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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.

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 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.

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.

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 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.

The article explains synthesis, as well as the characterization of a series of condensed oligopyrazine analogues of acenes, the pyrazinacenes. These compounds are shown to be widely used in the organic thin film transistors.

The synthesis, crystallization, and solid-state NMR dynamics of molecular structures composed of polar pyridine and pyridazine groups as well as their corresponding N-oxides, proposed to emulate the structure and function of macroscopic compasses are described. The preparation and crystal structure of pyridine-[d.sub.2] and pyridazine-[d.sub.10], one labeled with deuteria in the pyridine rotator and the other fully deuterated in the two trityl groups in the stator are also described.

Allylic esters bearing the (2-pyridine)C[O.sub.2]-, (2-pyrazine)C[O.sub.2]-, [(EtO).sub.2]P[O.sub.2]-, [C.sub.6][F.sub.5]C[O.sub.2]-, o-([Ph.sub.2]P)[C.sub.6][H.sub.4]C[O.sub.2]-, MeOC[O.sub.2]-, or AcO- group were examined to establish allylic substitution of secondary allylic alcohol derivatives with alkynyl copper reagents. Mg[Br.sub.2] (3 equiv) promoted the reaction to produce the anti [S.sub.N]2' product in 93% yield with 94% regioselectivity and 99% chirality transfer.

A study was conducted to examine drug-like inhibitors of tau assembly using quantitative high-throughput screening (qHTS) of [approximately]292000 compounds. Findings revealed classes of inhibitors, including novel aminothienopyridazines (ATPZs).

The ultrafast photoelectron imaging technique was used to perform ultrafast internal conversion of [S.sub.2] to [S.sub.1] of pyrazine through a conical intersection of the potential energy surfaces. The two-dimensional time-energy map of the photoelectron angular anisotropy revealed a clear signature of the internal conversion (the color change of green-orange-green in the horizontal direction).

The reaction of Cr[([C.sub.6][H.sub.6]).sub.2] and 1,2,4,5-tetracyanopyrazine (TCNP) has formed [Cr[([C.sub.6][H.sub.6]).sub.2][TCNP], and TCNP is reduced and has formed the eclipsed [pi]- [[TCNP].sub.2.sup.2-] dimer. [[TCNP].sub.2.sup.2-] is described by an atoms-in-molecules analysis as having a 2[e.sup.-]/8c C-C bond that has involved the four C atoms from each six-member ring.

The coordination-driven self-assembly of ortho-linked oxacalix[2]benzene[2]pyrazine as well as oxacalix[2]arene[2]pyrazene with a silver cation has helped in the formation of a discrete molecular cage and a one-dimensional chain of coordination cages. The void of the metal-containing cage is occupied by a methyl group of an acetonitrile bonded to a neighboring cage in the solid state and one benzene ring is present in the void of each one-dimensional chain of coordination cage complex.

o-C-H silylation of arylboronic acids is achieved by using 2-pyrazol-5-ylaniline as an ortho-directing agent, which is temporarily attached to the boronyl group via Ru-catalyzed silylation with hydrosilanes. The silylated products are used in Suzuki-Miyaura coupling, which is followed either by iodination with ICl or by Tamao oxidation to furnish iodine- or hydroxyl-substituted biaryls.

The synthesis, and structural and magnetic characterization of three [Cu.sup.2+]-containing coordination polymers, [Cu(H[F.sub.2])[(pyrazine).sub.2]]Sb[F.sub.6], [[Cu.sub.2]F(HF)(H[F.sub.2])[(pyrazine).sub.4]][(Sb[F.sub.6]).sub.2], and [CuAg([H.sub.3][F.sub.4])[(pyrazine).sub.5]][(Sb[F.sub.6]).sub.2] are presented. The polymers are found to crystallize in either tetragonal or orthorhombic space groups, have structures consisting of 2D square layers of [[M[(pyz).sub.2]].sup.n+] that are linked in the third dimension by either H[F.sub.2.sup.-] or [H.sub.3][F.sub.4.sup.-] and the variations observed in magnetic behaviors are driven by differences in the H***F hydrogen-bonding motifs.

A cobalt-catalyzed cross-coupling of aryl halides with 2-chloropyrimidines or 2-chloropyrazines is described. The main step in this method is the formation of aromatic organozinc reagents and their coupling with 2-chlorodiazines using the same cobalt halide as catalyst and Zn duster under mild conditions.

The 2,3,5,6-tetra(2'-pyridyl)pyrazine molecule (TPPZ) as the counterpart of anilic acids was examined to explore the extensive variety of H-bonding modes compatible with ferroelectricity. The observed slow polarization reversal with modest pyroelectricity provide evidence for the room-temperature ferroelectricity of the ([H.sub.2]-TPPZ)[(Hba).sub.2] crystal.

The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N,N-dihydrodiazatetracenes are reported. The results from experimental and computational studies provide insight into reduced aromaticity of the dihydroheteroacenes.

The synthesis and physicochemical properties of newly obtained palladium(II) complexes of the free-base tetrakis(2,3-(5,6-di-2-pyridylpyrazino)porphyrazine], [Py.sub.8]TPyzPz[H.sub.2] is described. The results provide insights into the behavior of palladium(II) species as photosensitizers for the production of singlet oxygen.

The development of a novel and convenient orthogonal method for the synthesis of asymmetrically tri- and tetrasubstituted pyrazines starting from para-methoxybenzyl-protected 3,5-dichloro-2(1H)-pyrazinones is demonstrated. The key step in the sequence involves mild, one-step iodine-catalyzed methylation with concomitant aromatization of the generated thiopyrazinone.

A comprehensive study of the reactivity of the C-B bond in Suzuki-Miyaura palladium-catalyzed cross-couplings with aryl/heteroaryl halides is described. The reaction involves the facile conversion of substituted 3-methoxypyridazines into the corresponding pyridazin-3(2H)-ones with good functional group tolerance.

A detail study was conducted to study the molecular structures of pyrazine in the 3s Rydberg states and the ground and excited states of its cations. The findings offered insights into the dynamics of ionic states and high-n Rydberg states converging to the [D.sub.1] vibronic states which are short-lived due to electronic autoinization to the [D.sub.0] continuum.

Two flexible thioether-containing heterocyclic ligands bis(2-pyrazylmethyl)sulfide and 2-benzylsulfanylmethylpyrazine have arene rings with changing [pi]-acidities that are used to probe anion-[pi] binding in five 1-dimensional (1-D) coordination polymers formed from the metal salts. The comparison between the polymers formed with these ligands has shown that only the more [pi]-acidic ring is involved in the anion-[pi] interactions.

The wave function-based method namely the CASSCF/MS-CASPT2 method was used to study the Creutz-Taube complex [[[(N[H.sub.3]).sub.5]-Ru-pyrazine-Ru-[(N[H.sub.3]).sub.5]].sup.5+]. The g-factors of the CT ion are shown to be independent of the coupling between the centers and to be the same as in the monomer, because the coupling acts in symmetry different from the one of the ground state.

An OFF-OFF-ON fluorescence sensor is described for metal ions by stepwise complex formation equilibria of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (TPPZ) with metal ions. The OFF-OFF-ON fluorescence sensor can be used in biological applications because its concept should be general enough to be applied to other probe molecules that display stepwise complex formation with metal ions.

The preparation of tetrahydropyrazines, dihydropyrazines, pyrazines, piperazinones, and quinoxalines by 1,4-addition of 1,2-diamines to 1,2-diaza-1,3-butadienes bearing carboxylate, carboxylate, carboxamide or phosphorylated groups at the terminal carbon and subsequent internal heterocyclization is described. The solid-phase reaction of polymer-bound 1,2-diaza-1,3-butadienes with 1,2-diamines has produced pyrazines.