High-affinity alpha-aminobutyric acid A/benzodiazepine ligands: synthesis and structure-activity relationship studies of a new series of tetracyclic imidazoquinoxalines

Recent Developments in the Synthesis, Chemistry and Applications of the Fully Unsaturated 1,2,4-Oxadiazoles

Recent major developments in the synthesis (including solid phase methodologies), chemistry and applications of the fully unsaturated 1,2,4-oxadiazole nucleus are reviewed. The review covers the years 1995–2000.

Radical Ions of 3-Styryl-quinoxalin-2-one Derivatives Studied by Pulse Radiolysis in Organic Solvents

The absorption-spectral and kinetic behaviors of radical ions and neutral hydrogenated radicals of seven 3-styryl-quinoxalin-2(1 H)-one (3-SQ) derivatives, one without substituents in the styryl moiety, four others with electron-donating (R = -CH₃, -OCH₃, and -N(CH₃)₂) or electron-withdrawing (R = -OCF₃) substituents in the para position in their benzene ring, and remaining two with double methoxy substituents (-OCH₃), however, at different positions (meta/para and ortho/meta) have been studied by UV-vis spectrophotometric pulse radiolysis in neat acetonitrile saturated with argon (Ar) and oxygen (O₂) and in 2-propanol saturated with Ar, at room temperature. In acetonitrile solutions, the radical anions (4R-SQ^•-) are characterized by two absorption maxima located at λ_max = 470-490 nm and λ_max = 510-540 nm, with the respective molar absorption coefficients ε_470-490 = 8500-13 100 M^-1 cm^-1 and ε_510-540 = 6100-10 300 M^-1 cm^-1, depending on the substituent (R). All 4R-SQ^•- decay in acetonitrile via first-order kinetics, with the rate constants in the range (1.2-1.5) × 10⁶ s^-1. In 2-propanol solutions, they decay predominantly through protonation by the solvent, forming neutral hydrogenated radicals (4R-SQH^•), which are characterized by weak absorption bands with λ_max = 480-490 nm. Being oxygen-insensitive, the radical cations (4R-SQ^•+) are characterized by a strong absorption with λ_max = 450-630 nm, depending on the substituent (R). They are formed in a charge-transfer reaction between a radical cation derived from acetonitrile (ACN^•+) and substituted 3-styryl-quinoxalin-2-one derivatives (4R-SQ) with a pseudo-first-order rate constant k = (2.7-4.7) × 10⁵ s^-1 measured in solutions containing 0.1 mM 4R-3-SQ. The Hammett equation plot gave a very small negative slope (ρ = -0.08), indicating a very weak influence of the substituents in the benzene ring on the rate of charge-transfer reaction. The decay of 4R-SQ^•+ in Ar-saturated acetonitrile solutions occurs with a pseudo-first-order rate constant k = (1.6-6.2) × 10⁴ s^-1 and, in principle, is not affected by the presence of O₂, suggesting charge-spin delocalization over the whole 3-SQ molecule. Most of the radiolytically generated transient spectra are reasonably well-reproduced by semiempirical PM3-ZINDO/S (for 4R-SQ^•-) and density functional theory quantum mechanics calculations employing M06-2x hybrid functional together with the def2-TZVP basis set (for 4R-SQ^•+).

Design, Synthesis, and Biological Evaluation of Imidazo[1,5-a]quinoline as Highly Potent Ligands of Central Benzodiazepine Receptors

A series of imidazo[1,5-a]quinoline derivatives was designed and synthesized as central benzodiazepine receptor (CBR) ligands. Most of the compounds showed high CBR affinity with Ki values within the submicromolar and subnanomolar ranges with interesting modulations in their structure-affinity relationships. In particular, fluoroderivative 7w (Ki = 0.44 nM) resulted in the most potent ligand among the imidazo[1,5-a]quinoline derivatives described so far. Overall, these observations confirmed the assumption concerning the presence of a large though apparently saturable lipophilic pocket in the CBR binding site region interacting with positions 4 and 5 of the imidazo[1,5-a]quinoline nucleus. The in vivo biological characterization revealed that compounds 7a,c,d,l,m,q,r,w show anxiolytic and antiamnestic activities without the unpleasant myorelaxant side effects of the classical 1,4-BDZ. Furthermore, the effect of 7l,q,r, and 8i in lowering lactate dehydrogenase (LDH) release induced by ischemia-like conditions in rat brain slices suggested neuroprotective properties for these imidazo[1,5-a]quinoline derivatives.

1,4-Bis(2-diazo-acet-yl)piperazine

The asymmetric unit of the title compound, C8H10N6O2, contains one-half mol-ecule, which is completed by a crystallographic center of symmetry. The piperazine ring adopts a chair conformation. In the crystal, weak C-H⋯O inter-actions link the mol-ecules into layers parallel to the bc plane. The crystal packing also exhibits short N⋯N contacts of 3.0467 (16) Å between the terminal diazo N atoms from neighbouring mol-ecules.

Characterization of Tunable Radical Metal-Carbenes: Key Intermediates in Catalytic Cyclopropanation

A new class of radical metal-carbene complex has been characterized as having Fischer-like orbital interactions and adjacent π acceptor stabilization. Density Functional Theory (DFT) along with Natural Bond Orbital (NBO) analysis and Charge Decomposition Analysis (CDA) has given insight into the electronics of this catalytic intermediate in an open-shell cobalt-porphyrin, [Co(Por)], system. The complex has a single bond from the metal to the carbene and has radical character with localized spin density on the carbene carbon. In addition, the carbene carbon is found to be nucleophilic and "tunable" through the introduction of different α-carbon substituents. Finally, based on these findings, rational design strategies are proposed which should lead to the enhancement of catalytic activity.

tert-Butyl 4-(2-diazo-acet-yl)piperazine-1-carboxyl-ate

The title crystal structure, C(11)H(18)N(4)O(3), is the first diazo-acetamide in which the diazo-acetyl group is attached to an N atom. The piperazine ring is in a chair form and hence the mol-ecule has an extended conformation. Both ring N atoms are bonded in an essentially planar configuration with the sum of the C-N-C angles being 359.8 (2) and 357.7 (2)°. In the crystal structure, the O atom of the diazo-acetyl group accepts two H atoms from C-H donors, thus generating chains of weak hydrogen-bonded R(2) (1)(7) rings.

A modified strategy for Pictet-Spengler reaction leading to the synthesis of imidazoquinoxalines on solid phase

An alternative strategy for Pictet-Spengler reaction involving an N-1 linked aromatic amine of imidazole and aldehydes is described. This is in contrast to a typical Pictet-Spengler reaction, which involves an aliphatic amine attached to the carbon of an activated aromatic nucleus and an aldehyde. Our strategy commences with the nucleophilic replacement of fluorine in resin-bound o-fluoro-nitrobenzoic acid with mono- or disubstituted imidazole, followed by reduction of the nitro group to give an N1 linked aromatic amine of the resin-bound imidazole. This was subsequently treated with an aldehyde in toluene at 80 degrees C and then oxidized in the presence of DDQ to give resin-bound imidazoquinoxalines. Finally, acidolytic cleavage furnished the desired compounds in high yields and purities.

Intramolecular 1,3-dipolar cycloadditions of dihydroimidazolium ylides: synthesis of pyrrolo[1,2,3-de]quinoxalines and imidazo[1,2-a]indoles

N-Alkylation of 4,5-dihydroimidazoles with alkene-containing bromomethyl ketones and treatment of the so-formed 4,5-dihydroimidazolium ions with DBU gives rise to an intramolecular 1,3-dipolar cycloaddition reaction that affords (via a reaction cascade involving eliminative ring-opening, recyclisation and prototropic tautomerism) unexpected hexahydropyrrolo[1,2,3-de]quinoxaline products. Steric bulk in both the dihydroimidazole and the dipolarophile allows isolation of an imidazo[1,2-a]indole, the initial product of cycloaddition. When the bromomethyl ketone contains no other functionality, or when cycloaddition is inhibited due to steric constraints, the dihydroimidazolium ion undergoes ring-opening hydrolysis followed by recyclization of the exposed amino ketone to afford either 3-alkyl-1-formylpiperazine-2-ones or 3-aryl-1-formyl-1,4,5,6-tetrahydropyrazines.

Synthesis and antimicrobial activity of certain novel quinoxalines

In this study, certain 3-methyl-2-[4-(substituted amino carbonyl)anilino] quinoxalines, (2a-d) and (3a-d), were synthesized from the new key compound 2-[4-(ethoxycarbonyl)anilino]-3-methyl quinoxaline (1). In addition, a series of 2-[4-(arylidene hydrazinocarbonyl)anilino]-3-methyl quinoxalines (5a-e), as well as their cyclized oxadiazolinyl derivatives (6a-e), and a series of 2-[4-N2-acylhydrazinocarbonyl) anilino]-3-methyl quinoxalines (7a-d), as well as their cyclized oxadiazoiyl derivatives (8a-d) were also prepared. Some of these derivatives were evaluated for antimicrobial activity in vitro. It was found that all the selected compounds exhibit antimicrobial activity and that compound 5b had a broad spectrum of activity.

Dihydroquinolines with amine-containing side chains as potent n-NOS inhibitors

Dihydroquinolines with aminoalkyl side chains have been synthesized and have been shown to be potent n-NOS inhibitors. A marked selectivity versus e-NOS of up to approximately 300-fold was observed, whereas i-NOS was moderately inhibited.

High affinity central benzodiazepine receptor ligands. Part 2: quantitative structure-activity relationships and comparative molecular field analysis of pyrazolo[4,3-c]quinolin-3-ones

A large series of 2-aryl(heteroaryl)-2,5-dihydropyrazolo[4,3-c]quinolin-3(3H)-ones (PQ, 106 compounds), carrying appropriate substituents at the quinoline and N2-phenyl rings, were designed, prepared and tested as central benzodiazepine receptor ligands. Compounds with an affinity significantly higher than the parent compound CGS-8216 were obtained, the most active ligand showing a pIC50 = 10.35. Hansch and comparative molecular field analyses gave coherent results suggesting the main structural requirements of high receptor binding affinity. The possible formation of a three-centred hydrogen bond (HB) at the HB donor site H2, as a key interaction for high receptor binding affinity, was assessed by the calculation and comparison of the molecular electrostatic potentials of a series of selected ligands.

High affinity central benzodiazepine receptor ligands: synthesis and structure-activity relationship studies of a new series of pyrazolo[4,3-c]quinolin-3-ones

A large series of 2-aryl(heteroaryl)-2,5-dihydropyrazolo[4,3-c]quinolin- 3-(3H)-ones, carrying appropriate substituents at the quinoline and N2-phenyl rings, were prepared and tested as central benzodiazepine receptor ligands. Results from structure-affinity relationship studies were in full agreement with previously proposed pharmacophore models and, in addition, quantitative structure-activity analysis gave further significant insight into the main molecular determinants of high benzodiazepine receptor affinity. The intrinsic activity of some active ligands was also determined and preliminary discussed.