Photochemical Z-->E isomerization of a hemithioindigo/hemistilbene omega-amino acid.

The molecule HTI, which combines hemithioindigo and hemistilbene molecular parts, allows reversible switching between two isomeric states. Photochromic behaviour of the HTI molecule is observed by irradiation with UV/Vis light. The photochemical reaction, a Z/E isomerization around the central double bond connecting the two molecular parts, is investigated by transient absorption and emission spectroscopy. For a special HTI molecule, namely, an omega-amino acid, the Z-->E isomerization process occurs on a timescale of 30 ps. In the course of the reaction fast processes on the 1-10 ps timescale are observed which point to motions of the molecule on the potential-energy surface of the excited state. The combination of transient absorption experiments in the visible spectral range with time-resolved fluorescence and infrared measurements reveal a photochemical pathway with three intermediate states. Together with a theoretical modelling procedure the experiments point to a sequential reaction scheme and give indications of the nature of the involved intermediates.     

The crystal and molecular structure of phenothiazine-10-propionic acid

The crystal structure of phenothiazine-10-propionic acid, C₁₂H₈SNC₂H₄COOH, was determined from three-dimensional X-ray diffraction data collected with a manual diffractomer using MoKα (λ 0.71069 Å) radiation. The space group is P2₁/c with a = 7.888 (2)Å, b = 8.703 (2)Å, c = 19.700 (8)Å, β = 101.42 (1)°, Z = 4, D_meas = 1.35(2) g. cm^?3 and D_calc = 1.36 g. cm^?3 at 23°. The structure was determined by the direct method and refined with 500 observed reflections by full-matrix least squares to an R of 0.072. The molecule is folded along the S-N axis and the dihedral angle is 136.5°. The C-S-C angle is 98.5(7)° and the average C-S bond is 1.77(2)Å. The shortening of the C-S bond, the small value of the C-S-C angle and the folding of the molecule are typical of the phenothiazine class of compounds and are assumed to be due to sulfur d orbital participation in ring bonding.     

RuCl3/CeCl3/NaIO4: a new bimetallic oxidation system for the mild and efficient dihydroxylation of unreactive olefins

[reaction: see text] The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO(4) has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO(4) has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO(4)-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO(4)-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl(3) allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.     

Synthesis of extended spacer-linked neooligodeoxysaccharides by metathesis olefination and evaluation of their RNA-binding properties

The preparation of linear 1,4-butanediol-linked oligodeoxysugars 50-53, 58 and 60 is described which are potential binders to polynucleotides. Various aminodeoxymonosaccharides 9, 13-18, 30, 31 and 40-42 which are either allylated at the anomeric center or at C4 were subjected to the metathesis olefination protocol. Depending on the position of allylation E/Z-mixtures of C₂-symmetric head-to-head or tail-to-tail homodimers were formed. Among them, saccharides 13, 30, 31 and 40 were transformed into the corresponding 1,4-butanediol linked disaccharides 50-53 by catalytic hydrogenation of the central olefinic double bond and exhaustive deprotection. In order to target extended spacer-linked neooligosaccharides homodimeric aminoglycoside 37 was bisallylated and subjected to cross metathesis conditions using methyl 4-O-allyl daunosamide 40 as reaction partner which yielded two desired trimeric and tetrameric linearly spacer-linked daunosamine derivatives. After hydrogenation and deprotection two additional probes 58 and 60 for nucleic acid binding studies were at hand.     

Molecular structure of [CpRu(PPh3)2(tht)]BF4, tht=tetrahydrothiophene

The title compound [CpRu(PPh₃)₂(tht)]BF₄,1, tht=tetrahydrothiophene, is obtained by the reaction of CpRu(PPh₃)₂Cl, tht and AgBF₄ in CH₂Cl₂ solution. Yellow crystals of [CpRu(PPh₃)₂(tht)]BF₄ were grown by the slow diffusion of pentane into a CH₂Cl₂ solution of1 and have been studied by X-ray crystallography: space group is P2₁/n,a=9.507(2),b=18.336(3),c=22.372(6) Å, =94.28(2)°,V=3889(1) ų,Z=4. The tht ligand binds to the Ru atom through the sulfur atom at a distance of 2.395(2) Å. The tht ring is in the puckered configuration.     

Structure of trans-aqua-bis(pyridine)dibenzoatocopper(II)

Crystals oftrans-aqua-bis(pyridine)dibenzoatocopper(II), Cu(NC₅H₅)₂(C₇H₅O₂)₂(H₂O), are monoclinic,P2₁/n, witha=5.9847(10),b=18.874(5),c=19.728(10)A,=92.97(4)°,Z=4, andV=2225.5(13)A³. The full-matrix least-squares refinement used 3030 reflections withI> 2.5(I) and converged toR=0.067. The benzoate ions bond as monodentate ligands in this five-coordinate complex. The copper ion is in a square pyrimidal environment with trans pyridine groups and benzoate ions in a slightly distorted square planar arrangement and a water ligand in the axial position. The Cu-O distance to the water molecule is significantly longer (2.273(4) Å) than the distances to the O atoms of the carboxyl groups (1.934(3) and 1.930(4)Å).     

Synthesis of Indoloquinolines: An Intramolecular Cyclization Leading to Advanced Perophoramidine-Relevant Intermediates

The bioactive natural product perophoramidine has proved a challenging synthetic target. An alternative route to its indolo[2,3-b]quinolone core structure involving a N-chlorosuccinimde-mediated intramolecular cyclization reaction is reported. Attempts to progress towards the natural product are also discussed with an unexpected deep-seated rearrangement of the core structure occurring during an attempted iodoetherification reaction. X-ray crystallographic analysis provides important analytical confirmation of assigned structures.     

Vertex pancyclic in‐tournaments

An in‐tournament is an oriented graph such that the negative neighborhood of every vertex induces a tournament. The topic of this paper is to investigate vertex k‐pancyclicity of in‐tournaments of order n, where for some 3 ≤ k ≤ n, every vertex belongs to a cycle of length p for every k ≤ p ≤ n. We give sharp lower bounds for the minimum degree such that a strong in‐tournament is vertex k‐pancyclic for k ≤ 5 and k ≥ n − 3. In the latter case, we even show that the in‐tournaments in consideration are fully (n − 3)‐extendable which means that every vertex belongs to a cycle of length n − 3 and that the vertex set of every cycle of length at least n − 3 is contained in a cycle of length one greater. In accordance with these results, we state the conjecture that every strong in‐tournament of order n with minimum degree greater than is vertex k‐pancyclic for 5 < k < n − 3, and we present a family of examples showing that this bound would be best possible. © 2001 John Wiley & Sons, Inc. J Graph Theory 36: 84–104, 2001