Structural investigations of a series of petrified woods of different origin

Since the investigations on petrified wood performed so far have not proved the assumption that the ring structure is coupled to the changes in the crystalline form of silica, attention was paid to other possible reasons. Using small angle X-ray scattering methods we determined the Porod curves and pore size distributions for five different samples of petrified wood, of different origin. We observed clear porosity differences in different samples. In addition, the normal X-ray diffractometric measurements were carried out, which gave a totally uniform image of the crystallographic structure of all the samples. All of them were made of pure α-quartz. The differentiation of rings in dark and bright zones should be put down to the changes in porosity rather than to other factors. The pores seem to be much more concentrated in those ring locations, which correspond to the original dark wood positions in living wood.     

Structural investigations of a lead(IV) tetraacetate-pyridine complex

A 1 : 1 crystalline complex of lead(IV) tetraacetate and pyridine (LTA-py) has been prepared. The single-crystal X-ray structure, at 296 and 150 K, establishes the presence of a relatively short Pb-N bond (2.307 A) within an intriguing seven-coordinate lead inner sphere consisting of the pyridine ligand and two bidentate and two monodentate acetate ligands. The pyridine occupies a surprising amount of the available coordination space and has induced a dramatic change in coordination compared to the four chelating acetate ligands found in lead tetraacetate (LTA). Thermal measurements (TGA/DSC) indicate the de-coordination of pyridine and its loss from the solid between 360 and 380 K. (207)Pb CP/MAS NMR spectroscopy also demonstrates the existence of the Pb-N bond through observation of (1)J((207)Pb,(14)N)= 63 Hz and a (207)Pb-(14)N dipolar coupling constant, of 149 Hz. The solid-state (207)Pb NMR parameters are used to give insight into the coordination environment of Pb(iv) in LTA-py. In solution, ligand exchange is rapid on chemical shift and J-coupling time scales. A (207)Pb NMR study of the titration of an LTA solution by pyridine yields a stability constant for LTA-py of K = 1.5 M(-1) and predicts it to have a (207)Pb NMR chemical shift essentially identical to that observed by CP/MAS NMR in the solid state. This correlation between the solid state and solution indicates that the seven-coordinate LTA-py structure found in the crystalline state does persist in solution, and this could further explain why the addition of pyridine has such profound effects on lead(IV) carboxylate-mediated organic reactions. Simulations of exchange-broadened line shapes of (13)C CP/MAS NMR spectra in the temperature regime above 280 K indicate local motion of the pyridine rings in the form of 180 degrees jumps (activation energy 72.5 kJ mol(-1)); these are first such ring flips reported for a coordinated pyridine ligand.     

Synthesis of spiro oxirane derivatives of the decahydroquinoline series

It was established that 1-methyl-, 1-ethyl-, 1,2-dimethyl-, and 1,2,2-trimethyl-4-oxiranyl-trans-decahydro-4-quinolols undergo isomerization under the influence of aqueous or alcohol solutions of sodium hydroxide, barium hydroxide, and thiourea with migration of the oxide ring to give the corresponding spiro epoxy compounds. A comparison of the IR spectra of the spiro epoxy compounds with the spectra of alternative products, viz., the corresponding keto alcohols, showed that keto alcohols are not formed under the given reaction conditions. The indicated conclusions are confirmed by the IR, PMR, and mass spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 72–77, January, 1982.     

Asymmetric synthesis of chiral spiro bis(isoxazoline) and spiro (isoxazole–isoxazoline) ligands

Asymmetric synthesis of novel chiral ligands, bis(isoxazoline) on a spiro[4.4]nonane scaffold and isoxazole–isoxazoline on a spiro[4.5]decane scaffold, has been achieved by utilizing an enantiomerically pure alcohol as the starting material.     

Structural analysis of spiro beta-lactone proteasome inhibitors

Spiro beta-lactone-based proteasome inhibitors were discovered in the context of an asymmetric catalytic total synthesis of the natural product (+)-lactacystin (1). Lactone 4 was found to be a potent inhibitor of the 26S proteasome, while its C-6 epimer (5) displayed weak activity. Crystallographic studies of the two analogues covalently bound to the 20S proteasome permitted characterization of the important stabilizing interactions between each inhibitor and the proteasome's key catalytic N-terminal threonine residue. This structural data support the hypothesis that the discrepancy in potency between 4 and 5 may be due to differences in the hydrolytic stabilities of the resulting acyl enzyme complexes.     

Polar effect in columnar unsymmetric 1,3,4-oxa(thia)diazoles

Three new series of heterocyclic 1,3,4-oxa(thia)diazoles 1a–c were prepared and their mesomorphic behavior studied. All compounds 1a exhibited monotropic columnar phases, which were confirmed by powder XRD diffractometer. Compounds 1b were not mesogenic. The formation of columnar phases was sensitive to the substituent groups attached on the phenyl moiety; those with electron-donating groups (R=H, OCH₃) were not liquid crystals, in contrast, other derivatives with electron-withdrawing groups (R=F, NO₂) formed enantiotropic columnar phases. A N_cell and R_ar value equal to 4.73 and 20.20 Å² of compound 1a (n=16) within a slice of 9.0 Å thick were obtained, indicating that two molecules was correlated to give a supplemental more disc-like molecule within columns. This is the first example, showing that the formation of columnar phases could be controlled by polar groups. The PL spectra of two compounds 1a–c showed one intense peak at λ_max=505–511 nm, and these photoluminescent emissions originated from quinoxaline moiety.     

Structural investigations of palladium(II) and platinum(II) complexes of salicylhydroxamic acid

Complexes of salicylhydroxamic acid (shaH) with palladium(II) and platinum(II) were investigated. The synthesis of [Pt(sha)(2)] was attempted via a number of methods, and ultimately (1)H NMR investigations revealed that salicylhydroxamate would not coordinate to chloro complexes of platinum(II). However, [Pt(sha-H)(PPh(3))(2)] was successfully synthesized and the crystal structure determined (orthorhombic, space group Pca2(1) a = 17.9325(19) A, b = 11.3102(12) A, c = 18.2829(19) A, Z = 4, R = 0.0224). The sha binds via an [O,O] binding mode, in its hydroximate form. In contrast the palladium complex [Pd(sha)(2)] was readily synthesized and crystallized as [Pd(sha)(2)](DMF)(4) in the triclinic space group P(-)1,a = 7.066(1) A, b = 9.842(2) A, c = 12.385(2) A, alpha = 99.213(3)(o), beta = 90.669(3), gamma = 109.767(3)(o), Z = 1, R = 0.037. The unexpected [N,O'] binding mode of the salicylhydroxamate ligand in [Pd(sha)(2)] prompted investigation of the stability of a number of binding modes of salicylhydroxamic acid in [M(sha)(2)] (M = Pd, Pt) by density functional theory, using the B3LYP hybrid functional at the 6-311G* level of theory. Geometry optimizations were carried out for various binding modes of the ligands and their relative energies established. It was found that the [N,O'] mode gave the more stable complex, in accord with experimental observations. Stabilization of hydroxamate binding to platinum is evidently afforded by soft ligands lying trans to them.     

Structural investigations of poly(amido amine) dendrimers in methanol using molecular dynamics

The molecular structure of poly(amido amine) dendrimers is investigated with molecular dynamics simulations using the Amber 7 modeling package. A method for defining residues for complex molecules is developed, and it enables the study of the effects of protonation of the primary and tertiary amines. The effects of implicit solvents versus explicit solvents as well as the pH of the solution on the molecular structure are calculated. Good agreement with experimental results for the radius of gyration measured in methanol by X‐ray scattering is observed for simulations with an explicit solvent and protonation of the primary amines. Calculations of the intramolecular atomistic pair correlation function show a dense core, as well as the presence of voids filled with the solvent inside the molecule. The primary amines (end groups) are shown to access the molecular interior by backfolding. Comparisons with experiments and other reported simulation results highlight the advantages of the approach developed here. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3062–3077, 2006     

Structural and mechanistic investigations of the oxidation of dimethylplatinum(II) complexes by dioxygen

The oxidation of (tmeda)Pt(II)(CH(3))(2) (1, tmeda = N,N,N',N'-tetramethylethylenediamine) to (tmeda)Pt(IV)(OH)(OCH(3))(CH(3))(2) (3) by dioxygen in methanol proceeds via a two-step mechanism. The initial reaction between (tmeda)Pt(CH(3))(2) and dioxygen yields a hydroperoxoplatinum(IV) intermediate, (tmeda)Pt(OOH)(OCH(3))(CH(3))(2) (2), which reacts with a second equivalent of (tmeda)Pt(CH(3))(2) to afford the final product 3. Both 2 and 3 have been fully characterized, including X-ray crystallographic structure determinations. The effect of ligand variation on the oxidation of several dimethylplatinum(II) complexes by 2 as well as by dioxygen has been examined.     

Synthesis of optically active 6-substituted 2-(aminomethyl)chromans

Three novel, optically active, 6-substituted 2-(aminomethyl)chromans were synthesized from readily available chroman 2-carboxylic acid precursors. These chroman-containing primary amines are useful building blocks for the synthesis of chroman-derived pharmaceutical agents.     

Structural investigations of copper(II) complexes containing fluorine-substituted β-diketonate ligands

The crystal structures of the β-diketonate complexes Cu(pta)₂, Cu(pta)₂·EtOH, and Cu(F₆-thd)₂ (where pta is the anion of 1,1,1-trifluoro-5,5-dimethylhexane-2,4-dione and F₆-thd is the anion of 2,2-bis(trifluoromethyl)-6,6-dimethylheptane-3,5-dione) were determined. The solid-state structures of Cu(pta)₂ and Cu(F₆-thd)₂ are square planar, while Cu(pta)₂·EtOH is approximately square pyramidal with the EtOH ligand occupying the apical position. In each case, only one geometrical isomer (cis or trans) was observed in the crystals; arguments are presented that both isomers are present in bulk samples. Calculations of molecular volumes for structurally related Cu(II) complexes containing non-fluorinated versus fluorinated ancillary ligands show that fluorine substitution does not significantly affect packing efficiency in the solid-state; however, solvent coordination decreases packing efficiency slightly. [Cu(tdf)(py)(μ-C₃F₇CO₂)]₂ (where tdf is the anion of 1,1,1,2,2,3,3,7,7,8,8,9,9,9-tetradecafluorononane-4,6-dionate), a derived impurity from preparations of Cu(tdf)₂, was isolated in low yield. The copper coordination geometries in the centrosymmetric structure are intermediate between square pyramidal and trigonal bipyramidal, with two unsymmetrically bridging μ,η¹:η¹-C₃F₇CO₂ ligands.     

Structural investigations of mixed metal compounds; the stereochemistry of two dilithium-tris(olefin)nickel(0) complexes

The structures of bis(lithium-N,N,N′,N′-tetramethylethylenediamine) (all-trans-1,5,9-cyclododecatrienenickel) (I) and tris(N,N,N′,N′,-tetramethyl-2-butene-1,4-diamine)dilithiumnickel (II) have been determined from single cyrstal X-ray data measured by counter methods. Crystals of I belong to the orthorhombic space group Pbca with a 13.776(2), b 14.090(2), c 58.374(6) », Z = 16 and d_c 1.10 g cm^-3. Compound II crystallizes in the monoclinic space group C2/c with a 22.960(2), b 8.9860(3), c 15.1984(6) », β 109.015(5)°, Z = 4 and d_c 1.12 g cm^-3. Refinement of I (II) coverged with R = 0.097 (0.037) for the 4281 (2424) reflections with I > 2σ(I).Two unique and essentially identical molecules not having crystallographic symmetry were found for I while molecules of II possess crystallographic C₂ symmetry, the two fold axis passing through the Ni atom and bisecting one olefinic CC bond. The trigonal bipyramidal geometry of the Ni atoms, the centers of three olefinic double bonds in the trigonal plane and Li atoms in apical positions, is distorted in I but nearly exact in II. The NiC (olefinic) bond lengths average 1.99(3) and 2.000(3) » in I and II respectively. The shorter bond distances to the Ni a toms formed by the three-coordinate Li atoms in I, average 2.400(6) », compared to those formed by the four-coordinate Li atoms in II, 2.561(3) », may be due to Ni → Li d_π → pπ backbonding in I. The LiNiLi angles are (average) 164(2)° in I and 178.9(1)° in II. The strength of the Ni olefin interactions in these compounds is most clearly shown by the long mean CC distances (1.452(9) ») and 40(2)° bending back of the olefinic substituents from the Ni atom in II.     

Condensation of Benzoxa(thia)zolyl-2-guanidines with Dicarbonyl Compounds

Heterocyclizations of benzoxa(thia)zolyl-2-guanidines with β-dicarbonyl compounds, such as diketones and derivatives of acetoacetic ether, were studied.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 322–325.Original Russian Text Copyright © 2005 by Shikhaliev, Kryl’skii, Kovygin, Verezhnikov.     

Synthesis and photochromic properties of spiro (benzindoline-naphthoxazines)

A new photochromic compound belonging to the class of spiro(indoline-naphthoxazines) was synthesized. Nitration of this compound was carried out and the structures of two photochromic nitro-substituted products were determined. The photochromic properties of the products were studied in solvents of different polarity.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 732–736, April, 1995.     

New designs and structure activity relationships in antitumoral aziridinyl cyclophospha(thia)zenes

Abstract

The partial aziridinolysis of (NPCl₂)_3,4 and (NPCl₂)₂ NSOCl yields non-geminal and geminal chloro-aziridinyl derivatives. Compounds N₃P₃Az_6-nR_n (R[dbnd]amino, alkoxy, aryloxy, id.) show in vitro and in vivo cytostatic activity. Structure-activity relationships are proposed. Phase I clinical trials of (NPAz₂)₂ NSOAz (Az[dbnd]1-aziridinyl) are summarized.