide position. Natural enzyme T4 DNA ligase is slightly less selective. On the basis of this reaction a fully homogeneous and sensitive assay for sequence-specific DNA detection has been developed (10 fmol DNA). Identification of one of four DNAs (variation at one position) can be done in a single experiment. Since the Cu2+ ion is tightly bound in an associate containing the ester PNA, the metal complex PNA, and the template DNA, application of this method in buffers containing other Cu2+-binding ligands, e.g., PCR buffer and physiological buffer, is possible.     

New aurone epoxide and auronolignan from the heartwood of Cotinus coggygria Scop

New aurone epoxide, 2,10-oxy-10-methoxysulfuretin (14), and new auronolignan (15), named cotinignan A, were isolated by silica gel column and semipreparative HPLC chromatography from the methylene chloride/methanol extract of Cotinus coggygria Scop. heartwood. In addition, thirteen known secondary metabolites namely sulfuretin, 2,3-trans-fustin, fisetin, butin, butein, taxifolin, eriodictyol, 3',5,5',7–tetrahydroxyflavanone, 3',4',7-trihydroxyflavone, 3-O-methyl-2,3-trans-fustin, 3-O-galloyl-2,3-trans-fustin, β-resorcylic acid and 3-O-β-sitosterol glucoside were isolated as well. Their structures were elucidated by 1D and 2D NMR, HR-ESI-MS, IR and UV. Ten out of eleven isolated flavonoids possess 7, 3' and 4' hydroxy groups. These structural features could be considered as chemotaxonomic characteristic of flavonoids from C. coggygria. Cotinignan A (15) represents new subclass of secondary metabolites - auronolignans.

     

Near-infrared spectroscopy of ethylene and ethylene dimer in superfluid helium droplets

The nu(5)+nu(9) spectra of ethylene, C(2)H(4), and its dimer, solvated in helium nanodroplets, have been recorded in the wavelength region near 1.6 microm. The monomer transitions show homogeneous broadening of approximately 0.5 cm(-1), which is interpreted as due to an upper state vibrational relaxation lifetime of approximately 10 ps. Nearly resonant vibrational energy transfer (nu(5)+nu(9)-->2nu(5)) is proposed as the relaxation pathway. The dimer gives a single unresolved absorption feature located 4 cm(-1) to the red of the monomer band origin. The scaling of moments of inertia upon solvation in helium is 1.18 for the monomer and >2.5 for the dimer. In terms of the adiabatic following approximation, this classifies the monomer as a fast rotor and the dimer as a slow rotor.     

Zweifach und dreifach intramolekular verbrückte p‐tert‐Butylcalix[8]aren‐triphosphate – Synthese,Struktur und Stereochemie

Two‐ and Threefold Intramolecular Brigdging p‐tert‐Butylcalix[8]arene Triphosphates – Synthesis, Structure and Stereochemistry [1] The phosphorylation of p‐tert‐butylcalix[8]arene ( 1 ) with phosphorus pentachloride and hydrolysis gives intramolecular bridging tert‐butylcalix[8]arene triphosphates. The reactivity (esterification, dehydratisation, complexation), the structure (nmr and x‐ray), and the stereochemical behaviour of the phosphates will be discussed.     

13C‐NMR spectra of santalol derivatives: a comparison of DFT‐based calculations and database‐oriented prediction techniques

A systematic investigation of a series of santalol and epi‐santalol derivatives by means of ab initio and density functional theory (DFT) calculations together with database‐oriented prediction methods leads to a configurational reassignment within this compound class. The DFT calculations as well as the HOSE‐code and neural network‐based predictions allow deriving a general rule set for unambiguous assignment within this compound class. The methyl group in position 2′ serves as an indication for the configuration at this stereocenter allowing easy differentiation between santalol derivatives and their diastereomers belonging to the epi‐santalol series. Copyright © 2009 John Wiley & Sons, Ltd.     

Hydrogen bonded dimers of triurea derivatives of triphenylmethanes

Tri-(2-alkoxy-5-ureido-phenyl)methanes represent a novel self-complementary motif forming hydrogen bonded homo- and heterodimers in nonpolar, aprotic solvents as evidenced by 1H NMR and ESI-mass spectra and by the formation of heterodimers. MD simulations suggest the formation of hydrogen bonds of different strength in agreement with NMR data. The dimerization does not interfere with that of tetraurea calix[4]arenes. A combination of both motifs may be used therefore to build up larger structures via self-assembly processes. [structure: see text]     

Efficient methods for the synthesis of 2-hydroxyphenazine based on the Pd-catalyzed N-arylation of aryl bromides

[reaction: see text] Substituted diphenylamines can be synthesized by Pd(0)-catalyzed N-arylation using o-nitroanilines and nitro-substituted aryl bromides for a substrate. Cyclization of the diphenylamines by various methods, including the intramolecular Pd(0)-catalyzed N-arylation, produces 2-methoxyphenazine which can easily be deprotected to give 2-hydroxyphenazine. This phenazine is required to synthesize methanophenazine, a novel redoxactive cofactor isolated from methanogenic archaea.