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Benzalaminoacetals have led to 2-benzylpyrazines after refluxing with trifluoroacetic acid instead of the expected isoquinolines. This reaction has represented a useful way for preparing a variety of 2-benzylpyrazines from the corresponding benzaldehydes.

The treatment of the 90 degree angular precursor trans-[Ru[Cl.sub.2][(dmso-S).sub.4]] with an equivalent amount of the linear and rigid pyrazine (pyz) linker has produced the molecular triangle [[{trans,cis-Ru[Cl.sub.2][(dmso-S).sub.2]([mu]-pyz)}.sub.3]] instead of the molecular squares. The results have shown that the entropically favored molecular triangles are preferred over the expected molecular squares with metal corner fragments that have favored [N.sub.pyz]-M-[N.sub.pyz] angles narrower than 90 degrees due to the presence of ancillary ligands.

A general synthetic route is developed for the resonance-stabilized pyrazine-bridged bisdithiazolyl framework, involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl chloride. Variable-temperature single-crystal conductivity measurements have shown that the conductivity is activated at room temperature.

The description of the electronic effects, due to the noncovalent host-guest interactions between calix [6] arene and a ruthenium dimer, [[[Ru.sub.3] O[(OAc).sub.6](CO)(ppy)].sub.2]-mu-pz where ppy is 4-phenyl pyridine and pz is pyrazine, is given.

The syntheses, crystal structures, and magnetic susceptibilities of a family of copper pyrazine (pz)-based antiferromagnets with moderate in-plane magnetic exchange is reported. The compounds are well described by the two-dimensional Heisenberg model over most of the temperature range but undergo transition to an ordered state at low temperature.

The density functional theory (DFT) framework is used to study the main features of double group transfer reactions. The results have shown that a wide range of structure-types and processes including type II-dyotropic reactions and the Meerwein-Ponndorf-Verley reduction have taken place through highly synchronous in-plane aromatic transition structures, where the orbital topology is equivalent to that which corresponds to a [D.sub.26]-symmetric aromatic molecule such as pyrazine.

A theoretical mechanistic study of the formation of imidazo[1,2-a]pyrazine-3,6-diones derived from alpha-amino acids using pyrolysis and silica-catalyzed process is presented. The cyclocondensation of N-acylated diketopiperazincs into bicyclic amidines is endothermic for Gly and exothermic for [alpha]-aminoisobutic acid.

The preparation of the symmetrical heptacyclic core of the spiro 6-6-5-5 ritterazines is reported. It is expected that the key Diels-Alder/Wittig/diene cyclozirconation approach illustrated, supported by ZINDO calculations, would be a general strategy for the stereocontrolled construction of polycarbacyclic target structures.

A series of pyrazine (pz) complexes containing cis-(NH3)(sub 2)Pt(super II), (tmeda)Pt(super II), and trans-(NH3)(sub 2)Pt(super II) entities are synthesized and characterized by X-ray crystallography and/or (super 1)H NMR spectroscopy. The formation of the cyclic tetramer [Pt(super II)](sub 4) complex conforms to expectations of the molecular library approach, which predicts a molecular box when two ditopic building blocks with 90 degree and 180 degree angles, respectively, are combined.

The study of the electrochemical and spectroscopic properties of the pi-conjugated ligand tetra-2-pyridyl-1, 4-pyrazine (tppz) as well the mixed ligand complexes with terpyridine end caps provides a better characterization of the series of multimetallic oligomers of ruthenium (Ru). The results show that these compounds can be excellent wire materials, as they yield low HOMO-LUMO gaps, high delocalization, and the onset of quasi-bond features.

Report of the experimental UV absorbance behavior of the supercritical ethane-pyrazine system, the quantum-chemically determined structures for pyrazine and ethanes, the computed transition energies, predictions of the supercritical ethane densities and a two-dimensional computational probe of the pyrazine-ethane potential surface is illustrated. The combination of experiment and computation allows prediction of supercritical ethane bulk densities at which the pyrazine primary solvation shell contains an average of one, two, three, and four ethane molecules.

The usefulness of intervalence-resonant Raman spectroscopy in exploring the localized-to-delocalized transition in mixed-valence chemistry is demonstrated. The results presented give evidence for vibronic coupling of totally symmetric bridging vibrations of pyrazine to the 'intervalence' charge transfer that confirms a near-delocalized description for the mixed-valence cluster dimer.

The density functional theory was used to investigate the energetics of free radical generation via the sequence of reactions proposed by Shul'pin and co-workers. The studies showed that vanadate anions in combination with pyrazine-2-carboxylic acid produce an exceptionally active complex that promote the oxidation of alkanes and other organic molecules.

Representative polyfunctional tetrahydropyrido[3,4-b]pyrazine scaffolds are synthesized very readily by a one-pot annelation reaction of pentafluoropyridine with appropriate diamines. The trifluorinated pyridopyrazine products react sequentially with various nucleophiles to give polysubstituted tetrahydropyridopyrazines, demonstrating the potential of the polyfluorinated ring fused pyridine system as a scaffold for the synthesis of polysubstituted pyridopyrazine derivatives.

Classical trajectory calculations are performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine and CO2, which are conducted using a high-resolution transient infrared absorption spectrometer. The goal is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results.

The enthalpy of binding of 2-methoxy-3-isobutylpyrazine (IBMP) to the mouse major urinary protein (MUP) is examined using a combination of isothermal titration calorimetry (ITC), NMR, X-ray crystallography, all-atom molecular dynamics simulations, and site-directed mutagenesis. The characteristic signature of hydrophobic association in solution depends on the degree of solvation of the binding pocket and the architecture of this pocket is as important as hydrophobicity in determining the relative contribution of enthalpic versus entropic effects in biomolecular associations.

Two isolated pyrazine alkaloids from Botryllus leachi, botryllazines A and B exhibit cytotoxicity against human tumor cells. Regioselective metalation of pyrazines and cross-coupling reactions provide an easy access to botryllazines A and B and to an isomer of botryllazine A with good yields from chloropyrazine.

A general and selective methods for partial reduction of pyridine, pyrimidine, and pyrazine rings of various heteroaromatic compounds with bridgehead nitrogen via Birch reduction motif is reported. This method could serve as a new and effective tool for quick and easy access toward various partially reduced fused heterocyclic compounds.

6-Aryl-4-pyrazol-4-yl-pyridazin-3-one (1) reacted with a [PCl.sub.5]--PO[Cl.sub.3] mixture to give the 3-chloropyridazine derivative 3. Reaction of 3 with 2-hydroxybenzoylhydrazide and semicarbazide hydrochloride afforded 4 and 5. Reaction of 1 with ethyl chloroacetate gave 8. Reaction of 8 with hydrazine hydrate yielded the hydrazide 9. The hydrazide 9 condensed with the acetylenic ketones and esters 10a-10d and acetylacetone to give the adducts 11a, 11b, 12, 13, and 14. Reacting 1 with formaldehyde and piperidine, morpholine, or piperazine, 3-bromopropanoic acid, acetic anhydride, p-toluenesulphonyl chloride, succinyl chloride, oxalyl chloride, ethanolamine, or ethyl chloroacetate gave the adducts 15-22. The structures of all newly synthesized compounds were evidenced from their spectral and microanalytical data. Key words: 6-aryl-4-pyrazol-l-yl-pyridazinone derivative, 6-aryl-3-chloropyridazine derivative, 3,6-diaryl-1,2,4-triazino [4,3-b]pyridazine derivative, 6-aryl-3-ethoxycarbonylmethoxypyridazine derivative, 6-aryl-3-([omega]-benzoylacetophenone) hydrazinocarbonylmethoxypyridazine.

The synthesis and chemical reactivity of pyridazinedione-based enediynes are presented. The single-crystal X-ray structures showed similar c,d distances (distance between the acetylenic carbons undergoing covalent connection in Bergman cyclization).

Some evidence for the directing effect of the 2-pyridylsilyl groups in Ru-catalyzed intermolecular Pauson-Khand-type reaction (PKR) is provided. Most importantly, the study succeeded in isolating several monometallic Ru complexes relevant to the catalytic reaction.

The properties of a series of complexes trans-[LRu(N[H.sub.3]).sub.4]-NC-Ru(N[H.sub.3]).sub.4] [(methylpyrazinium).sup.n+] where L is an anion ([Cl.sup.-], [F.sup.-], C[N.sup.-], O[H.sup.-]) or neutral ligand (CO. [H.sub.2]O, pyridine, C[H.sub.3]N[O.sub.2]) were analyzed using density functional theory. The objective was to probe the change in electronic coupling between the two ruthenium atoms and between ruthenium and the non-innocent methylpyrazinium ligand as L is varied. Molecular orbital coefficients and optical spectra were derived. The electronic interactions involved are seen to be quite sensitive to the charge on the ruthenium to which the ligand L is attached as L is varied. Exchange (K) and Coulomb (J) energies are also calculated, using the INDO/S model, for the various metal-to-ligand charge-transfer transitions. K correlates well with the energy and intensity of the transitions and specifically with the variation in delocalization induced by variation of L. Key words: DFT, ruthenium, non-innocent ligand, methylpyrazinium, ZINDO.

The catalytic reduction of molecular oxygen O(sub 2) by four kinds of pyrazine derivatives-2,3,5,6-tetramethylpyrazine, 2,3-dimethylquinoxaline,2,3-diphenylquinoxaline, and phenazine is studied by cyclic and rotating disk voltammetry and molecular orbital calculations. It was possible to compare the reactivity of both one-electron reduced forms (the radicals) and two-electron reduced forms of the derivatives toward O(sub 2) in this acidic solution.

The electrochemical, spectroscopic, and electroluminescent properties of a family of diimine complexes of Ru featuring various aliphatic side chains as well as a more extended pi-conjugated system are investigated. The incorporation of di-tert-butyl side chains on the dipyridyl ligand is found to be the most beneficial substitution in terms of reducing self-quenching of luminescence.

Quasielastic neutron scattering is used to probe the solid-state ligand dynamics in the coordination polymer Mn [N (CN)(sub 2)](sub 2)(pyz) {pyz = pyrazine} which has double-interpenetrating 3D lattices. The pyrazine ligand is seen to liberate in a deep potential in the low temperature monoclinic phase.

Classical trajectory calculations made for collisions between vibrationally hot pyrazine and cold collision partner CO appear to show qualitative agreement with preliminary experimental results. On doubling the length of the CO artificially, results in angular momentum transfer to the CO molecule that look like angular momentum transfer to CO2 in analogous pyrazine-CO2 experiments.

The energy gain in specific rational levels of vibrationally excited H(sub 2)O(010) which resulted from collisions with highly vibrationally excited pyrazine in a low-pressure, 298 K environment are investigated using high-resolution transient infrared absorption spectroscopy of water at Lambda approximately equal to 2.7 mm. The molecular properties of the energy acceptor that influence the relaxation of highly virbationally excited molecules are explored.

Byline: R. Wagner (1), M. Czerny (1), J. Bielohradsky (1), W. Grosch (1) Keywords: Pyrazine; Odour threshold; Structure; Molecular modelling; Theoretical receptor Abstract: The odour thresholds (i.e. the lowest concentration at which an odour can be detected) of 80 alkylpyrazines, most of them synthesized, were determined by gas chromatography-olfactometry. Trimethylpyrazine (1) had the lowest threshold (50ng/l air) amongst mono-, di-, tri- and tetramethylpyrazine. Substitution of the methyl group in position 2 of 1 by an ethyl group yielded 2-ethyl-3,5-dimethylpyrazine (2) showing a 4500-fold lower odour threshold than 1. The thresholds of 2-ethenyl-3,5-dimethylpyrazine (3), 2,3-diethyl-5-methylpyrazine (4) and 2-ethenyl-3-ethyl-5-methylpyrazine (5) were as low as that of 2. The threshold of 3-ethenyl-2-ethyl-5-methylpyrazine was 8000 times higher than that of 5, indicating that an ethenyl group was only tolerated in position 2, but not in position 3. A further increase in the odour threshold was found when the ethenyl or the ethyl group was located in position 5. Substitution of the ethyl group of 2 by a (Z)-1-propenyl group increased the threshold only by a factor of 5, whereas the (E)-isomer and the 2-propenyl group enhanced the threshold by factors of 160 and 27000, respectively. Also a propyl, butyl, pentyl, isobutyl or hexyl group in position 2 of 2 was too bulky, and in these molecules the odour threshold was at least 2200 times higher than that of 2. The geometrical structure of a theoretical receptor was obtained by superimposing the minimized structures of pyrazines with low thresholds (Chem-X force-field minimization). Sterically forbidden regions in the resulting model were found by superimposing pyrazines with high thresholds. Author Affiliation: (1) Deutsche Forschungsanstalt fur Lebensmittelchemie, Lichtenbergstrasse 4, D-85748 Garching, Germany e-mail: Werner.Grosch@lrz.tu-muenchen.de, DE Article note: Received: 20 August 1998

The study describes a novel multi-component synthesis of highly substituted 1,6-dihydropyrazine-2,3-dicarbonitrile derivatives. The isocyanide-based three-component reaction gives very high yield of the derivatives at ambient temperature.

A new approach is described for measuring absolute rates for molecular collisions along with contributions of strong and weak collisions. The method is used for the collisional relaxation of highly vibrationally excited pyrazine with HOD and the observed collision rate is nearly twice the Lennard-Jones collision rate.

A volatile route is developed for symmetrical and unsymmetrical pyrazines. The Swern oxidation of the aminodiols is carried out without the protection of the basic amines, showing that the amino diketones are not isolated, but are condensed directly with hydroxylamine to give the substituted pyrazines.

Two new magnetic 3D NaCl-like networks were synthesized in which the metal ions are linked by pyz into (4, 4) sheets and further linked by a [[mu].sub.1,3]-nitrito group. The magnetic investigation reveals that the [[mu].sub.1,3]-nitrite bridge could transmit antiferromagnetic interaction in the case of [Co.sup.2+].

A report on the one-pot synthesis of new 9-alkyl-6-chloropyrido[3,2-e]-[1,2,4] triazolo[4,3-a]pyrazines, which was obtained from 2,3-dichloropyrido[2,3-b]pyrazine in a single step, is presented. The key step was the regioselective nucleophilic aromatic substitution that gave rise to unique isomers before ring closure.

The article explains various mechanisms that are involved in the catalysis and inhibition of nicotinamidase/pyrazinamidase (PncA) from Mycobacterium tuberculosis. Pyrazinecarbonitrile is shown to be one compound that acts as an irreversible inactivator of PcnA.

Cycloparaphenylenes and nitrogen analogues formed from the building blocks pyridinyl, pyrazinyl, pyridazinyl and pyrimidinyl are investigated. Nitrogen substitution has reduced the o,o' steric interactions between neighboring rings and nanohoops pyrazinyl and pyrimidinyl have ribbon-like structure with dihedral angles between neighboring rings near zero.

A supramolecular three-component double-porphyrin tweezer (PT) is prepared by using heteroleptic complex formation along the HETTAP method. The higher stability of PT-(DABCO)(py) is the result of charge transfer from DABCO to the vacant [pi]* orbital of pyrazine across the porphyrin plane.

Asymmetric scandium(III)-catalyzed rearrangement of 3-allyloxyflavones is used for preparing chiral, nonracemic 3,4-chromanediones in high enantioselectivities. The initial studies of the mechanism have supported an intramolecular arrangement pathway that has proceeded via benzopyrylium intermediate.

The mechanism study of C-F reductive elimination from a transition metal complex and the evaluation of C-F bond formation from three Pd(IV) fluoride complexes are described. The ancillary pyridyl-sulfonamide ligand has played a main role for C-F reductive elimination in which an oxygen atom of the sulfonyl group has coordinated to Pd.

Photolysis of triazolo[1,5-b]pyridazine isolated in Ar matrix has generated diazomethylpyridazines and diazopentenynes as detected by IR spectroscopy. Three possible mechanisms of ring-contraction in arylnitrenes are identified by ring-opening to dienylnitrenes, concerted ring-contraction and by spiroazirenes.

A novel [PPh.sub.3]-catalyzed (4+n) annulation of 2-(acetoxymethyl)buta-2,3-dienoates with 1,n-bisnucleophiles (n = 1, 2) is developed for providing a facile synthetic method for cyclopentene and 1,2,3,6-tetrahydropyridazine derivatives. The acetate group of 2-(acetoxymethyl)buta-2,3-dienoate is very important for the formation of 1,4-biselectrophilic intermediate and deuterium studies have shown that the reaction pathway has involved a proton-transfer process.

Several experimental and first-principles studies are conducted to explain the 1D to 3D magnetic transformation taking place in the copper pyrazine dinitrate S = 1/2 antiferromagnetic crystal. The magnetic susceptibility and heat capacity curves of the crystal are shown to remain unchanged with the spin and magnetic chains.