Synthesis, molecular structure and reactivity of 5-methylidene-1,2,3,5-tetrahydroimidazo[2,1-b]quinazolines

Synthesis of novel 5-methylidene-1,2,3,5-tetrahydro[2,1-b]-quinazoline derivatives 2-4 with potential biological activities mediated by alpha-adrenergic and/or imidazoline receptors was performed by reacting 2-chloro-4,5-dihydroimidazole (1) with the corresponding 2-aminoacetophenones. Compound 2, which incorporates an enamine moiety, underwent a 1,3-dipolar cycloaddition reaction with the appropriate nitrones 5-9 to give 1,2,3,5-tetrahydro-imidazo[2,1-b]quinazolin-5,5'-spiro-2',3'-diphenylisoxazol-idines 10-14. Reactions of the title compounds 2 and 4 with dimethyl acetylene-dicarboxylate (DMAD) afforded dimethyl 2-(2,3-dihydroimidazo[2,1-b]quinazolin-5(1H)-ylidenemethyl)but-2-enedioates 15, 16. Imidazo[2,1-b]quinazoline 2 was further treated with acetyl chloride, benzoyl chloride and mesyl chloride to give the 1-substituted derivatives 17, 18 and 19, respectively. The structures of all new compounds obtained were confirmed by elemental analysis and spectral data (IR, (1)H- and (13)C-NMR) as well asX-ray crystallographic analysis of 3 and 18.     

Interactions of porphyrin covalently attached to poly(methacrylic acid) with liposomal membranes

The interactions between the 5-(4-acryloyloxyphenyl)-10,15,20-tritolylporphyrin covalently attached to poly(methacrylic acid) chain (PMA-Po) and phosphatidylcholine liposomes in aqueous solution at different pH values were studied. The binding constants (K(b)) for the liposome- PMA-Po in solutions in the pH range from 6.5 to 9.2 were determined using fluorescence spectroscopy. The binding was found to be efficient. The acid-base properties of the porphyrin chromophores were also studied. Both pK values associated with imine-N protonation of the porphyrin core were found to be 6.4. The quantum yield (Phi(Delta)) of singlet oxygen production by Po in the lipid-PMA-Po system was found to be high (0.88 +/- 0.05).     

Electrosorption of hydrogen into palladium-gold alloys

Hydrogen electrosorption into Pd-Au alloys has been studied in acidic solutions (1 M H₂SO₄) using cyclic voltammetry. Pd-Au electrodes with limited volume were prepared by electrochemical co-deposition. It was found that the maximum H/(Pd+Au) ratios decrease monotonically with increasing gold content and reach zero at ca. 70 at% Au. Similarly to the case of Pd limited volume electrodes, two peaks in the hydrogen region, corresponding to two types of sorbed hydrogen, are observed on voltammograms for Pd-rich alloys. The hydrogen capacity, H/(Pd+Au), measured electrochemically, depends on the sweep rate in the cyclic voltammetry experiments, which suggests that two different mechanisms for hydrogen desorption from the Pd-Au alloy are possible. After a strong decrease of Pd concentration at the electrode surface, caused by long cyclic polarization to sufficiently anodic potentials, the amount of absorbed hydrogen is still significant for alloys initially rich in Pd. The results obtained from CO adsorption experiments suggest that only Pd atoms are active in the hydrogen absorption/desorption process. Electronic Publication     

Interferences in ultratrace speciation of organolead and organotin by gas chromatography with atomic spectrometric detection

Interferences in trace and ultratrace speciation analysis of organotin and organolead compounds in various samples by gas chromatography coupled with atomic absorption spectrometry (AAS) and/or microwave induced plasma atomic emission spectrometry (MIP AES) are investigated. Particular attention is given to the effects of matrix co-extractives and reagents impurities introduced during sample preparation. Their influence on the detection limits is discussed in terms of baseline noise level, blank value, formation of artefacts and signal suppression. Loss of column resolution during the analysis of some matrices is observed.     

Quasi-column chromatography of barbituric acid derivatives II. Separation on Non-polar adsorbents using binary water-organic solvent mixtures as the mobile phase

Summary Twenty barbituric acid derivatives having four different types of substitution were separated in TLC-S chambers. Using non-polar adsorbents (silanized silica gel or silica gel coated with paraffin oil) and binary water-organic solvent mixtures as the mobile phase. Linear relationships between R_M and the concentration of the organic solvent were observed for the majority of the investigated compounds. The non-polar absorbents ensure a better separation than untreated silica gel especially for the therapeutically useful C₅ disubstituted barbiturates. The results can be used for the optimization of the systems for the chromatography of barbiturates. The R_f values were correlated with the number of carbon atoms of the substitutions, molecular connectivity and a parameter associated with the molecular volume. The best correlations were obtained for this last parameter.     

Reactions of Polycyclic Ketones with Dimethoxycarbene; a Convenient Route for a ‘One‐Pot’ Preparation of Some α‐Hydroxycarboxylic Acid Esters

Polycyclic ‘cage’ ketones, such as pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecan‐8‐one ( 10 ), pentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane‐8,11‐dione ( 11 ), and adamantan‐2‐one ( 16 ) were treated with the nucleophilic dimethoxycarbene (DMC; 1 ), which was generated thermally from 2,5‐dihydro‐2,2‐dimethoxy‐5,5‐dimethyl‐1,3,4‐oxadiazole ( 4a ) in boiling toluene. In this ‘one‐pot’ procedure, the α‐hydroxycarboxylic acid ester 12 or a corresponding derivative 15 or 17 was obtained (Schemes 4–7). Additionally, ‘cage’ thione 21 was treated with DMC under the same conditions yielding dimethoxythiirane 22 (Scheme 8). Subsequent hydrolysis or desulfurization (followed by hydrolysis on silica gel) of 22 gave α‐mercaptocarboxylate 25 and the corresponding desulfurized ester 24 , respectively. In all cases, the addition of DMC occurred stereoselectively, and the addition from the exo‐face is postulated to explain the structures of the isolated products.     

Stabilization of very rare tautomers of 1-methylcytosine by an excess electron

We characterized valence anionic states of 1-methylcytosine using various electronic structure methods. We found that the most stable valence anion is related to neither the canonical amino-oxo nor a rare imino-oxo tautomer, in which a proton is transferred from the N4 to N3 atom. Instead, it is related to an imino-oxo tautomer, in which the C5 atom is protonated. This anion is characterized by an electron vertical detachment energy (VDE) of 2.12 eV and it is more stable than the anion based on the canonical tautomer by 1.0 kcal/mol. The latter is characterized by a VDE of 0.31 eV. Another unusual low-lying imino-oxo tautomer with a VDE of 3.60 eV has the C6 atom protonated and is 3.6 kcal/mol less stable than the anion of the canonical tautomer. All these anionic states are adiabatically unbound with respect to the canonical amino-oxo neutral, with the instability of 5.8 kcal/mol for the most stable valence anion. The mechanism of formation of anionic tautomers with carbon atoms protonated may involve intermolecular proton transfer or dissociative electron attachment to the canonical neutral tautomer followed by a barrier-free attachment of a hydrogen atom to the C5 or C6 atom. The six-member ring structure of anionic tautomers with carbon atoms protonated is unstable upon an excess electron detachment. Indeed the neutral systems collapse without a barrier to a linear or a bicyclo structure, which might be viewed as lesions to DNA or RNA. Within the PCM hydration model, the anions become adiabatically bound with respect to the corresponding neutrals, and the two most stable tautomers have a carbon atom protonated.     

1,3-Dipole mit zentralem S-Atom aus der Umsetzung von Aziden mit Thiocarbonyl-Verbindungen: Eine unerwartete MeS-Wanderung im Abfangprodukt eines ‘Thiocarbonyl-aminids’ mit Dithiobenzoesäure-methylester

1,3-Dipoles with a Central S-Atom from the Reaction of Azides and Thiocarbonyl Compounds: An Unexpected MeS Migration in the Trapping Product of a ‘Thiocarbonyl-aminide’ with Methyl Dithiobenzoate Reaction of PhN₃ with O-methyl thiobenzoate ( 11a ) and thioacetate ( 11c ) as well as with the dithio esters 11b,d at 80° yields the corresponding imidates and thioimidates 12 (Scheme 3). The formation of 12 is rationalized by a 1,3-dipolar cycloaddition of the azide and the C?S group followed by successive elimination of N₂ and S. In the three-component reaction of 11b , PhN₃, and the sterically crowded thioketone 1a , 1,2,4-trithiolane 13a and 1,4,2-dithiazolidine 3a are formed in addition to 12b (Scheme 4). The heterocycles 13a and 3a are trapping products of 1a and ‘thiocarbonyl-thiolate’ 5a and ‘thiocarbonyl-aminide’ 2a (Ar?Ph), respectively (Scheme 6). These 1,3-dipoles are formed as reactive intermediates. Surprisingly, in the presence of catalytic amounts of acids, the major product is the (methyldithio)cyclobutyl thioimidate of type 14 (Scheme 5), formed by an acid-catalyzed MeS migration in dithiazolidine 17 . A reaction mechanism is proposed in Scheme 7.