Synthesis of 1H-imidazo[1,2-a]imidazole

The synthesis and structure analysis of the unknown 1H-imidazo[1,2-a]imidazole ( 1 ) is described. The preparation involves alkylation of 2-aminoimidazole with bromoacetaldehyde diethyl acetal and subsequent hydrolysis and cyclization with hydrochloric acid. The structure was characterized by mass spectrometry and by ¹H-, ¹⁵N- and ¹³C-nmr of 1 and by ¹H-nmr of its 1-benzyl derivative 8 . An independent synthesis of 8 was accomplished via cyclization of 2-(N-dichloroethyl-N-benzyl)aminoimidazole ( 11 ).     

New Explanation for Ligand Bending in Transition Metal Tris(dithiolate) Complexes

The results of Fenske-Hall molecular orbital calculations are reported for the trigonal prismatic complexes Mo(S(2)C(2)H(2))(3) and Mo(S(2)C(6)H(4))(3). Both complexes exhibit a bend of the S-C-C-S ligand plane away from the S-Mo-S plane. A series of calculations which systematically follow the changes in electronic structure as the bend angle alpha is varied between 0 and 30 degrees indicates that the bend can be attributed to a second order Jahn-Teller distortion. The driving force for this distortion, which allows mixing between a set of ligand pi orbitals and the metal d(z)()()2 orbital, should be greatest for d(0) systems. In these systems the bent geometry leads to the stabilization of the doubly occupied HOMO. The driving force for ligand bending should be lower in systems having more or fewer electrons (e.g. Re(S(2)C(2)Ph(2))(3) or V(S(2)C(2)Ph(2))(3), respectively). While the steric bulk of the dithiolate ligands in the latter complexes may also influence the degree of ligand bending, this is probably a secondary effect.     

Thermal decomposition mechanisms of sequential bipolyesters based on propeneglycol and hydroxybenzoic/phthalic diacid derivatives

Some bipolyesters consisting of 1,2-propene glycol residue (P) and symmetric aromatic diacid residue constituted by two p-oxybenzoyl groups coupled by a terephthaloyl (HTH), phthaloyl (HOH) and isophthaloyl (HIH) group, and a nonsymmetric diacid residue constituted by a p-oxybenzoyl and a terephthaloyl group (HT), have been investigated by direct pyrolysis–mass spectrometry. The results show that intramolecular exchange reactions occurring at the pyrolysis stage cause a reorganization of the copolymer sequences. Sequential bipolyesters I–IV are ideal materials for the investigation of the extent of thermal reorganization induced in the bipolymer sequences by the pyrolytic process.     

Conformation of bicyclo [n.1.0] Derivatives: Part 3. Dioxa- and trioxa-bicyclo[5.1.0] octanes

The conformational geometries and possible interconversion paths for some oxa derivatives of bicyclo[5.1.0] octane have been studied by the molecular mechanics method. The theoretical results are compared with the experimental data for the molecular geometry of bicyclo[5.1.0] octane and 3,5,8-trioxabicyclo[5.1.0] octane, the free energy of activation for cycloheptene epoxide and 3,5-dioxabicyclo[5.1.0] octane, the dipole moments and molar Kerr constants in solution for cycloheptene epoxide, 3,5-dioxa- and 3,5,8-trioxabicyclo[5.1.0] octane.     

Influence of the coordination number of nitrogen on the structure of 6aλ4-thia-1,3,4,6-tetraazapentalenic systems. Crystal structure analyses of 3-(2-pyridylimino)-3H-[1,2,4]thiadiazolo[4,3-a]pyridine and its 1-methylated salt

Structural data were obtained by X-ray crystallography for the title compounds which show that they are essentially planar and exhibit an approximately linear N2-S1-N8 arrangement. In compound 3 the separation between the sulfur atom and the pyridine nitrogen atom (2.61 Å) is larger than the Huggins constant energy distance (2.58 Å), suggesting that there is little or no bonding between them. The methylated salt 4 , on the contrary, has a closer S…N(pyridine) distance (2.19 Å) with an estimated bond dissociation energy of 6 kcal/mole.     

Protective effects of cyanidin-3-O-beta-glucopyranoside against UVA-induced oxidative stress in human keratinocytes

Ultraviolet-A (UVA) radiation causes significant oxidative stress because it leads to the generation of reactive oxygen species (ROS), leading to extensive cellular damage and eventual cell death either by apoptosis or necrosis. We evaluated the protective effects of cyanidin-3-O-beta-glucopyranoside (C-3-G) against UVA-induced apoptosis and DNA fragmentation in a human keratinocyte cell line (HaCaT). Treatment of HaCaT cells with C-3-G before UVA irradiation inhibited the formation of apoptotic cells (61%) and DNA fragmentation (54%). We also investigated antioxidant properties of C-3-G in HaCaT cells against ROS formation at apoptotic doses of UVA; C-3-G inhibited hydrogen peroxide (H2O2) release (an indicator of cellular ROS formation) after UVA irradiation. Further confirmation of the potential of C-3-G to counteract UVA-induced ROS formation comes from our demonstration of its ability to enhance the resistance of HaCaT cells to the apoptotic effects of both H2O2 and the superoxide anion (O2*-), two ROS involved in UVA-oxidative stress. Furthermore, in terms of Trolox Equivalent Antioxidant Activity, C-3-G treatment led to a greater increase in antioxidant activity in the membrane-enriched fraction than in the cytosol (55% vs 19%). The protective effects against UVA-induced ROS formation can be attributed to the higher membrane levels of C-3-G incorporation. These encouraging in vitro results support further research into C-3-G (and other anthocyanins) as novel agents for skin photoprotection.     

Structure-Reactivity Correlations in the Dissociative Hydrolysis of 2',4'-Dinitrophenyl 4-Hydroxy-X-benzenesulfonates

The hydrolysis reactions of several title esters in water at 60 degrees C follow the rate law k(obs) = (k(a) + k(b)[OH(-)])/(1 + a(H)/K(a)), where K(a) is the ionization constant of the hydroxy group of the ester and k(b) is the second-order rate constant for the S(N)2(S) attack of hydroxide ion on the ionized ester. Hammett and Br?nsted correlations are consistent with a previous proposal that the mechanism related to k(a) is dissociative. An unusual relationship between k(a) values and redox equilibrium constants for substituted quinones is found to hold: this finding further supports the dissociative nature of the pathway related to k(a).     

Synthesis, structure, and kinetics and stereochemistry of base-catalyzed hydrolysis of meso- and rac-[Co2(tmpdtne)Cl2]4+, bis(pentaamine) complexes devoid of deprotonatable NH centers

The bis(pentadentate) ligand tmpdtne binds two Co(II) centers, and the entity is readily oxidized to the dicobalt(III) derivative [Co(2)(tmpdtne)Cl(2)](4+) which has been separated into two isomeric forms. NMR studies establish these as meso and rac isomers arising from the different or same absolute configurations for the asym configuration about each Co(III) center. Each dinuclear ion base hydrolyses to the dihydroxo derivative [Co(2)(tmpdtne)(OH)(2)](4+) with retained asym configurations about each metal ion and also retained rac or meso configurations. The kinetics for the stepwise loss of the two Cl(-) ligands is uniphasic, and data are presented to show that the loss of the first chloride is rate determining and is followed by very rapid intramolecular and loss of the second Cl(-) via a hydroxo-bridged species to yield the observed dihydroxo derivative. Meso and rac forms of the latter have been crystallized. The X-ray crystal structure of the rac-dihydroxo complex is reported, and it establishes the configurations of all the complexes reported. The (1)H NMR spectra for the hydroxo ions show very high field Co-OH resonances (ca. delta-0.5 ppm) not observed previously for such ions, and this result is discussed in the context of published (1)H NMR data for bridged Co-OH-Co species. The base hydrolysis kinetics for the dichloro ions are first order in [OH(-)], and deprotonation at an alpha-CH(2) center (alpha to a pyridyl) is identified as the source of the catalysis, since there is no NH center available for deprotonation on the ligand. These data further support the new pseudoaminate base hydrolysis mechanism first reported in 2003. The values of k(OH) for the second-order base-catalyzed reaction are ca. 4.0 M(-1) s(-1) for both the rac and meso isomers, and these results are discussed in terms of the increased acidities of these 4+ cations compared to their 2+ ion counterparts.     

Anisotropic quantum spin fluid and quantum spin glass in La cuprate-based catalyst

Evidence of quantum spin fluid and of quantum spin glass behaviour has been detected by extended EPR analysis in some La-cuprate catalysts of general formula La_2−xEu_xCuO_{4 + δ}. Some features of the EPR spectrum, appearing in a few cases, are also discussed and attributed to a pseudo Jahn-Teller effect due to a particular coordination of up to five oxygen ions around surface copper ions at low temperature.     

A class of completek-caps inPG(3,q) forq an odd prime

The main problem on caps, posed originally by Segre in the fifties, is to determine the values of k for which there exists a complete k-cap. Very few results on this problem are known. The cardinality of the largest cap(s) and the smallest complete cap(s) are crucial. In this paper it is shown that there exist complete k-caps in PG(3, q), q an odd prime 5 or q = 9, such that k = (q² + q + 6)/3 or k = (q² + 2q + 6)/3. These complete caps are smaller than those currently known for q odd.In memoriam Giuseppe Tallini